Primary Science — How to Answer Experiment Questions

1 Start with the aim, then name the three variables

Every experiment question is answered the same way. Learn the order once:

state the aim,
name the changed, measured and controlled variables,
write the result, then the conclusion.

Aim: “To find out how the size of the food affects how long it takes to be digested.”

CHANGED	The one thing you change on purpose: the size of the food — 1, 2 or 4 pieces.
MEASURED	The thing you measure or observe: the mass of food left after one hour, in g.
CONTROLLED	Everything else that could affect the measured variable, kept the same in every set-up — the same 5 g of food, the same 10 mℓ of digestive juices, the same test tube and the same one hour.

Ask yourself: What am I changing? What am I measuring? What else could change my result? Your school worksheet may write the first two as “changed/independent” and “measured/dependent” — they are the same things.

2 A fair test: only one thing changed

A fair test changes only the changed variable. Then any difference in the measured variable is solely due to it.

Smaller pieces expose more surface to the digestive juices, so less food is left behind.

FairOnly the size of the food is different, so the difference in the mass of food left can only be due to the size of the food.

UnfairIf one tube also gets more digestive juices, two things changed at once — you cannot tell which one caused the difference.

3 More fair-test examples

An aim starts with To find out and then uses if, how or which — never “whether”. The three variables fall straight out of it.

if — a yes-or-no question: To find out if seeds need light to germinate.

Changed light or dark · Measured number of seeds that sprout · Controlled same seeds, same damp cotton wool, same number of days

how — how much the change matters: To find out how the amount of water affects how tall a plant grows.

Changed amount of water · Measured height of the plant (cm) · Controlled same plant, pot, soil, place and days

which — choosing between options: To find out which type of paper absorbs the most water.

Changed type of paper · Measured mass of water absorbed (g) · Controlled same size strip, same volume of water, same time

4 Control set-up

A control set-up is a second set-up with the changed variable not applied — everything else treated the same, so the two can be compared.

Not always needed. In Section 1, both tubes digest the food, so nothing is left out.
Needed when the question asks “is this needed?” Ask the same apparatus a new aim: “To find out if the digestive juices are needed to break the food down.”

Set-up A
Food + 10 mℓ of digestive juices.

Set-up B — the control
Same food, same one hour, water only.

Only Set-up A breaks the food down → the digestive juices are needed.
Both break it down equally → the juices are not needed.

Set-up with the change	The control set-up
To find out if the digestive juices are needed. (Set-up A: juices)	The same food and the same one hour — but water only, no digestive juices.
To find out if seeds need water to germinate. (Set-up A: damp cotton wool)	Same seeds on dry cotton wool, kept in the same place for the same number of days.
To find out if seeds need light to germinate. (Set-up A: in the dark)	Same seeds on damp cotton wool, kept in the light instead.

Purposes of the controlTo compare, and to confirm that any difference in the measured variable is solely due to the changed variable.

Do not confuse the twoControlled variables — kept the same in both set-ups. Control set-up — the set-up with no change.

5 Getting more accurate results

Questions often say: “Suggest how Sam can obtain more accurate results.” Give the improvement, then say why it makes the reading more accurate.

Read the scale more often.

Measure the temperature of the water every minute instead of every five minutes.

Add a pointer or guide to a scale.

The pointer allows the length of the spring to be measured more accurately.

Stop the object moving sideways.

A narrow passage lets the ball move only upwards, so the height it reaches can be measured more accurately.

Make it easier to see what you are counting.

White shirts instead of black ones let her see what landed on them more clearly, so she can count it more accurately.

Slow down the change you are measuring.

A double-walled container has a layer of air, a poor conductor of heat, so heat is lost more slowly, increasing the accuracy of the results.

Use the right instrument.

Measure volume with a measuring cylinder, not a beaker, and read it at eye level.

Finish every one of these answers with the same words: “This allows the ___ to be measured more accurately.” or “… so that the ___ can be counted more accurately.”

6 Answers in the school's wording

Aim if / how / which	if — a yes-or-no question: “To find out if seeds need light to germinate.” how — how much the change matters: “To find out how the amount of water affects how tall a plant grows.” which — choosing between options: “To find out which liquid allows more light through.”
Why it is fair	“This ensures that any difference in the measured variable is solely due to the changed variable and not the controlled variable.”
Purpose of the control set-up	“It acts as a control to compare and confirm that any difference in the measured variable is solely due to the changed variable.”
More accurate results	“This allows the ___ to be measured more accurately.” “… so that the ___ can be counted more accurately.” “… increasing the accuracy of the results of the experiment.”

Inferred variables. When the variable you want cannot be measured directly, you measure something else that stands for it.

Mass of food left after an hour → the rate of digestion.
Number of pieces the food is cut into → its surface area.

7 Worked example — spot the variables

Lynette wanted to find out how the size of food affects how long it takes to be digested.

Tubes P, Q, R
5 g of food in each, cut into 1, 2 and 4 pieces.

Added to each
10 mℓ of digestive juices.

After one hour
Food left: P 3 g, Q 2 g, R 1 g.

Aim	To find out how the size of the food affects how long it takes to be digested.
Changed variable	The size of the food (1, 2 or 4 pieces)
Measured variable	The mass of food left after one hour (g) — inferred: the rate of digestion
Controlled variables	The mass of food (5 g), the volume of digestive juices (10 mℓ), the same test tube and the same one hour
Conclusion	The smaller the pieces of food, the less food is left after one hour, so smaller pieces are digested faster.

8 Before you hand it in

Run through these five checks on every experiment question.

Aim — did you start it with “To find out…” and then use if, how or which?
Changed variable — only one thing, and you say what it is.
Measured variable — the thing you measure, with its unit.
Controlled variables — named, and genuinely kept the same.
Results and conclusion — does the conclusion name the changed and the measured variable?
More accurate results — did you suggest an improvement, and say why it helps?
Marks — one point for each mark, so a 3-mark question needs three separate points.

Say it like this“Only the size of the food was changed, while the mass of food and the volume of digestive juices were kept the same, so any difference in the mass of food left is solely due to the size of the food.”

Watch out for“It is a fair test.” on its own — it earns no marks until the controlled variables are named. And do not invent a control set-up when the question only asks if one thing affects another.