Primary Science — Fair Test & Variables Practice (Answer Key)
Model answers for the four scenarios
Created by Miss Clarissa Ng · www.clartutors.com
Answer key
Model answers. Where a question asks for several things, any scientifically correct set that names the variables properly is acceptable.
1. Seeds in the light and in the dark
(a) Changed The amount of light — one dish is on the windowsill, one is in a dark cupboard.
(b) Kept the same The type and number of seeds (5 green beans), the amount of water, the type of cotton wool, the temperature, the size of the dish.
(c) Measured Whether the seeds germinate, and how many germinate (a count).
(d) Predict and explain Both should germinate, because seeds need water, warmth and oxygen to germinate — light is not needed. (If a pupil predicts the light dish germinates first, the reason must be a scientific one, not “the dark is bad”.)
(e) What it tells her Light is not needed for germination, because the seeds in the dark germinated too. Light will matter later, when the seedling needs it to photosynthesise.
2. Spot the unfair test
(a) Two reasons (i) Ben used two sheets and Ali used one — the amount of towel must be the same. (ii) two different brands are being compared, so the test measures the brand and the amount at once.
(b) Make it fair Ben should use ONE sheet of Brand B — the same number of sheets, the same 20 ml of water, the same soaking time — and then compare the water absorbed.
3. Completing a results table
(a) Table 60 °C → 30 s; 80 °C → 15 s.
(b) Conclusion The hotter the water, the faster the sugar dissolves, because the water at the higher temperature has more heat energy, so its particles move faster and the sugar dissolves in less time.
(c) Keep the same Any one of: the amount of sugar (one spoon), the volume of water, the amount of stirring, the time measured from the moment the sugar is added.
4. Design your own fair test
(a) Changed The material wrapped around the cup (wool, aluminium foil, cotton or paper).
(b) Keep the same (any three) The starting temperature of the water; the volume of water in each cup; the type and size of cup; the amount (thickness) of material used; the room temperature; how the thermometer is placed.
(c) Measured The temperature of the water with a thermometer — starting at 0 minutes, then every 5 minutes for 20 minutes.
(d) Prediction The cup wrapped in the thickest material — for example wool — will keep the water warm the longest, because it traps air, and air is a poor conductor of heat.