An investigator designed an experiment to determine how pH affects the rate of starch hydr… — ACT

An investigator designed an experiment to determine how pH affects the rate of starch hydrolysis by the enzyme amylase. The investigator prepared four test tubes, each containing an identical concentration of starch and amylase at a specific pH. To establish and maintain each pH level, the investigator used different buffer systems, as summarized in the table below:

Tube	pH	Buffer System Components	Rate of Starch Hydrolysis ( $\text{mg/min}$ )
1	$4.0$	Citric acid / Sodium citrate	$0.2$
2	$6.0$	Phosphate buffer / Sodium chloride	$1.8$
3	$8.0$	Tris-HCl / Potassium chloride	$1.2$
4	$10.0$	Carbonate / Bicarbonate	$0.1$

Given that amylase activity is stimulated by the presence of chloride ( $Cl^-$ ) ions, which of the following statements best identifies the confounding variable in this experiment and its potential impact on the results?

The presence of chloride ions in the buffers for Tube 2 and Tube 3, which may artificially elevate the measured hydrolysis rates at pH 6.0 and pH 8.0.Answer
B
The variation in pH values across the tubes, which prevents the investigator from establishing a clear baseline control group at neutral pH.
C
The use of potassium ions in Tube 3 instead of sodium ions, which would inhibit enzyme activity and cause the rate at pH 8.0 to be lower than it should be.
D
The change in the independent variable (pH) across the tubes, which makes it impossible to determine if the rate of starch hydrolysis is the dependent variable.

Answer

The presence of chloride ions in the buffers for Tube 2 and Tube 3, which may artificially elevate the measured hydrolysis rates at pH 6.0 and pH 8.0.

The correct answer identifies chloride ions as the confounding variable. Because chloride ions stimulate amylase activity and are only present in the buffer systems of two of the treatment groups (Tubes 2 and 3), the rates of hydrolysis at pH

6.0

and

8.0

are artificially elevated, preventing an accurate comparison of the effect of pH alone.

Step-by-Step Solution

Identify the intended independent variable and the dependent variable.

The independent variable is pH (varying from

4.0

10.0

), and the dependent variable is the rate of starch hydrolysis.

This establishes what the experiment is designed to measure and helps isolate any unintended variables.

Analyze the buffer components for each tube to identify any differences that do not correlate with pH.

Tube 2 contains sodium chloride (supplying

Cl^-

ions) and Tube 3 contains Tris-HCl and potassium chloride (both supplying

Cl^-

ions), while Tube 1 and Tube 4 do not contain chloride components.

Any variable that differs systematically between experimental groups other than the independent variable is a potential confounding variable.

Evaluate the impact of the identified difference using the given scientific fact.

Since chloride ions stimulate amylase activity, their presence in Tubes 2 and 3 will increase the reaction rates in those tubes, making the rates at pH

6.0

and pH

8.0

appear higher due to the chloride ions rather than the pH alone.

This determines how the confounding variable distorts the experimental conclusions.

Key Concept

A confounding variable is an uncontrolled factor that varies systematically with the independent variable, making it impossible to determine whether the observed effects are due to the independent variable or the uncontrolled factor.

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