A student conducts an experiment to investigate how the concentration of hydrochloric acid… — ACT

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?

A
The concentration of the hydrochloric acid ( $\text{HCl}$ ) solution
B
The volume of carbon dioxide ( $CO_2$ ) gas produced
The surface area of the calcium carbonate ( $CaCO_3$ ) particlesAnswer
D
The initial mass of calcium carbonate ( $CaCO_3$ ) added to each flask

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

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.

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.

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

Answer

Step-by-Step Solution

Key Concept

Common Mistakes

Topics

More questions from this topic