A student conducted an experiment to investigate the effect of pH on the rate of yeast fer… — ACT

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.Answer
B
The mass of glucose was kept constant at 5.0 g across all four flasks.
C
The student did not include a flask with a pH of 7.0 to serve as a neutral control.
D
The volume of carbon dioxide gas produced was not controlled and varied between 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

Identify the intended independent variable and the dependent variable.

The independent variable is pH (varied from

4.0

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.

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.

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

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