Comparing and Contrasting Multiple Experimental Designs

An investigator conducted two experiments to study the factors affecting the rate of a chemical reaction between zinc metal ($\text{Zn}$) and hydrochloric acid ($\text{HCl}$).

In Experiment 1, the investigator added $1.0\text{ g}$ of zinc powder to $50\text{ mL}$ of $1.0\text{ M}$ $\text{HCl}$ solution in a beaker. The reaction was carried out at different temperatures ($20^\circ\text{C}$, $30^\circ\text{C}$, $40^\circ\text{C}$, and $50^\circ\text{C}$), and the time required for the zinc to completely dissolve was recorded for each trial.

In Experiment 2, the investigator added $1.0\text{ g}$ of zinc powder to $50\text{ mL}$ of $\text{HCl}$ solution of different concentrations ($0.5\text{ M}$, $1.0\text{ M}$, $1.5\text{ M}$, and $2.0\text{ M}$) in a beaker. The reaction was carried out at a constant temperature of $20^\circ\text{C}$, and the time required for the zinc to completely dissolve was recorded for each trial.

Based on the descriptions of these experimental designs, is the statement that 'the temperature of the reaction was varied in Experiment 1 but was kept constant in Experiment 2' true or false?

Students conducted two experiments to study the growth of *Arabidopsis thaliana* plants over a 30-day period.

Experiment 1
Five groups of 10 plants were grown in identical pots containing the same soil type. Each group received a different concentration of Fertilizer X (0%, 1%, 2%, 3%, or 4% by volume) dissolved in water. All plants were kept under continuous white light at a constant temperature of 22°C and watered daily with 50 mL of their respective solution. The average height of the plants in each group was measured at the end of 30 days.

Experiment 2
Five groups of 10 plants were grown under the same conditions as in Experiment 1, except that all plants received a 2% solution of Fertilizer X, and each group was exposed to a different color of light (blue, green, red, yellow, or white).

Based on the descriptions of the two experiments, which of the following variables was kept constant in Experiment 1 but was varied in Experiment 2?

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.

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?

To investigate the factors affecting the rate of transpiration in plants, students set up two distinct experiments using identical bean plants.

In Experiment 1, three plants were placed in separate chambers with varying levels of relative humidity ($30\%$, $50\%$, and $70\%$) at a constant temperature of $22^\circ\text{C}$ and a constant light intensity of $800\text{ lux}$. The mass of water lost by each plant was measured after 6 hours.

In Experiment 2, three plants were placed in separate chambers at varying temperatures ($18^\circ\text{C}$, $24^\circ\text{C}$, and $30^\circ\text{C}$) at a constant relative humidity of $50\%$ and a constant light intensity of $800\text{ lux}$. The mass of water lost by each plant was measured after 6 hours.

Determine whether the following statement is true or false:

The relative humidity of the environment in Experiment 2 was held constant at a value that was also tested as an experimental condition in Experiment 1.

Students conducted two experiments to study the decomposition of hydrogen peroxide ($H_2O_2$) into water and oxygen gas ($O_2$) in the presence of a yeast catalyst suspension.

Experiment 1
A test tube was filled with $10\text{ mL}$ of $3\%$ $H_2O_2$ solution. A yeast suspension with a concentration of $1.0\text{ g/L}$ was prepared. The students added $1\text{ mL}$ of the yeast suspension to the test tube at a temperature of $25^\circ\text{C}$ and measured the volume of $O_2$ gas produced over $5\text{ minutes}$. This procedure was repeated at temperatures of $35^\circ\text{C}$, $45^\circ\text{C}$, and $55^\circ\text{C}$.

Experiment 2
Using the same apparatus at a constant temperature of $35^\circ\text{C}$, the students added $1\text{ mL}$ of yeast suspensions of varying concentrations ($0.5\text{ g/L}$, $1.0\text{ g/L}$, $1.5\text{ g/L}$, and $2.0\text{ g/L}$) to separate test tubes, each containing $10\text{ mL}$ of $3\%$ $H_2O_2$ solution. The volume of $O_2$ gas produced was measured over $5\text{ minutes}$.

Based on the experimental designs described, is the statement that the concentration of the yeast suspension served as the independent variable in Experiment 1 and as a controlled variable in Experiment 2 true or false?