A student conducted an experiment to determine how the concentration of a sodium chloride … — ACT

A student conducted an experiment to determine how the concentration of a sodium chloride ( $\text{NaCl}$ ) solution affects the rate of rust formation on iron nails. The student prepared $4$ beakers with different $\text{NaCl}$ concentrations, placed $1$ identical iron nail in each beaker, and positioned the beakers at various locations in the laboratory. The experimental setup is summarized in the table below:

Beaker	$\text{NaCl}$ Concentration (\%)	Volume of Solution (mL)	Location in Laboratory
$1$	$0$	$100$	Next to a sunny window
$2$	$5$	$100$	Inside a closed wooden cabinet
$3$	$10$	$100$	Directly above a heating vent
$4$	$15$	$100$	On an open laboratory bench

After $5$ days, the student measured the mass of rust that had accumulated on each nail. Which of the following factors represents an uncontrolled variable in this experiment that could confound the results?

A
The mass of rust accumulated on each nail after $5$ days
The location of each beaker in the laboratoryAnswer
C
The concentration of sodium chloride ( $\text{NaCl}$ ) in the solutions
D
The volume of the salt solution in each beaker

Answer

The location of each beaker in the laboratory

The correct answer is the location of each beaker in the laboratory. In a controlled experiment, only the independent variable (in this case, the sodium chloride concentration) should be changed between groups. By placing the beakers in different locations, the student introduced additional variables such as temperature and light intensity, which can also affect the rate of rust formation. This makes it impossible to determine if any difference in rust mass was caused by the salt concentration or the environmental conditions of the location.

Step-by-Step Solution

Identify the independent variable and the dependent variable in the experimental setup.

The independent variable is the sodium chloride (

\text{NaCl}

) concentration, and the dependent variable is the mass of rust accumulated on the nails.

Knowing which variables are intentionally changed (independent) and measured (dependent) helps separate them from the variables that need to be controlled.

Examine the variables that are kept constant (controlled variables) and those that vary unintentionally.

The volume of the solution is kept constant at

100\text{ mL}

for all trials. However, the location of each beaker varies (window, cabinet, heating vent, bench).

To ensure a fair test, all conditions other than the independent variable must be kept identical across all trials.

Determine which varying factor introduces a confounding variable.

The different locations introduce varying temperature and light conditions, which are known to affect chemical reaction rates like rusting.

An uncontrolled variable that changes along with the independent variable is a confounding factor, as it makes it impossible to isolate the cause of the observed changes.

Key Concept

Identifying confounding variables (uncontrolled factors that vary across experimental groups) and understanding how they compromise the validity of experimental conclusions.

Question

Answer

Step-by-Step Solution

Key Concept

Common Mistakes

Topics

More questions from this topic