13.1 - Variables & Controls

What a "keep this the same" question looks like

• Practical questions often ask you to state what should be kept the same in an experiment, or to explain why.
• The mark always goes to a variable named specifically, never to a vague answer. For example, when investigating how acid concentration affects the rate of reaction with magnesium:
  • The volume of acid used in each experiment
  • The length (or mass) of magnesium ribbon used in each experiment
• A variable must stay constant so that the factor being investigated is the only one affecting the result.

Key terms

• Independent variable - The variable you change on purpose.
• Dependent variable - The variable you measure.
• Control variables - The variables you keep constant to ensure a valid test.

A typical question

A student investigates how the rate of reaction of magnesium ribbon with dilute acid changes as the concentration of the acid is changed. The student uses five solutions of the same acid, each with a different concentration.

Suggest two things that should be kept the same in each experiment. [2]

What to keep constant, and why

• In an experiment, you change one variable on purpose (the independent variable) and measure what happens to another (the dependent variable).
• All other variables must be kept constant. If two things change at once, you can't tell which one caused the result.

A rate experiment

A student investigates how acid concentration affects the rate of reaction with magnesium ribbon. Five different concentrations are used, adding a coil of magnesium ribbon to 30 cm³ of acid and timing until the ribbon disappears.

• Changed on purpose - Concentration of the acid.
• Kept constant:
  • Volume of acid (30 cm³ each time)
  • Length (or mass) of magnesium ribbon
  • Starting temperature of the acid (since warmer acid reacts faster)

A temperature-change experiment

A student investigates how the length of magnesium ribbon affects the temperature rise when reacting with dilute sulfuric acid. Five different lengths of ribbon are added to 20 cm³ of acid in a boiling tube, and the maximum temperature reached is recorded.

• Changed on purpose - Length of magnesium ribbon.
• Kept constant:
  • Volume of acid (20 cm³ each time)
  • Concentration of acid

How to name control variables

• Identify what is being changed, then name any other factor that could affect the outcome specifically.
• Stating "the acid" isn't specific enough - specify its volume, concentration, or temperature.

"Fair test" in metal-reactivity planning

• Planning questions (such as finding the order of reactivity of three metals with dilute acid) often state that your plan must make it clear how your investigation will be a fair test.
• To make a test fair, you must state the specific variables kept constant across all trials:
  • The same (or a stated) volume of acid
  • The same (or a stated) concentration of acid
  • The same (or a measured) starting temperature of the acid
  • The same (or a stated) mass (or surface area) of each metal

Common exam questions

Question 1

A student compares the rate of reaction of magnesium with acids of five different concentrations. Suggest one thing that should be kept the same in each experiment.

The volume of acid used (or the length of magnesium ribbon used), so that only the concentration affects how quickly the magnesium reacts.

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Question 2

A student writes "keep everything else the same" as their answer to a constant-variable question. Why might this not score?

Because the examiner needs to see which quantity is meant; a controlled variable has to be named specifically, such as "the volume of acid" or "the mass of metal", not described in general terms.

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Question 3

In a temperature-change experiment where the length of magnesium ribbon is changed, why must the volume of acid be the same in every experiment?

Because a larger volume of acid could produce a different temperature change on its own, so if the volume varied as well as the length of ribbon, the change in temperature could not be attributed to the length of ribbon alone.

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Question 4

A plan asks you to find the order of reactivity of three metals with dilute hydrochloric acid, and says your plan must make it clear how the investigation will be a fair test. What should your answer include?

Named quantities kept the same for each metal - the same volume of acid, the same concentration of acid, and the same mass of metal - alongside how you place the metals in order from your results, such as by timing how long each takes to finish reacting.

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Question 5

A student names "the time taken" as a variable that should be kept constant in a rate experiment. Why is this wrong?

The time taken is what is being measured to find the effect of the change, so it is the dependent variable, not something held constant; only the variable being investigated is changed on purpose, and everything else that could affect it is what gets kept the same.