9.1 - Variables & Controls

What a variables question looks like

• Questions often describe an experiment and ask what should be changed, what should be measured, or what should be kept the same.
• Example (a simple cell made from a metal strip and a copper strip in an electrolyte): "Suggest one factor that should be kept constant to make the experiment fair."
  • "Kept constant" asks for a controlled variable.
  • "Fair" means any difference in the result is down only to the one thing deliberately changed.

The three types of variable

1. The variable changed - The one factor deliberately altered, such as the metal used.
2. The variable measured - The quantity read off an instrument to see the effect, such as the voltage produced.
3. A variable kept constant - A specific factor kept the same throughout, such as the volume of electrolyte or the surface area of the metal strips, rather than vaguely "the conditions".

A typical question

Suggest one factor that should be kept constant to make the experiment fair.

Keeping conditions the same - A rate experiment

A student investigated how the mass of calcium affects the rate of its reaction with water, by measuring the volume of hydrogen gas collected in the first minute.

Ca(s) + 2H₂O(l) → Ca(OH)₂(aq) + H₂(g)

The three variables here

1. The variable changed - The mass of calcium.
2. The variable measured - The volume of hydrogen gas collected in the first minute.
3. The variables kept constant - Everything else that could affect the volume of gas: the volume of water, the temperature of the water, and the particle size of the calcium.

Naming and fairness

• Controlled variables must be named in full. "The volume of water" and "the temperature of the water" score marks; "the conditions" does not.
• Keeping those variables constant is what makes the comparison fair: if the mass of calcium is the only factor changed, any difference in the volume of gas must be caused by the mass of calcium alone.

Keeping conditions the same - An energy experiment

• An experiment to find the energy released when propanol burns uses a metal can of water heated by the burning alcohol, with a thermometer and a draught shield around the flame.
• The variable measured is the temperature change of the water, and the mass of propanol burned is measured on a balance before and after.

What has to be kept constant

• Volume of water in the can - A smaller volume of water heats up more for the same energy input, which would alter the temperature rise independently of the fuel.
• Distance between the flame and the can, and the type of can - Keeps the fraction of heat transferred to the water the same in each run.
• The draught shield - Stops draughts carrying heat away before it warms the water.

Explaining, not just naming

When asked to explain why a control is used (e.g. "Explain why the use of a draught shield improves the accuracy of the results"), state both:

1. What the feature does - Stops heat escaping to the surroundings.
2. Why that matters - So the measured temperature rise accurately reflects the energy released by the fuel.

Safety

• Keep the propanol away from open flames when not in use.
• Never leave a lit burner unattended.

Scientific vocabulary

In exam questions, name the specific variable directly (e.g. "volume of acid") rather than just writing "independent variable" or "to make it a fair test".

The aim

• An aim states the purpose of the investigation in one sentence (e.g. "To investigate how the mass of calcium affects the rate of its reaction with water").
• The conclusion of an investigation must always refer directly back to the aim.

Common exam questions

Question 1

A student investigated how the concentration of hydrochloric acid affects the rate of its reaction with magnesium ribbon, by measuring the volume of hydrogen gas produced in the first minute. Name the variable that was changed and the variable that was measured.

The variable changed is the concentration of the hydrochloric acid. The variable measured is the volume of hydrogen gas produced in the first minute.

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Question 2

Suggest one factor that should be kept constant to make the experiment fair.

Any named factor kept the same, such as the length or mass of magnesium ribbon, the volume of acid, or the temperature. Keeping it the same means any difference in the volume of gas produced can be put down to the concentration of the acid.

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Question 3

A student burns a fuel to heat 100 cm³ of water and measures the temperature rise. Give one reason why the volume of water should be the same in every repeat.

A smaller volume of water would heat up more for the same amount of energy released, so changing the volume of water would change the temperature rise for a reason that has nothing to do with the fuel being tested.

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Question 4

A student says the volume of gas produced should be kept constant in the calcium and water experiment. What is wrong with this?

The volume of gas produced is the variable being measured, so it is the thing expected to change. What should be kept constant is the volume of water, its temperature, and the size of the pieces of calcium.

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Question 5

Why is naming "the conditions" as the controlled variable not enough?

Because it does not say which condition is meant. A marker can only give credit for a variable named specifically, such as the temperature or the volume of acid.

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Question 6

What does the assignment's "Aim" section do, and what should your conclusion do with it?

The aim states the purpose of the investigation in one sentence, at the start of the report. The conclusion has to refer back to that aim and say whether the results support it.