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Difficulty: EasyIdentifying Sources of Error and Confounding Variables

A student wants to investigate how the volume of water affects the time it takes for the water to boil. In Trial 1, the student heats 100 mL100\text{ mL} of water in a glass beaker on a hot plate set to High (Level 1010). In Trial 2, the student heats 200 mL200\text{ mL} of water in an identical glass beaker on a different hot plate set to Medium (Level 55). Which of the following is an uncontrolled variable in this experiment that prevents the student from drawing a valid conclusion?

  1. A
    The boiling time of the water
  2. B
    The volume of water used in each trial
  3. The heat setting of the hot platesAnswer
  4. D
    The material of the beakers

Answer

The heat setting of the hot plates
To determine how water volume affects boiling time, all other variables, such as the heat setting of the hot plates, must be kept constant. Because the hot plates were set to different levels (High in Trial 1 and Medium in Trial 2), the heat setting is an uncontrolled variable that confounds the results.

Step-by-Step Solution

1
Identify the independent variable (what is intentionally changed to test its effect) and the dependent variable (what is measured).
The independent variable is the volume of water (100 mL100\text{ mL} vs. 200 mL200\text{ mL}). The dependent variable is the boiling time.
This establishes the core relationship the student intends to study.
2
Examine the experimental procedure for any variables other than the independent variable that changed between trials.
The heat setting changed between trials (Level 1010 in Trial 1, Level 55 in Trial 2).
Any variable that changes alongside the independent variable acts as a confounding variable, making it impossible to determine which factor caused the observed change in the dependent variable.
3
Identify the variable that was kept constant to confirm it is not confounding.
The beaker material was kept constant (both were identical glass beakers).
Controlled variables do not introduce experimental error or confound the results.

Key Concept

Identifying Sources of Error and Confounding Variables
Estimated Time:45s
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Identifying Sources of Error and Confounding Variables

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