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Difficulty: EasyIdentifying Sources of Error and Confounding Variables

A student wanted to test how temperature affects the rate of a chemical reaction between baking soda and vinegar. The student performed two trials:

* Trial 1: 10 g10\text{ g} of baking soda was mixed with 50 mL50\text{ mL} of vinegar at 20∘C20^\circ\text{C} in a beaker, and the mixture was stirred.
* Trial 2: 10 g10\text{ g} of baking soda was mixed with 50 mL50\text{ mL} of vinegar at 40∘C40^\circ\text{C} in an identical beaker, and the mixture was not stirred.

The reaction rate was determined by measuring the volume of carbon dioxide gas produced. Which of the following describes a flaw in this experimental design?

  1. A
    Changing the temperature of the vinegar between the trials prevents a proper comparison.
  2. B
    Keeping the mass of baking soda at 10 g10\text{ g} in both trials makes the results invalid.
  3. Stirring the mixture in only one of the trials introduces a confounding variable.Answer
  4. D
    Measuring the volume of carbon dioxide gas produced acts as an uncontrolled factor.

Answer

Stirring the mixture in only one of the trials introduces a confounding variable.
Stirring affects how quickly baking soda and vinegar mix and react. By stirring only the first trial, the student introduced an uncontrolled variable (stirring), making it impossible to determine if the difference in reaction rate was caused by the temperature difference or the stirring.

Step-by-Step Solution

1
Identify the goal of the experiment and the independent variable.
The goal is to test the effect of temperature (independent variable) on the reaction rate.
Understanding what is being tested helps separate the variable that should change from those that should be controlled.
2
Examine the experimental setup of both trials to see if any other variables changed.
In Trial 1, the mixture was stirred, but in Trial 2, it was not stirred. Both temperature and stirring differed between the trials.
A valid experiment must only vary the independent variable while keeping all other potential variables constant.
3
Determine the impact of the uncontrolled variable on the conclusion.
Stirring is a confounding variable because it changes between the trials and affects reaction rate, meaning any difference in results cannot be solely attributed to temperature.
This identifies the experimental flaw and the source of error.

Key Concept

An experimental design must control all variables except the independent variable to prevent confounding results.
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Identifying Sources of Error and Confounding Variables

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