Designing Follow-Up Experiments and Test Modifications Practice Questions — ACT

Question 1Question →

A group of students investigated the catalytic decomposition of hydrogen peroxide ( $H_2O_2$ ) into water and oxygen gas using yeast as a source of the enzyme catalase:

2H_2O_2(aq) \xrightarrow{\text{catalase}} 2H_2O(l) + O_2(g)

In Study 1, they mixed $10\text{ mL}$ of $3\%\ H_2O_2$ with $2\text{ mL}$ of a $5\%$ yeast suspension in a sealed flask at $20^\circ\text{C}$ and recorded the volume of $O_2$ gas collected in a gas syringe over 5 minutes. They repeated this procedure using $10\%$ , $15\%$ , and $20\%$ yeast suspensions.

In Study 2, they used a $10\%$ yeast suspension and repeated the procedure at temperatures of $10^\circ\text{C}$ , $30^\circ\text{C}$ , $40^\circ\text{C}$ , and $50^\circ\text{C}$ . The volume of $O_2$ collected at 5 minutes increased with temperature up to $40^\circ\text{C}$ but was extremely low at $50^\circ\text{C}$ .

The students want to perform a follow-up experiment to determine if the low $O_2$ production at $50^\circ\text{C}$ is due to the permanent thermal denaturation of catalase, or if the enzyme is simply temporarily less active at $50^\circ\text{C}$ but remains functional when returned to a lower temperature. Which of the following procedures would best allow the students to test this hypothesis?

Preheat a sample of the

3\%\ H_2O_2

solution to

50^\circ\text{C}

for 10 minutes, cool the sample back to

20^\circ\text{C}

, and then mix it with the

10\%

yeast suspension at

20^\circ\text{C}

to measure the volume of

O_2

produced over 5 minutes.

Mix the

10\%

yeast suspension with

3\%\ H_2O_2

at

50^\circ\text{C}

, allow the reaction to proceed for 5 minutes, and then cool the entire mixture to

20^\circ\text{C}

to measure the volume of

O_2

produced over the subsequent 5 minutes.

Preheat a sample of the

10\%

yeast suspension to

50^\circ\text{C}

for 10 minutes, cool the sample back to

20^\circ\text{C}

, and then mix it with

3\%\ H_2O_2

at

20^\circ\text{C}

to measure the volume of

O_2

produced over 5 minutes.

Mix the

10\%

yeast suspension with

3\%\ H_2O_2

at

45^\circ\text{C}

and measure the

O_2

produced, then immediately transfer the reaction flask to a

55^\circ\text{C}

water bath and measure the volume of

O_2

produced.

Show answer & explanation

Answer: Preheat a sample of the $10\%$ yeast suspension to $50^\circ\text{C}$ for 10 minutes, cool the sample back to $20^\circ\text{C}$ , and then mix it with $3\%\ H_2O_2$ at $20^\circ\text{C}$ to measure the volume of $O_2$ produced over 5 minutes.

Answer

Preheat a sample of the

10\%

yeast suspension to

50^\circ\text{C}

for 10 minutes, cool the sample back to

20^\circ\text{C}

, and then mix it with

3\%\ H_2O_2

at

20^\circ\text{C}

to measure the volume of

O_2

produced over 5 minutes.

The correct procedure targets the yeast suspension containing the catalase enzyme, subjects it to the high temperature of

50^\circ\text{C}

, and cools it back down to the baseline temperature of

20^\circ\text{C}

before mixing it with fresh

H_2O_2

substrate. If the enzyme is permanently denatured, it will remain inactive at

20^\circ\text{C}

and produce little to no

O_2

. If the inactivation is temporary, the rate will recover to normal levels. By keeping the reactants separate during heating, this method avoids the confounding effect of reactant depletion.

Step-by-Step Solution

1

Identify the source of the enzyme (catalase) within the experimental setup.

The catalase enzyme is in the yeast suspension, not in the hydrogen peroxide substrate.

To test the thermal denaturation of catalase, the heat treatment must target the yeast suspension.

2

Formulate a method to test the reversibility of the high-temperature inactivation.

The yeast suspension must be heated to the target high temperature (

50^\circ\text{C}

) and then returned to a lower baseline temperature (

20^\circ\text{C}

) where it normally functions well.

If the inactivation is temporary, activity will recover at

20^\circ\text{C}

; if it is permanent (denaturation), activity will not recover.

3

Isolate the heat treatment phase from the reaction phase to control for confounding variables.

Keep the yeast suspension separate from the hydrogen peroxide while heating and cooling, and only mix them once the temperature is back to

20^\circ\text{C}

.

Mixing them during the heating phase would consume the substrate, introducing reactant depletion as a confounding variable that could explain the lack of subsequent gas production.

Key Concept

Isolating variables and controlling for reactant depletion when designing follow-up experiments to test the reversibility of temperature-induced enzyme inactivation.

Estimated Time:3m 0s

Question 2Question →

Suppose a scientist wants to modify an electroplating procedure to isolate the specific effect of temperature on the deposition rate of copper and determine the activation energy of the reaction. The scientist must ensure that concentration depletion and current fluctuations do not confound the results. Arrange the following steps in the correct chronological order to design and execute this modified follow-up experiment.

Drag items to arrange them in the correct order

Show answer & explanation

Answer

The correct chronological sequence begins with preparing the high-volume electrolyte bath to maintain constant concentration. Next, preliminary trials are run to determine the optimal constant current. Once the current is established, the temperature-controlled trials are executed. After the trials, the deposited mass is measured to calculate rates. Finally, these rates are plotted against the reciprocal of absolute temperature to calculate the activation energy.

The correct sequence begins with preparing the high-volume electrolyte bath to ensure concentration remains constant. Next, preliminary trials must be run to determine the optimal current. Once the current is established, the temperature-controlled trials are executed. After completing the trials, the mass of deposited copper is measured to calculate rates. Finally, these rates are plotted to calculate activation energy.

Step-by-Step Solution

1

Prepare the constant concentration electrolyte bath.

Maintains a stable chemical environment.

This must be done first so that all subsequent trials, including preliminary calibration, use the same electrolyte concentration.

2

Run preliminary trials to select the operating current.

Determines the optimal constant current value.

A constant current must be selected prior to running the main experimental trials to properly control this variable.

3

Execute the temperature-controlled trials.

Generates copper deposition at different temperatures.

This step uses the selected current and prepared bath to collect raw data across the independent temperature variable.

4

Measure mass and calculate deposition rates.

Obtains the rate of deposition for each temperature.

The rate data is the dependent variable required for the final activation energy calculation.

5

Construct an Arrhenius plot.

Determines the activation energy.

This final analytical step uses the rates calculated from the trials to perform the mathematical analysis.

Key Concept

Isolating independent variables and controlling confounding factors in a multi-step sequence for follow-up experimental design.

Estimated Time:3m 0s

Question 3Question →

A student conducted an experiment to measure the distance a $50\text{ g}$ toy car traveled along a flat floor after rolling down a $1\text{ m}$ wooden ramp set at an angle of $30^\circ$ . The student performed 3 trials using the same car and ramp. Suppose the student wants to design a follow-up experiment to determine how the mass of the car affects the distance it travels. Which of the following modifications to the procedure should the student make to test this relationship?

Test the original

50\text{ g}

toy car on ramps of different materials and different lengths.

Test toy cars of different masses while increasing the ramp angle for each heavier car.

Test toy cars of different masses while keeping the ramp angle, ramp length, and ramp material the same.

Measure the time it takes the original

50\text{ g}

toy car to reach the bottom of the ramp at various release heights.

Show answer & explanation

Answer: Test toy cars of different masses while keeping the ramp angle, ramp length, and ramp material the same.

Answer

Test toy cars of different masses while keeping the ramp angle, ramp length, and ramp material the same.

The correct answer proposes varying only the mass of the toy car while keeping all other parameters—such as ramp angle, length, and material—constant. This isolates the car's mass as the single independent variable, which is necessary to determine its direct effect on the distance traveled.

Step-by-Step Solution

1

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

The independent variable is the mass of the toy car.

The student wants to determine the specific effect of the car's mass on the distance it travels.

2

Identify the controlled variables that must remain constant.

The ramp angle (

30^\circ

), ramp length (

1\text{ m}

), and ramp material (wooden) must remain unchanged across trials.

To ensure a fair test, all variables except the one being tested must be controlled.

3

Evaluate the choices to find the one that alters only the mass of the car while keeping the ramp conditions constant.

Testing cars of different masses with the same ramp setup is the correct design.

This isolates the mass of the car as the single independent variable affecting the dependent variable (distance traveled).

Key Concept

To isolate the effect of a new independent variable in a follow-up experiment, only that variable should be changed, while all other variables from the original design must be controlled.

Question 4Question →

A student wants to modify an experiment measuring sugar solubility in water to determine the effect of higher temperatures ( $40^\circ\text{C}$ , $60^\circ\text{C}$ , and $80^\circ\text{C}$ ) on the mass of dissolved sugar. Arrange the steps of this modified procedure in the correct chronological order from start to finish.

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Answer

The correct order of steps for the modified experiment is first heating the water to a target temperature, next adding sugar until it no longer dissolves, then measuring and recording the mass of the dissolved sugar, and finally repeating these steps for the other temperatures.

The correct order follows a logical experimental procedure for measuring solubility at different temperatures: first, establishing the independent variable (temperature) by heating the water; second, performing the test by adding sugar to saturation; third, measuring the dependent variable (mass of dissolved sugar); and fourth, repeating the process for other levels of the independent variable (other temperatures) while controlling other factors.

Step-by-Step Solution

1

Identify the first step in conducting a solubility test at a specific temperature.

The water sample must be heated to the target temperature (

40^\circ\text{C}

,

60^\circ\text{C}

, or

80^\circ\text{C}

) before adding sugar.

Dissolving sugar at the correct starting temperature ensures the solubility measurement is accurate for that temperature.

2

Determine the next physical step in the dissolution process.

Sugar is added incrementally and stirred until the solution becomes saturated.

Saturating the solution is necessary to find the maximum limit of solubility at that temperature.

3

Determine the measurement step that follows saturation.

Measure and record the total mass of sugar dissolved.

Recording the mass at the end of the trial provides the data point for that specific temperature.

4

Determine the final step to complete the study across the full temperature range.

Repeat the entire procedure for the remaining temperatures while keeping the water volume constant.

Testing all temperatures under controlled conditions allows for a valid comparison of solubility trends.

Key Concept

To modify an experiment to test a new range of an independent variable (temperature), one must systematically heat the solvent, dissolve the solute to saturation, measure the mass, and repeat the process under controlled conditions for all remaining values of the independent variable.

Question 5Question →

A student conducted an experiment to measure the rate of water loss from a certain plant species under different relative humidity levels. The experimental design is summarized in the table below:

Trial	Relative Humidity (%)	Temperature ( $^\circ\text{C}$ )	Plant Species	Exposure Time (hours)
1	20	25	Fern	2
2	40	25	Fern	2
3	60	25	Fern	2
4	80	25	Fern	2

Suppose the student wants to perform a follow-up experiment to determine how temperature affects the water loss of this same plant species. Which of the following modifications to the experimental design would best allow the student to isolate the effect of temperature?

Vary the temperature across trials while keeping the relative humidity, plant species, and exposure time constant.

Vary both the temperature and the relative humidity across trials while keeping the plant species and exposure time constant.

Vary the temperature and use a different plant species for each trial while keeping the relative humidity constant.

Vary the temperature across trials while increasing the exposure time for each increase in temperature.

Show answer & explanation

Answer: Vary the temperature across trials while keeping the relative humidity, plant species, and exposure time constant.

Answer

Vary the temperature across trials while keeping the relative humidity, plant species, and exposure time constant.

To investigate the effect of a new independent variable (temperature), that variable must be varied while holding all other variables constant. The correct option describes changing the temperature across trials while maintaining constant relative humidity, plant species, and exposure time, which successfully isolates the effect of temperature.

Step-by-Step Solution

1

Identify the goal of the follow-up experiment.

The goal is to determine the specific effect of temperature on water loss (transpiration).

This establishes temperature as the new independent variable.

2

Apply the rule of experimental control.

To isolate the effect of temperature, only temperature should vary, while all other potential independent variables (such as relative humidity, plant species, and exposure time) must remain constant.

Varying multiple factors simultaneously introduces confounding variables, which prevents a clear conclusion about which variable caused the change.

3

Evaluate the choices to find the one that varies only temperature.

The option to vary the temperature while holding relative humidity, plant species, and exposure time constant correctly isolates the variable of interest.

This aligns with proper scientific method and experimental design.

Key Concept

Scientific Control of Variables in Follow-Up Experiments

Question 6Question →

Suppose a student wants to modify an experiment measuring the evaporation rate of salt water ( $200\text{ mL}$ of $5\%$ saline solution heated by a $100\text{ W}$ heat lamp placed $30\text{ cm}$ above the beaker in a draft-free room) to determine the specific impact of wind speed on the evaporation rate, while ensuring that the thermal energy input and other variables remain controlled. Arrange the following steps in the correct chronological sequence to successfully conduct this follow-up experiment.

Drag items to arrange them in the correct order

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Answer

The correct chronological sequence is: first, prepare the three identical saline solution beakers; second, position the heat lamp and the variable-speed fan at fixed distances relative to the beaker; third, select the wind speed setting and start the fan and lamp; and fourth, record the mass of the beaker at ten-minute intervals over one hour.

The correct sequence begins with preparing the identical solutions to establish controlled initial parameters. Next, the physical apparatus must be positioned to control the heat lamp distance and fan distance. Then, the specific wind speed is chosen and the trial is started by turning on the equipment. Finally, the mass is measured at regular intervals to track the evaporation rate.

Step-by-Step Solution

1

Prepare identical samples to control initial conditions.

Three beakers with equal volume (

200\text{ mL}

) and concentration (

5\%

) of saline solution.

Ensures that differences in evaporation are due only to the wind speed, not variations in initial volume or salinity.

2

Set up the physical apparatus with controlled physical dimensions.

The beaker, heat lamp (

30\text{ cm}

away), and fan (

50\text{ cm}

away) are positioned in a stable, repeatable configuration.

Ensures the heat energy delivered to the solution is constant across all trials, and the fan's physical distance does not vary.

3

Apply the independent variable (wind speed) and start the experimental conditions.

The trial begins under a specific, constant wind speed with simultaneous heating.

Allows the evaporation process to begin under the selected test condition.

4

Measure and record the dependent variable over the specified duration.

Mass data at

10

,

20

,

30

,

40

,

50

, and

60

minutes.

Provides the raw data necessary to calculate the rate of mass loss (evaporation rate) over time for that specific wind speed.

Key Concept

Designing a controlled follow-up experiment requires isolating the new independent variable (wind speed) by maintaining all original variables (lamp distance, initial salinity, and volume) constant, and sequencing the steps from sample preparation to final measurement.

Question 7Question →

A scientist conducted two experiments to study soil respiration (measured as the rate of carbon dioxide release, $R_s$ , in $\text{g CO}_2/\text{m}^2/\text{day}$ ) from a forest soil sample under different conditions.

Experiment	Soil Temperature ( $^\circ\text{C}$ )	Soil Moisture Content (SMC)	Observed Trend in $R_s$
1	$20^\circ\text{C}$ (constant)	Varied ( $10\%$ , $20\%$ , $30\%$ )	$R_s$ increases as SMC increases
2	Varied ( $15^\circ\text{C}$ , $25^\circ\text{C}$ , $35^\circ\text{C}$ )	$20\%$ (constant)	$R_s$ increases as temperature increases

Suppose a scientist wants to determine if the positive relationship between SMC and $R_s$ observed at $20^\circ\text{C}$ remains positive at a near-freezing temperature of $2^\circ\text{C}$ . Which of the following modifications to the experimental design would best allow the scientist to test this hypothesis?

Measure

R_s

of soil samples at a constant temperature of

2^\circ\text{C}

while varying the SMC at

10\%

,

20\%

, and

30\%

.

Measure

R_s

of soil samples as the temperature is decreased from

20^\circ\text{C}

to

2^\circ\text{C}

while keeping the SMC constant at

20\%

.

Measure

R_s

of soil samples at temperatures of

2^\circ\text{C}

,

10^\circ\text{C}

, and

20^\circ\text{C}

while allowing the SMC to vary freely for each sample.

Measure the rate of water evaporation from the soil samples at

2^\circ\text{C}

as the SMC is varied at

10\%

,

20\%

, and

30\%

.

Show answer & explanation

Answer: Measure $R_s$ of soil samples at a constant temperature of $2^\circ\text{C}$ while varying the SMC at $10\%$ , $20\%$ , and $30\%$ .

Answer

Measure

R_s

of soil samples at a constant temperature of

2^\circ\text{C}

while varying the SMC at

10\%

,

20\%

, and

30\%

.

To determine if the positive relationship between soil moisture content (SMC) and soil respiration rate (

R_s

) is preserved at a new constant temperature (

2^\circ\text{C}

), the scientist must isolate SMC as the independent variable. This is accomplished by holding the temperature constant at

2^\circ\text{C}

and measuring

R_s

across the same range of SMC values (

10\%

,

20\%

, and

30\%

) used in the original experiment.

Step-by-Step Solution

1

Identify the goal of the proposed follow-up experiment.

The goal is to determine if the positive relationship between SMC and

R_s

holds at

2^\circ\text{C}

.

Understanding the hypothesis is necessary to determine which variables must be manipulated and measured.

2

Identify the independent and dependent variables required to test the hypothesis.

The independent variable must be SMC (varied at

10\%

,

20\%

, and

30\%

) and the dependent variable must be

R_s

(soil respiration rate).

To see if the relationship between moisture and respiration is altered, moisture must be varied while measuring respiration.

3

Identify the necessary control variables.

The temperature must be held constant at the new temperature of interest (

2^\circ\text{C}

).

If temperature is not held constant at

2^\circ\text{C}

, the effect of temperature will confound the results.

4

Select the option that matches these design criteria.

Varying SMC while maintaining a constant temperature of

2^\circ\text{C}

and measuring

R_s

is the correct experimental design.

This setup isolates the effect of SMC at the specific temperature of

2^\circ\text{C}

without introducing confounding variables.

Key Concept

To test the relationship between an independent variable and a dependent variable under a new constant condition, the independent variable must be varied while keeping all other conditions constant.

Estimated Time:2m 0s

Question 8Question →

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 9Question →

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.

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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 10Question →

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 11Question →

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 12Question →

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 13Question →

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 14Question →

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 15Question →

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.

Designing Follow-Up Experiments and Test Modifications

Answer

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