Experimental Design and Scientific Method Practice Questions — ACT — Page 5

A student proposed the following hypothesis regarding soil drainage:

*Hypothesis*: Soil permeability (the rate at which water flows through soil) is determined by the average particle size of the soil, such that soils with larger average particle sizes will always have higher water flow rates, regardless of the compaction level of the soil.

To test this hypothesis, the student measured the water flow rate, in milliliters per minute ( $\text{mL/min}$ ), through three different soil samples under both uncompacted and compacted conditions. The results are shown in the table below.

Soil Sample	Average Particle Size ( $\text{mm}$ )	Flow Rate - Uncompacted ( $\text{mL/min}$ )	Flow Rate - Compacted ( $\text{mL/min}$ )
X	$0.1$	$15$	$3$
Y	$0.5$	$45$	$12$
Z	$2.0$	$120$	$8$

Based on these results, do the data support the student's hypothesis?

Yes; under both uncompacted and compacted conditions, the flow rate increased as the average particle size increased from Soil X to Soil Z.

Yes; the flow rate of each soil sample was higher under uncompacted conditions than under compacted conditions.

No; under compacted conditions, Soil Z had a lower flow rate than Soil Y, even though Soil Z has a larger average particle size.

No; under uncompacted conditions, Soil Y had a higher flow rate than Soil X, which contradicts the hypothesis.

Show answer & explanation

Answer: No; under compacted conditions, Soil Z had a lower flow rate than Soil Y, even though Soil Z has a larger average particle size.

Answer

No; under compacted conditions, Soil Z had a lower flow rate than Soil Y, even though Soil Z has a larger average particle size.

The correct answer is the option stating that under compacted conditions, Soil Z had a lower flow rate than Soil Y, even though Soil Z has a larger average particle size. The student's hypothesis states that soils with larger average particle sizes will always have higher flow rates, regardless of compaction level. However, the data show that in compacted conditions, Soil Z (average particle size

2.0\text{ mm}

) has a water flow rate of

8\text{ mL/min}

, which is lower than that of Soil Y (average particle size

0.5\text{ mm}

, flow rate

12\text{ mL/min}

). This direct contradiction means the hypothesis is not supported by the data.

Step-by-Step Solution

1

Identify the student's hypothesis and the conditions it applies to.

The hypothesis asserts that soils with larger average particle sizes will always have higher flow rates, regardless of the compaction level.

This establishes the rule that the experimental data must satisfy in order to support the hypothesis.

2

Examine the data for both uncompacted and compacted conditions.

In uncompacted conditions, flow rates increase as particle size increases (

15 < 45 < 120\text{ mL/min}

). In compacted conditions, the flow rate increases from Soil X (

3\text{ mL/min}

) to Soil Y (

12\text{ mL/min}

), but decreases for Soil Z (

8\text{ mL/min}

), which has the largest particle size.

Evaluating each set of conditions separately is required because the hypothesis states the relationship must hold true regardless of compaction level.

3

Compare the behavior under compacted conditions to the hypothesis.

Under compacted conditions, Soil Z (particle size

2.0\text{ mm}

) has a lower flow rate than Soil Y (particle size

0.5\text{ mm}

), showing that a larger particle size does not always lead to a higher flow rate.

A single contradiction is sufficient to refute the student's hypothesis.

Key Concept

Formulating and Modifying Hypotheses based on experimental results and identifying counterexamples.

Question 82Question →

A student proposed the following hypothesis regarding the decomposition of hydrogen peroxide ( $H_2O_2$ ) in the presence of the catalyst catalase:

*Hypothesis*: The rate of $H_2O_2$ decomposition increases linearly as the concentration of catalase increases, because more catalyst molecules are available to speed up the reaction.

To test this hypothesis, the student measured the rate of oxygen ( $O_2$ ) gas production (in mL/min) at various catalase concentrations (in percent, $\%$ ) while keeping the substrate concentration and temperature constant. The results are shown in the table below:

Catalase concentration ( $\%$ )	$O_2$ production rate (mL/min)
0.0	0.0
1.0	5.4
2.0	10.8
3.0	16.2
4.0	16.3
5.0	16.3

Based on these results, how should the student modify the hypothesis?

The hypothesis should be modified to state that the reaction rate is inversely proportional to catalase concentration at all tested concentrations.

The hypothesis should be modified to state that the reaction rate increases continuously without limit as catalase concentration increases.

The hypothesis should be modified to state that the reaction rate increases linearly with catalase concentration up to a threshold, above which the rate remains constant.

The hypothesis should be modified to state that the reaction rate decreases once the catalase concentration exceeds 3.0\%.

Show answer & explanation

Answer: The hypothesis should be modified to state that the reaction rate increases linearly with catalase concentration up to a threshold, above which the rate remains constant.

Answer

The hypothesis should be modified to state that the reaction rate increases linearly with catalase concentration up to a threshold, above which the rate remains constant.

The experimental results demonstrate a linear relationship between catalase concentration and oxygen production rate from 0.0\% to 3.0\%, with the rate increasing by exactly 5.4 mL/min for each 1.0\% increase in concentration. However, at concentrations of 4.0\% and 5.0\%, the rate levels off and remains nearly constant at 16.3 mL/min. The correct option reflects this transition by stating that the rate increases linearly up to a threshold (around 3.0\%), after which it remains constant.

Step-by-Step Solution

1

Analyze the student's initial hypothesis.

The initial hypothesis predicts a linear, unlimited increase in the reaction rate as the catalase concentration increases.

To evaluate a hypothesis, we must first identify its specific predictions.

2

Examine the relationship between catalase concentration and oxygen production rate in the data table.

From 0.0\% to 3.0\% catalase, the rate increases linearly by 5.4 mL/min for every 1.0\% increase in catalase concentration (

0.0 \rightarrow 5.4 \rightarrow 10.8 \rightarrow 16.2

).

This step determines the range over which the original hypothesis holds true.

3

Analyze the trend in the data table above the 3.0\% catalase concentration.

At 4.0\% and 5.0\% catalase, the rate remains constant at approximately 16.3 mL/min.

Identifying where the data deviates from the predicted trend shows how the hypothesis must be modified.

4

Synthesize the two trends to modify the hypothesis.

The rate increases linearly up to 3.0\% (a threshold) and then plateaus, meaning the hypothesis must be modified to include this threshold and constant behavior.

This produces the final modified hypothesis that is fully supported by all experimental data.

Key Concept

Modifying a hypothesis to align with data that shows a transition from a linear increase to a plateau (saturation effect).

Estimated Time:1m 30s

Question 83Question →

Experiment 1
Yeast suspensions were incubated in flasks containing $10\%$ solutions of three different sugars (glucose, sucrose, and lactose) at a constant temperature of $35^\circ\text{C}$ . The volume of carbon dioxide ( $\text{CO}_2$ ) gas produced was measured every $5$ minutes for a total of $30$ minutes.

Experiment 2
Yeast suspensions were incubated in flasks containing a $10\%$ glucose solution at four different temperatures ( $15^\circ\text{C}$ , $25^\circ\text{C}$ , $35^\circ\text{C}$ , and $45^\circ\text{C}$ ). The volume of $\text{CO}_2$ gas produced was measured only once, exactly $30$ minutes after incubation began.

Statement to evaluate:
The two experiments differed in their independent variables (sugar type vs. temperature) and also in how the dependent variable ( $\text{CO}_2$ production) was monitored over time.

True

False

Show answer & explanation

Answer: True

Answer

True

The statement is correct because Experiment 1 manipulates sugar type (independent variable) and collects data dynamically at

5

-minute intervals, while Experiment 2 manipulates temperature (independent variable) and collects a single endpoint data point at

30

minutes.

Step-by-Step Solution

1

Identify and compare the independent variables of both experiments.

In Experiment 1, the sugar type (glucose, sucrose, lactose) is varied, while in Experiment 2, the temperature (

15^\circ\text{C}

,

25^\circ\text{C}

,

35^\circ\text{C}

,

45^\circ\text{C}

) is varied. These independent variables are different.

To determine if the independent variables differ between the experimental designs.

2

Identify and compare the methods of monitoring the dependent variable.

In Experiment 1, the volume of

\text{CO}_2

is measured repeatedly (every

5

minutes for

30

minutes) to observe the rate of reaction. In Experiment 2, the volume is measured only once at the end (

30

minutes). These monitoring intervals/methods are different.

To determine if the method of tracking the dependent variable differs between the experimental designs.

3

Evaluate the statement based on the findings from steps 1 and 2.

Since both the independent variables and the monitoring protocols differ, the statement is correct.

To arrive at the final true/false evaluation.

Key Concept

Comparing and Contrasting Multiple Experimental Designs

Question 84Question →

A student investigates how different colors of light affect the rate of photosynthesis in *Elodea* plants. The student places one *Elodea* plant in each of four separate glass beakers filled with water. Each beaker is exposed to a different color of light (red, blue, green, or white) by placing colored filters over the light source. To ensure enough light reaches each beaker, the student places the beakers at different distances from the light source: the beaker with the green filter is placed $10\text{ cm}$ away, the blue filter beaker is $20\text{ cm}$ away, the red filter beaker is $30\text{ cm}$ away, and the white light beaker is $40\text{ cm}$ away. After two hours, the student measures the volume of oxygen gas produced by each plant. Which of the following identifies a confounding variable in this experimental design that invalidates the student's conclusion about the effect of light color?

The volume of oxygen gas produced, because it varies depending on the color of light.

The species of plant used, because different plant species have different baseline rates of photosynthesis.

The varying distances of the beakers from the light source, which changes the light intensity received by each plant.

The duration of light exposure, because two hours may not be long enough to observe a measurable difference.

Show answer & explanation

Answer: The varying distances of the beakers from the light source, which changes the light intensity received by each plant.

Answer

The varying distances of the beakers from the light source, which changes the light intensity received by each plant.

The correct answer is the option stating that the varying distances of the beakers from the light source is a confounding variable. Changing the distance of the plant from the light source alters the light intensity it receives. Because both the color of light and the light intensity vary among the beakers, it is impossible to determine which factor caused any observed changes in the volume of oxygen gas produced.

Step-by-Step Solution

1

Identify the independent variable (the variable intended to be changed) and the dependent variable (the variable being measured).

The independent variable is the color of light (red, blue, green, white). The dependent variable is the volume of oxygen gas produced.

This establishes the relationship that the experiment is designed to test.

2

Scan the experimental setup to identify any other variables that changed between the different treatment groups.

The student placed the beakers at different distances (

10\text{ cm}

,

20\text{ cm}

,

30\text{ cm}

, and

40\text{ cm}

) from the light source.

Any factor other than the independent variable that differs across treatment groups is a potential confounding variable.

3

Determine if the varying factor (distance) could influence the dependent variable (photosynthesis rate).

Distance changes light intensity, which directly affects the rate of photosynthesis.

Because both the independent variable (color) and the confounding variable (distance/intensity) varied across groups, the student cannot determine which factor caused the observed changes in oxygen production.

Key Concept

Identifying Confounding Variables

Estimated Time:1m 30s

Question 85Question →

A student group conducts an experiment to compare the thermal conductivity of four different metal rods (copper, iron, aluminum, and brass). Each rod is placed on a stand, and one end is heated with a candle. The students measure the time ( $t$ , in seconds) it takes for a small wax sphere placed at the opposite end of the rod to melt. To ensure a fair comparison, several variables must be controlled. Match each potential experimental deviation on the left with the specific confounding effect or type of error it introduces on the right.

Click a left item, then click its matching right item

Show answer & explanation

Answer

The correct matches pair rod thickness differences with altered heat transfer rates (confounding comparison), candle positioning with changed conduction distance, insufficient cooling with elevated initial thermal states, and stopwatch drift with systematic measurement error.

Each experimental deviation is correctly matched to its physical or analytical consequence: rod diameter affects the conduction area, heating position affects conduction distance, insufficient cooling affects the starting thermal baseline, and stopwatch drift affects overall measurement accuracy.

Step-by-Step Solution

1

Analyze how physical dimensions affect heat conduction.

Using rods of different diameters alters the cross-sectional area, which directly changes the rate of heat transfer regardless of the metal type. This matches the description of altering heat transfer independently of conductivity.

Conduction rate is proportional to cross-sectional area.

2

Analyze how distance affects conduction time.

Changing the distance between the candle and the wax sphere changes the conduction path length, which directly alters the time it takes for heat to travel. This matches the description of changing the travel distance.

Heat transfer time depends on the distance over which conduction occurs.

3

Analyze thermal baseline conditions.

Failing to cool the rods between trials means they start with residual thermal energy, meaning less heat must be conducted to reach the melting point of wax. This matches the description of introducing an elevated initial thermal energy state.

The initial temperature of the rods must be controlled to ensure equal heat input is needed for all trials.

4

Analyze the nature of the measurement tool error.

A stopwatch that drifts systematically changes all time measurements in a predictable manner, representing a systematic measurement error rather than an uncontrolled physical variable of the setup. This matches the description of introducing systematic error.

Errors in measurement tools affect data collection accuracy but do not physically alter the experiment's process.

Key Concept

Identifying Confounding Variables and Measurement Errors

Estimated Time:1m 30s

Question 86Question →

A student conducted an experiment to determine how the concentration of a sodium chloride ( $\text{NaCl}$ ) solution affects the rate of rust formation on iron nails. The student prepared $4$ beakers with different $\text{NaCl}$ concentrations, placed $1$ identical iron nail in each beaker, and positioned the beakers at various locations in the laboratory. The experimental setup is summarized in the table below:

Beaker	$\text{NaCl}$ Concentration (\%)	Volume of Solution (mL)	Location in Laboratory
$1$	$0$	$100$	Next to a sunny window
$2$	$5$	$100$	Inside a closed wooden cabinet
$3$	$10$	$100$	Directly above a heating vent
$4$	$15$	$100$	On an open laboratory bench

After $5$ days, the student measured the mass of rust that had accumulated on each nail. Which of the following factors represents an uncontrolled variable in this experiment that could confound the results?

The mass of rust accumulated on each nail after

5

days

The location of each beaker in the laboratory

The concentration of sodium chloride (

\text{NaCl}

) in the solutions

The volume of the salt solution in each beaker

Show answer & explanation

Answer: The location of each beaker in the laboratory

Answer

The location of each beaker in the laboratory

The correct answer is the location of each beaker in the laboratory. In a controlled experiment, only the independent variable (in this case, the sodium chloride concentration) should be changed between groups. By placing the beakers in different locations, the student introduced additional variables such as temperature and light intensity, which can also affect the rate of rust formation. This makes it impossible to determine if any difference in rust mass was caused by the salt concentration or the environmental conditions of the location.

Step-by-Step Solution

1

Identify the independent variable and the dependent variable in the experimental setup.

The independent variable is the sodium chloride (

\text{NaCl}

) concentration, and the dependent variable is the mass of rust accumulated on the nails.

Knowing which variables are intentionally changed (independent) and measured (dependent) helps separate them from the variables that need to be controlled.

2

Examine the variables that are kept constant (controlled variables) and those that vary unintentionally.

The volume of the solution is kept constant at

100\text{ mL}

for all trials. However, the location of each beaker varies (window, cabinet, heating vent, bench).

To ensure a fair test, all conditions other than the independent variable must be kept identical across all trials.

3

Determine which varying factor introduces a confounding variable.

The different locations introduce varying temperature and light conditions, which are known to affect chemical reaction rates like rusting.

An uncontrolled variable that changes along with the independent variable is a confounding factor, as it makes it impossible to isolate the cause of the observed changes.

Key Concept

Identifying confounding variables (uncontrolled factors that vary across experimental groups) and understanding how they compromise the validity of experimental conclusions.

Question 87Question →

An investigator wants to identify potential confounding variables and sources of error in various scientific investigations. Match each experimental design setup on the left with the corresponding source of error or confounding factor on the right.

Click a left item, then click its matching right item

Show answer & explanation

Answer

Bean plant growth in different soils matches with uncontrolled light exposure. Evaporation rates of different liquids matches with varying thermal energy from sunlight. Temperature effect on chemical reaction matches with differing rates of reactant collision due to variable stirring speeds. Exercise intensity effect on heart rate matches with baseline cardiovascular differences due to age.

Each setup is correctly matched to the uncontrolled variable that systematically co-varies with the independent variable, thereby obscuring the true relationship between the independent and dependent variables.

Step-by-Step Solution

1

Analyze each experimental design setup on the left to identify the independent variable and locate any uncontrolled variables that change systematically alongside it.

For the first setup, the independent variable is soil type, but light exposure also changes between rooms. For the second, the independent variable is liquid type, but sunlight exposure varies. For the third, the independent variable is temperature, but stirring speed varies. For the fourth, the independent variable is exercise intensity, but participant age varies.

Identifying factors that vary alongside the independent variable helps determine the specific source of error or confounding factor.

2

Connect the uncontrolled variables to their potential physical or biological impact on the dependent variable.

Light exposure affects bean growth; sunlight thermal energy affects liquid evaporation; stirring speed affects reactant collision rate; and age affects cardiovascular performance.

Linking the uncontrolled variable to its direct impact allows for the correct matching pair to be established.

Key Concept

Identifying confounding variables and uncontrolled factors in experimental designs

Estimated Time:1m 30s

Question 88Question →

A group of students designed three separate experiments to investigate different physical and chemical processes. In each design, a specific uncontrolled variable or a systematic source of error was introduced.

* Experiment 1: To study how the concentration of reactant $A$ affects the rate of a chemical reaction, students combined reactant $A$ with reactant $B$ in three separate test tubes. They used $1.0\text{ M}$ , $2.0\text{ M}$ , and $3.0\text{ M}$ solutions of reactant $A$ . However, they used test tubes of different diameters ( $15\text{ mm}$ , $20\text{ mm}$ , and $25\text{ mm}$ ) for each concentration, measuring the time it took for the mixture to change color.
* Experiment 2: To study the effect of temperature on the rate of gas diffusion, students placed a gas canister at $20^\circ\text{C}$ , $40^\circ\text{C}$ , and $60^\circ\text{C}$ at one end of a closed horizontal tube and measured the time required for the gas to travel to the other end. Because the trials were performed on different days, the relative humidity in the room fluctuated between $30\%$ and $75\%$ during testing.
* Experiment 3: To compare the density of three different liquid samples ( $X$ , $Y$ , and $Z$ ), students used a graduated cylinder to measure $50\text{ mL}$ of each liquid and recorded their masses using a digital balance. However, the balance was not zeroed (tared) before measuring Liquid $Z$ , so the balance registered an initial reading of $+1.2\text{ g}$ before any mass was added.

Match each experiment with its primary source of error or confounding variable.

Click a left item, then click its matching right item

Show answer & explanation

Answer

Experiment 1 matches with varying test tube diameters; Experiment 2 matches with fluctuating relative humidity; Experiment 3 matches with a systematic calibration offset in the scale.

Each experimental setup contains a specific design flaw: Experiment 1 fails to control test tube geometry, Experiment 2 fails to control ambient environmental conditions, and Experiment 3 suffers from systematic scale calibration error.

Step-by-Step Solution

1

Analyze Experiment 1 to identify any parameters changed other than the independent variable.

The independent variable is reactant concentration, but the tube diameter is also varied. This changes the liquid's surface area and reaction geometry, representing an uncontrolled factor.

To determine the source of error, identify any variable that changes between trials other than the independent variable.

2

Analyze Experiment 2 for external environmental conditions that were not held constant.

The relative humidity in the room fluctuated between

30\%

and

75\%

across different testing days, which affects air density.

Environmental factors that vary during testing are uncontrolled variables that can confound the rate of gas diffusion.

3

Analyze Experiment 3 for measurement or instrumentation issues.

The digital balance was not zeroed before weighing Liquid

Z

, resulting in a

+1.2\text{ g}

offset.

Failure to zero a balance before measurement causes a systematic measurement error (offset bias).

Key Concept

Identifying sources of error (systematic error) and confounding variables (uncontrolled factors) in scientific experiments.

Estimated Time:1m 30s

Question 89Question →

A student conducted an experiment to investigate the effect of pH on the rate of yeast fermentation. The student prepared four flasks, each containing $2.0\text{ g}$ of yeast and $5.0\text{ g}$ of glucose dissolved in $100\text{ mL}$ of distilled water. Each flask was adjusted to a specific pH level and placed in a water bath at a designated temperature. The volume of carbon dioxide ( $\text{CO}_2$ ) gas produced in each flask was measured after $15\text{ minutes}$ :

Flask	pH	Temperature ( $^\circ\text{C}$ )	Yeast Mass (g)	Glucose Mass (g)	$\text{CO}_2$ Produced (mL)
1	$4.0$	$30.0$	$2.0$	$5.0$	$12.4$
2	$6.0$	$30.0$	$2.0$	$5.0$	$28.1$
3	$8.0$	$25.0$	$2.0$	$5.0$	$15.3$
4	$10.0$	$30.0$	$2.0$	$5.0$	$6.2$

Based on the table, which of the following factors represents a confounding variable that prevents the student from drawing a valid conclusion about the effect of pH on the fermentation rate?

The temperature of Flask 3 was lower than that of the other three flasks.

The mass of glucose was kept constant at 5.0 g across all four flasks.

The student did not include a flask with a pH of 7.0 to serve as a neutral control.

The volume of carbon dioxide gas produced was not controlled and varied between flasks.

Show answer & explanation

Answer: The temperature of Flask 3 was lower than that of the other three flasks.

Answer

The temperature of the third flask was lower than that of the other three flasks, introducing a confounding variable.

The temperature of the third flask was kept at

25.0^\circ\text{C}

while all other flasks were kept at

30.0^\circ\text{C}

. Because temperature directly affects yeast metabolic rates and enzyme activity, this difference introduces an uncontrolled variable. Consequently, it is impossible to determine whether the difference in carbon dioxide production between the third flask and the other flasks is due to the change in pH or the change in temperature.

Step-by-Step Solution

1

Identify the intended independent variable and the dependent variable.

The independent variable is pH (varied from

4.0

to

10.0

), and the dependent variable is the volume of

\text{CO}_2

gas produced (measured outcome).

Understanding the experimental goal helps identify which variables should vary and which should remain constant.

2

Examine the remaining columns in the data table to verify if all other variables were kept constant.

Yeast mass is constant at

2.0\text{ g}

and glucose mass is constant at

5.0\text{ g}

across all flasks. However, temperature is

30.0^\circ\text{C}

for Flasks 1, 2, and 4, but drops to

25.0^\circ\text{C}

for Flask 3.

A valid experiment must only vary one independent variable at a time; any other variable that changes acts as a confounding factor.

3

Evaluate the impact of this uncontrolled variable on the experimental validity.

Since temperature also affects yeast fermentation rates, the difference in temperature for Flask 3 prevents a clear determination of whether its gas production rate (

15.3\text{ mL}

) was due to pH

8.0

or the lower temperature of

25.0^\circ\text{C}

.

This confirms that temperature is the primary confounding variable in the design.

Key Concept

Identifying Sources of Error and Confounding Variables

Question 90Question →

A student conducts an experiment to investigate how the concentration of hydrochloric acid ( $\text{HCl}$ ) affects the rate of chemical reaction with calcium carbonate ( $CaCO_3$ ). The experimental conditions for the four trials are shown in the table below:

Trial	$\text{HCl}$ Concentration ( $\text{M}$ )	Mass of $CaCO_3$ ( $\text{g}$ )	Form of $CaCO_3$	Volume of $\text{HCl}$ ( $\text{mL}$ )
1	$0.5$	$5.0$	Large chips	$50$
2	$1.0$	$5.0$	Large chips	$50$
3	$1.5$	$5.0$	Fine powder	$50$
4	$2.0$	$5.0$	Fine powder	$50$

Which of the following is the primary confounding variable in this experiment that prevents the student from drawing a valid conclusion about the effect of acid concentration?

The concentration of the hydrochloric acid (

\text{HCl}

) solution

The volume of carbon dioxide (

CO_2

) gas produced

The surface area of the calcium carbonate (

CaCO_3

) particles

The initial mass of calcium carbonate (

CaCO_3

) added to each flask

Show answer & explanation

Answer: The surface area of the calcium carbonate ( $CaCO_3$ ) particles

Answer

The surface area of the calcium carbonate (

CaCO_3

) particles is the primary confounding variable because it varies between trials alongside the independent variable.

The correct answer is the surface area of the calcium carbonate (

CaCO_3

) particles. A confounding variable is an uncontrolled factor that changes along with the independent variable, making it impossible to determine which factor caused the change in the dependent variable. In this experiment, the student changes the physical form of the calcium carbonate from large chips (low surface area) in Trials 1 and 2 to a fine powder (high surface area) in Trials 3 and 4. Since a larger surface area increases the reaction rate, any differences in the rate of carbon dioxide gas production could be caused by either the higher acid concentration or the increased surface area.

Step-by-Step Solution

1

Identify the independent variable (the factor intentionally changed) and the dependent variable (the outcome measured).

The independent variable is the concentration of hydrochloric acid, and the dependent variable is the reaction rate (measured by gas production).

This establishes the core relationship being tested in the experiment.

2

Identify the controlled variables that are kept constant across all trials.

The mass of calcium carbonate (

5.0\text{ g}

), the volume of acid (

50\text{ mL}

), and the time duration are controlled variables.

Controlled variables ensure that any observed change in the dependent variable is due only to the independent variable.

3

Identify any uncontrolled factor that varies between trials and could affect the reaction rate.

The physical form of the calcium carbonate changes from large chips in Trials 1 and 2 to fine powder in Trials 3 and 4.

Changing the physical form changes the surface area of the reactant, which is a known factor influencing reaction rates, thereby introducing a confounding variable.

Key Concept

Identifying Confounding Variables in Experimental Design

Estimated Time:1m 30s

Question 91Question →

In scientific experiments, failing to control variables or using improper measurement techniques can introduce errors. Match each experimental scenario with the primary source of error or confounding variable it introduces.

Click a left item, then click its matching right item

Show answer & explanation

Answer

The experimental scenarios match their corresponding sources of error as follows: testing wind speed in different locations matches ambient environmental confounding factors; yeast respiration trials with different sugar brands matches uncontrolled chemical composition; a miscalibrated temperature probe matches systematic measurement error; and using plants of varying initial sizes matches biological variation confounding factors.

The correct matches align each scenario with its primary experimental flaw. Changing the location of the soil containers introduces ambient temperature and humidity variations (environmental factors). Varying the chemical composition of the sugar introduces an uncontrolled reactant ratio (chemical composition). The consistently offset digital thermometer demonstrates a calibration issue (systematic error). Finally, selecting seedlings of different initial states introduces baseline variation in the experimental units (biological variation).

Step-by-Step Solution

1

Analyze the wind speed experiment.

Placing soil containers in different physical environments (window vs. closet) changes temperature and air flow, which are ambient environmental confounders.

To test wind speed alone, ambient temperature and humidity must be kept constant.

2

Analyze the yeast respiration experiment.

Using different sugar brands with varying chemical ratios introduces an uncontrolled nutritional variable.

Yeast metabolizes glucose and fructose at different rates, so the type of sugar must be kept constant.

3

Analyze the temperature measurement setup.

A temperature probe reading consistently

2.0^\circ\text{C}

low represents a systematic calibration offset.

Systematic errors shift all measurements in the same direction by a consistent amount.

4

Analyze the plant growth experiment.

Using seedlings of varying initial heights introduces baseline biological differences.

Differences in starting height or health confound the final growth measurement since the starting point is not uniform.

Key Concept

Identifying sources of error and confounding variables is essential to establish clear cause-and-effect relationships and ensure validity in experimental designs.

Question 92Question →

An investigation is designed to examine the reaction rate between magnesium ( $\\text{Mg}$ ) and hydrochloric acid ( $\\text{HCl}$ ). In the baseline trial, a single $2.0\\text{ g}$ solid strip of $\\text{Mg}$ is placed into $100\\text{ mL}$ of $1.0\\text{ M}$ $\\text{HCl}$ solution at $22^\\circ\\text{C}$ , and the volume of hydrogen gas ( $\\text{H}_2$ ) released is recorded over $5\\text{ minutes}$ .

To isolate the effect of reactant surface area on the reaction rate, a second trial must be designed. Which of the following experimental setups would allow for a valid comparison to the baseline trial?

Reacting

4.0\\text{ g}

of solid

\\text{Mg}

strips with

100\\text{ mL}

of

1.0\\text{ M}

\\text{HCl}

at

22^\\circ\\text{C}

, and recording the volume of

\\text{H}_2

gas released over

5\\text{ minutes}

.

Reacting

2.0\\text{ g}

of ground

\\text{Mg}

powder with

100\\text{ mL}

of

1.0\\text{ M}

\\text{HCl}

at

22^\\circ\\text{C}

, and recording the volume of

\\text{H}_2

gas released over

5\\text{ minutes}

.

Reacting

2.0\\text{ g}

of ground

\\text{Mg}

powder with

200\\text{ mL}

of

2.0\\text{ M}

\\text{HCl}

at

30^\\circ\\text{C}

, and recording the volume of

\\text{H}_2

gas released over

5\\text{ minutes}

.

Reacting a

2.0\\text{ g}

solid strip of zinc (

\\text{Zn}

) with

100\\text{ mL}

of

1.0\\text{ M}

\\text{HCl}

at

22^\\circ\\text{C}

, and recording the volume of

\\text{H}_2

gas released over

5\\text{ minutes}

.

Show answer & explanation

Answer: Reacting $2.0\\text{ g}$ of ground $\\text{Mg}$ powder with $100\\text{ mL}$ of $1.0\\text{ M}$ $\\text{HCl}$ at $22^\\circ\\text{C}$ , and recording the volume of $\\text{H}_2$ gas released over $5\\text{ minutes}$ .

Answer

Reacting

2.0\\text{ g}

of ground

\\text{Mg}

powder with

100\\text{ mL}

of

1.0\\text{ M}

\\text{HCl}

at

22^\circ\text{C}

over

5\text{ minutes}

is the correct modification because it increases the surface area of the magnesium while keeping the mass, concentration, volume, temperature, and duration constant.

The correct answer is the setup that reacts

2.0\\text{ g}

of ground

\\text{Mg}

powder with

100\\text{ mL}

of

1.0\\text{ M}

\\text{HCl}

at

22^\\circ\\text{C}

over

5\\text{ minutes}

. Grinding the metal strip into powder increases the total surface area exposed to the acid. Because the mass of magnesium, the volume and molarity of the acid, the temperature, and the trial duration are kept identical to the baseline trial, any difference in the reaction rate can be attributed solely to the increased surface area.

Step-by-Step Solution

1

Determine the independent variable that needs to be altered and the control variables that must remain constant.

The independent variable to test is the surface area of the reactant. All other factors—reactant mass (

2.0\\text{ g}

), reactant type (

\\text{Mg}

), acid volume (

100\\text{ mL}

), acid concentration (

1.0\\text{ M}

), temperature (

22^\\circ\\text{C}

), and duration (

5\\text{ minutes}

)—must remain identical to the baseline trial.

To establish a causal relationship between surface area and reaction rate, only the surface area variable may differ between the trials.

2

Assess the options to find the setup that changes only the surface area of the magnesium.

Grinding the solid strip of magnesium into a powder increases its surface area. The option using

2.0\\text{ g}

of ground powder with the original acid concentration, volume, temperature, and time window successfully isolates this variable.

This configuration provides a valid experimental modification that conforms to scientific variable control.

Key Concept

Control of variables in follow-up experiments

Estimated Time:1m 30s

Question 93Question →

In a baseline experiment, a student measured the rate of yeast fermentation by recording the volume of $\text{CO}_2$ gas produced in a $10\%$ glucose solution at a constant temperature of $30^\circ\text{C}$ . The student wants to design a follow-up experiment to determine the effect of pH on the fermentation rate. Arrange the following steps in the correct chronological order to properly execute this modified procedure.

Drag items to arrange them in the correct order

Show answer & explanation

Answer

The correct chronological order is to first prepare the yeast cultures with different pH levels, then place them in the constant-temperature water bath, next allow the cultures to reach temperature equilibrium before sealing, and finally measure the volume of gas produced over the fixed 20-minute interval.

The correct sequence begins with preparing the experimental groups with varying pH (the independent variable) while maintaining constant concentrations. Next, the controlled variable (temperature) is applied by placing them in the water bath. The mixtures must then equilibrate to prevent thermal expansion from affecting measurements. Finally, the dependent variable (gas volume) is recorded over a standardized time interval.

Step-by-Step Solution

1

Vary the independent variable while holding other starting factors constant.

Yeast cultures are prepared with different pH values but identical yeast and glucose levels.

To isolate the effect of pH on fermentation.

2

Apply the temperature control variable to all test groups.

All cultures are incubated at

30^\circ\text{C}

.

To ensure that temperature does not act as a confounding variable.

3

Equilibrate the temperature of the mixtures before sealing the apparatus.

Thermal expansion of gas is prevented from skewing fermentation measurements.

To eliminate physical gas expansion errors from the biological gas production data.

4

Measure the accumulation of the dependent variable over a set time period.

The final volume of gas produced in 20 minutes is recorded.

To calculate and compare the fermentation rates across different pH values.

Key Concept

Designing and sequencing follow-up experiments with controlled variables.

Estimated Time:1m 30s

Question 94Question →

A group of students conducted an experiment to measure the electrical current produced by a monocrystalline silicon solar cell exposed to different wavelengths of light. For each trial, the cell was placed $30\text{ cm}$ from a light source, the light intensity at the source was kept constant at $500\text{ W/m}^2$ , and the cell temperature was maintained at $25^\circ\text{C}$ . The current output (in milliamperes, $\text{mA}$ ) was recorded for wavelengths ranging from $400\text{ nm}$ to $700\text{ nm}$ in increments of $50\text{ nm}$ .

Suppose the students want to conduct a follow-up experiment to determine how the distance from the light source affects the solar cell's current output when exposed to a single wavelength of light. Which of the following modifications to the procedure would allow the students to test this hypothesis?

Vary the distance from the light source from

15\text{ cm}

to

60\text{ cm}

while keeping the wavelength constant, and measure how the temperature of the cell changes.

Measure the current output at distances of

15\text{ cm}

,

30\text{ cm}

,

45\text{ cm}

, and

60\text{ cm}

while simultaneously increasing the wavelength of light at each distance.

Keep the wavelength, light intensity at the source, and cell temperature constant, and measure the current output at distances of

15\text{ cm}

,

30\text{ cm}

,

45\text{ cm}

, and

60\text{ cm}

.

Keep the distance constant at

30\text{ cm}

, vary the light intensity from

100\text{ W/m}^2

to

500\text{ W/m}^2

, and measure the current output at different wavelengths.

Show answer & explanation

Answer: Keep the wavelength, light intensity at the source, and cell temperature constant, and measure the current output at distances of $15\text{ cm}$ , $30\text{ cm}$ , $45\text{ cm}$ , and $60\text{ cm}$ .

Answer

The correct modification is to keep the wavelength, light intensity at the source, and cell temperature constant, while measuring the current output at distances of

15\text{ cm}

,

30\text{ cm}

,

45\text{ cm}

, and

60\text{ cm}

.

The correct answer is the option that keeps the wavelength, light intensity at the source, and cell temperature constant, while measuring the current output at distances of

15\text{ cm}

,

30\text{ cm}

,

45\text{ cm}

, and

60\text{ cm}

. This is because a valid scientific experiment must isolate the independent variable of interest (distance) by varying it systematically while holding all other potential variables constant, and directly measuring the target dependent variable (current output).

Step-by-Step Solution

1

Identify the independent and dependent variables required by the new hypothesis.

The independent variable is the distance from the light source, and the dependent variable is the current output of the solar cell.

The hypothesis focuses on how distance affects current output, which dictates what must be varied and what must be measured.

2

Determine which variables must be controlled (held constant) in the modified experiment.

The wavelength of light, light intensity at the source, and cell temperature must remain constant.

Controlling these variables ensures that any observed changes in current output are solely due to the change in distance, rather than confounding factors.

3

Select the experimental procedure that matches these variables and controls.

The procedure must vary distance (

15\text{ cm}

to

60\text{ cm}

), measure current output, and keep wavelength, light intensity, and temperature constant.

This setup is the only one that establishes a valid control group and isolates the effect of the single independent variable.

Key Concept

Isolating a single independent variable and controlling all other experimental conditions to test a specific follow-up hypothesis.

Estimated Time:1m 30s

Question 95Question →

In a study of ball dynamics, students dropped a standard tennis ball inflated to a pressure of $12\text{ psi}$ from heights of $1.0\text{ m}$ , $2.0\text{ m}$ , and $3.0\text{ m}$ onto a concrete floor. In each trial, the temperature was maintained at $21^\circ\text{C}$ , and the rebound height was recorded.

Suppose a student wants to conduct a follow-up experiment to determine how the internal air pressure of the tennis ball affects its rebound height. Which of the following procedures would be the most appropriate modification to the original study?

Drop a single tennis ball inflated to

12\text{ psi}

from

2.0\text{ m}

onto concrete, wood, and carpeted floors at

21^\circ\text{C}

.

Drop tennis balls inflated to

8\text{ psi}

,

12\text{ psi}

, and

16\text{ psi}

from a constant height of

2.0\text{ m}

onto the concrete floor at

21^\circ\text{C}

.

Drop a tennis ball, a basketball, and a soccer ball, each inflated to a different pressure, from heights of

1.0\text{ m}

,

2.0\text{ m}

, and

3.0\text{ m}

onto the concrete floor.

Drop tennis balls inflated to

8\text{ psi}

,

12\text{ psi}

, and

16\text{ psi}

from

1.0\text{ m}

,

2.0\text{ m}

, and

3.0\text{ m}

, respectively, onto different flooring surfaces.

Show answer & explanation

Answer: Drop tennis balls inflated to $8\text{ psi}$ , $12\text{ psi}$ , and $16\text{ psi}$ from a constant height of $2.0\text{ m}$ onto the concrete floor at $21^\circ\text{C}$ .

Answer

The correct option is the procedure that drops tennis balls inflated to different pressures (

8\text{ psi}

,

12\text{ psi}

, and

16\text{ psi}

) from a constant height (

2.0\text{ m}

) onto the concrete floor at a constant temperature (

21^\circ\text{C}

).

To determine the effect of internal air pressure on the rebound height, the internal air pressure must be the only independent variable that is varied. All other potential variables—such as the drop height, landing surface, ball type, and temperature—must be kept constant. Dropping tennis balls with different pressures from a constant height onto the same concrete floor at the same temperature meets this requirement.

Step-by-Step Solution

1

Identify the target independent variable to be tested in the follow-up experiment.

The target independent variable is the internal air pressure of the tennis ball.

The student wants to determine the specific effect of internal air pressure on rebound height.

2

Determine the necessary control variables that must remain constant.

The drop height, landing surface material, ball type, and ambient temperature must be kept constant across all trials.

If multiple variables are changed, the experimenter cannot attribute the results to any single factor.

3

Evaluate the choices to find the one that isolates the target independent variable.

Varying only the tennis ball pressure (

8\text{ psi}

,

12\text{ psi}

, and

16\text{ psi}

) at a fixed height (

2.0\text{ m}

) on concrete at

21^\circ\text{C}

successfully isolates this variable.

This setup holds all other variables constant while systematically changing the pressure.

Key Concept

Isolating a single independent variable in a follow-up experiment

Estimated Time:1m 0s

Question 96Question →

A student conducted a baseline experiment to measure the rate of heat transfer through a copper rod. The student attached wax beads at $5\text{ cm}$ intervals along the rod, heated one end with a Bunsen burner, and recorded the time taken for each bead to melt. The student now wants to design a follow-up experiment to compare the relative thermal conductivities of copper, aluminum, and iron rods. To ensure a scientifically valid comparison that controls variables and measures the rate of heat transfer accurately, in what order should the student perform the following procedural steps?

Drag items to arrange them in the correct order

Show answer & explanation

Answer

Select copper, aluminum, and iron rods that are identical in length, diameter, and surface texture; attach wax beads of equal mass and size at identical

5\text{ cm}

intervals along each of the three rods; expose one end of each rod to the same heat source simultaneously, ensuring equal contact area and heat input; record the elapsed time taken for each wax bead to melt on each rod.

The correct ordering begins with selecting identical rods to control the physical properties of the materials. Next, the wax beads must be attached uniformly at identical distances on the cold rods to ensure a standardized measurement system. After the setup is complete, heat must be applied simultaneously and equally to all three rods to initiate the independent variable. Finally, the elapsed melting times are recorded to gather the dependent variable data. This sequence ensures all variables are controlled except for the rod material.

Step-by-Step Solution

1

Identify the constant factors that must be controlled to isolate the effect of the rod material on heat transfer.

The student must select rods of copper, aluminum, and iron that are identical in dimensions (length, diameter, and surface texture).

Varying physical dimensions would introduce confounding variables, as a thicker or shorter rod would naturally transfer heat differently regardless of its material.

2

Ensure uniform indicators are applied to the rods before the experiment begins.

Wax beads of equal mass and size are attached at identical

5\text{ cm}

intervals on the cold rods.

Applying the beads to hot rods or placing them at unequal distances would distort the measurement of heat propagation rate.

3

Apply the independent variable (heat) uniformly to start the process.

A heat source of equal intensity is applied to one end of all three rods simultaneously.

Simultaneous and equal heating ensures that any differences in melting times are due solely to the differing thermal conductivities of the materials.

4

Observe and record the dependent variable.

The elapsed times for each wax bead to melt are recorded.

Recording these times provides the quantitative data needed to compare the rates of heat transfer and draw a valid scientific conclusion.

Key Concept

Controlling variables and establishing standard procedures in comparative follow-up experiments.

Estimated Time:1m 30s

Question 97Question →

In a baseline experiment, a chemist measured the rate of hydrogen peroxide ( $H_2O_2$ ) decomposition by adding $1.0\text{ g}$ of manganese dioxide ( $MnO_2$ ) to $100\text{ mL}$ of a $3\%\text{ }H_2O_2$ solution at $25^\circ\text{C}$ and recording the volume of oxygen ( $O_2$ ) gas produced over $5\text{ minutes}$ . The chemist wants to design a follow-up experiment to determine how temperature affects this reaction rate, while ensuring that the concentration of reactants, catalyst mass, and total volume remain controlled. Place the following steps in the correct chronological order to successfully perform this follow-up experiment.

Drag items to arrange them in the correct order

Show answer & explanation

Answer

The correct sequence of steps is: prepare the identical hydrogen peroxide solutions, adjust their temperatures in the different water baths, add the manganese dioxide catalyst to initiate the reaction, and then measure the oxygen gas volume produced over the five-minute interval.

The correct sequence starts with preparing the identical solutions, then bringing them to the target temperatures. The catalyst must only be added after the temperatures are established to ensure the entire reaction occurs at the target temperature. Finally, the volume of gas produced is measured to determine the rate.

Step-by-Step Solution

1

Identify the independent variable (temperature) and the controlled variables (reactants concentration, volume, and catalyst mass).

The setup must isolate temperature as the only variable that changes before the reaction begins.

To accurately measure the effect of temperature on the rate of decomposition.

2

Sequence the steps such that temperature is controlled before the reaction is initiated.

The solutions must be prepared and brought to their target temperatures prior to adding the catalyst.

Adding the catalyst first would start the reaction at an uncontrolled, transitional temperature.

3

Determine the final phase of the experiment.

Adding the catalyst starts the reaction, immediately followed by the measurement of gas volume.

The rate is determined by measuring gas production over time once the reaction is active.

Key Concept

Isolating the independent variable in a modified experimental procedure by establishing controlled conditions and adjusting the independent variable before initiating the reaction.

Estimated Time:1m 30s

Question 98Question →

In a study of reaction kinetics, a student investigates the reaction between sodium thiosulfate ( $Na_2S_2O_3$ ) and hydrochloric acid ( $HCl$ ). The reaction produces solid sulfur, which precipitates and clouds the solution.

Experiment 1
The student placed a beaker containing $50\text{ mL}$ of $0.1\text{ M}$ $Na_2S_2O_3$ over a sheet of paper marked with a black cross. The student then added $10\text{ mL}$ of $1.0\text{ M}$ $HCl$ to the beaker, immediately started a stopwatch, and recorded the time (in seconds) required for the liquid to become so opaque that the black cross was no longer visible. The reaction was conducted at $25^\circ\text{C}$ . The student repeated this procedure at temperatures of $35^\circ\text{C}$ , $45^\circ\text{C}$ , and $55^\circ\text{C}$ , keeping the volumes and concentrations of the reactants constant.

Suppose the student wants to design a follow-up experiment to determine how the concentration of $HCl$ affects the reaction rate, while ensuring that temperature does not act as a confounding variable. Which of the following modifications to the procedure of Experiment 1 would best achieve this goal?

Conduct multiple trials using different concentrations of

HCl

, and increase the reaction temperature by

10^\circ\text{C}

for each higher concentration tested.

Conduct multiple trials at

25^\circ\text{C}

keeping the

HCl

concentration at

1.0\text{ M}

while varying the concentration of the

Na_2S_2O_3

solution.

Conduct multiple trials at a constant temperature of

25^\circ\text{C}

using

50\text{ mL}

of

0.1\text{ M}

Na_2S_2O_3

, while varying the concentration of the

10\text{ mL}

of

HCl

added.

Conduct multiple trials at a constant temperature of

25^\circ\text{C}

using

1.0\text{ M}

HCl

and

0.1\text{ M}

Na_2S_2O_3

, but vary the total volume of the reaction mixture.

Show answer & explanation

Answer: Conduct multiple trials at a constant temperature of $25^\circ\text{C}$ using $50\text{ mL}$ of $0.1\text{ M}$ $Na_2S_2O_3$ , while varying the concentration of the $10\text{ mL}$ of $HCl$ added.

Answer

Conduct multiple trials at a constant temperature of

25^\circ\text{C}

using

50\text{ mL}

of

0.1\text{ M}

Na_2S_2O_3

, while varying the concentration of the

10\text{ mL}

of

HCl

added.

To investigate the effect of hydrochloric acid concentration on the reaction rate, the concentration of hydrochloric acid must be systematically varied as the independent variable. To prevent confounding variables, all other parameters—including the temperature, the volume of both reactants, and the concentration of the other reactant (sodium thiosulfate)—must be kept constant. Conducting trials at a constant temperature of

25^\circ\text{C}

while keeping the volume and concentration of sodium thiosulfate constant and varying only the concentration of the added hydrochloric acid achieves this design.

Step-by-Step Solution

1

Identify the target independent variable for the follow-up study.

The target independent variable is the concentration of hydrochloric acid (

HCl

).

The student wants to determine how the concentration of

HCl

affects the reaction rate.

2

Determine the control variables that must remain constant.

The reaction temperature must remain constant (e.g., at

25^\circ\text{C}

), and the volume and concentration of sodium thiosulfate (

Na_2S_2O_3

), as well as the volume of the

HCl

solution, must remain constant.

Controlling these factors prevents them from acting as confounding variables, ensuring that any variation in reaction time is due solely to the changes in

HCl

concentration.

3

Evaluate the proposed modifications and select the one that isolates the independent variable.

The only procedure that varies only the concentration of the

HCl

solution while holding the temperature and the properties of the sodium thiosulfate reactant constant is the correct choice.

This isolates the

HCl

concentration as the single variable under test.

Key Concept

Isolating a single independent variable while controlling other variables to study its specific effect in a follow-up experiment.

Estimated Time:1m 30s

Question 99Question →

Students conducted two studies to investigate heat conduction in metal rods. In both studies, a wax bead was placed at one end of a metal rod, and the other end was heated using a Bunsen burner. The time required for the wax bead to melt was recorded.

Study 1
Rods of copper, aluminum, and iron, all with a length of $15\text{ cm}$ and a diameter of $0.5\text{ cm}$ , were tested. The flame height of the burner was kept constant at $3\text{ cm}$ .

Study 2
Copper rods with a diameter of $0.5\text{ cm}$ and lengths of $10\text{ cm}$ , $20\text{ cm}$ , and $30\text{ cm}$ were tested. The flame height of the burner was kept constant at $3\text{ cm}$ .

Which of the following statements best describes a difference between the experimental designs of Study 1 and Study 2?

Study 1 varied the material of the rods while keeping their length constant, whereas Study 2 varied the length of the rods while keeping their material constant.

Study 1 varied the length of the rods while keeping their material constant, whereas Study 2 varied the material of the rods while keeping their length constant.

Study 1 varied the diameter of the rods while keeping the burner flame height constant, whereas Study 2 varied the burner flame height while keeping the rod diameter constant.

Study 1 varied the burner flame height while keeping the rod material constant, whereas Study 2 varied the rod material while keeping the burner flame height constant.

Show answer & explanation

Answer: Study 1 varied the material of the rods while keeping their length constant, whereas Study 2 varied the length of the rods while keeping their material constant.

Answer

Study 1 varied the material of the rods while keeping their length constant, whereas Study 2 varied the length of the rods while keeping their material constant.

In Study 1, the type of metal (copper, aluminum, iron) was varied while keeping the length of the rods constant at

15\text{ cm}

. In Study 2, the length of the copper rods was varied (

10\text{ cm}

,

20\text{ cm}

,

30\text{ cm}

) while keeping the material constant (copper). Thus, the correct answer correctly identifies that Study 1 varied the material with constant length, whereas Study 2 varied the length with constant material.

Step-by-Step Solution

1

Identify the independent variable (what is changed) in Study 1.

The rod material (copper, aluminum, iron) is varied, while rod length (

15\text{ cm}

), diameter (

0.5\text{ cm}

), and flame height (

3\text{ cm}

) are kept constant.

Understanding the design of Study 1 requires identifying which parameters are systematically changed and which are controlled.

2

Identify the independent variable (what is changed) in Study 2.

The rod length (

10\text{ cm}

,

20\text{ cm}

,

30\text{ cm}

) is varied, while rod material (copper), diameter (

0.5\text{ cm}

), and flame height (

3\text{ cm}

) are kept constant.

Understanding the design of Study 2 requires identifying which parameters are systematically changed and which are controlled.

3

Compare the independent and constant variables between the two designs to find the differences.

Study 1 changes the material while keeping length constant; Study 2 changes the length while keeping material constant.

Comparing the designs reveals how they target different variables to isolate their respective effects on heat conduction time.

Key Concept

Comparing and Contrasting Multiple Experimental Designs

Estimated Time:1m 30s

Question 100Question →

A group of students conducted a baseline experiment to measure the rate of transpiration in tomato plants under still-air conditions at a constant temperature of $25^\circ\text{C}$ and $50\%$ relative humidity. They measured transpiration by recording the mass loss of a plant over time using a digital scale.

Suppose the students want to modify this experiment to investigate the specific, independent effect of wind speed on transpiration rate using a variable-speed fan and an anemometer (wind gauge). To ensure a valid follow-up experiment, in what order should the students perform the following procedural steps?

Drag items to arrange them in the correct order

Show answer & explanation

Answer

The correct sequence starts with establishing control variables (temperature and relative humidity), followed by calibrating the wind speeds using the anemometer, placing the plant on the scale in the wind path, and finally recording the plant's mass loss over time.

The correct order begins with establishing the control variables (temperature and relative humidity) at their baseline levels to isolate the effect of wind speed. Next, the new independent variable must be calibrated by measuring the fan settings with the anemometer. Once calibrated, the plant is positioned on the scale in front of the fan. Finally, the dependent variable (mass loss over time) is measured to determine the transpiration rate.

Step-by-Step Solution

1

Identify the control variables that must match the baseline experiment.

Temperature must be held at

25^\circ\text{C}

and relative humidity at

50\%

.

To isolate the effect of wind speed, all other variables from the baseline experiment must remain constant.

2

Establish and measure the levels of the new independent variable.

Calibrate the fan speed settings using the anemometer.

The independent variable (wind speed) must be accurately measured and set before starting the trials.

3

Position the plant and scale within the experimental setup.

The plant is placed on the digital scale in front of the fan.

The physical setup must be assembled before data collection can begin.

4

Collect the dependent variable data.

Measure the mass of the plant over a

30\text{-minute}

period.

Transpiration rate is determined by mass loss per unit of time, which requires tracking mass change over a set interval.

Key Concept

Designing follow-up experiments requires isolating the new independent variable by holding all other variables constant at baseline levels, calibrating the independent variable, setting up the test subject, and then measuring the dependent variable.

Experimental Design and Scientific Method

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