Year 3 Science: Fair tests with plants
Part of the Year 3 Science what plants need chapter. Chapter overview · whole chapter on one page
Fair tests with plants
A fair test changes only one thing. Everything else is kept the same. To test whether light helps a bean plant to grow, the class:
- uses two bean plants of the same kind, and the same height at the start;
- puts both in the same size of pot, with the same soil;
- gives both the same amount of water;
- puts one on a sunny windowsill and one in a dark cupboard. This is the one thing that changes.
The class measures both plants with a ruler at the start and after 3 weeks. They write the results in a table, and show the growth on a bar chart. Then they write a conclusion, which says what the results show.
Key ideas
- A fair test changes only one thing and keeps the rest the same.
- Measure the plants at the start and at the end, using a ruler.
- A bar chart can show how much each plant grew.
- A conclusion says what the results show.
Method. Look for the one thing that is different between the two plants.
- Compare the two plants. Both plants have the same pot, soil and water, but one is in the light and one is in the dark.light differs
- Name the one thing. The one thing that changes is the amount of light.light
- Write the answer. light
Check. Only the light changes in a fair test. ✓
Method. Take the height at the start away from the height at the end.
- Read the two heights. The plant was 6 cm at the start and 21 cm at the end.6 and 21
- Find the growth. The growth is the end height minus the start height.21 − 6 = 15
- Write the answer. 15 cm
Check. 6 + 15 = 21, so the answer is right. ✓
Method. Take the smaller growth away from the bigger growth.
- Read the two growths. The window plant grew 15 cm and the cupboard plant grew 3 cm.15 and 3
- Find the difference. The difference is how much more the window plant grew.15 − 3 = 12
- Write the answer. 12 cm
Check. 3 + 12 = 15, so the answer is right. ✓
Watch out
Watch out: one test is not enough. A class repeats the test with more than one plant in each place, so that one unusual plant does not mislead them.
Try it at home or school (with a grown-up). With a grown-up, set up two identical small pots of bean or cress seeds: one by a sunny window and one in a dark cupboard. Use the same soil and the same amount of water for both. Measure the height of each plant with a ruler every few days, and write the results in a table. Do not put anything in the cupboard that could harm the plant. Wash your hands afterwards.
Exam tip
Grown-up tip. Ask: “What is the one thing that changes? What must stay the same?” Help your child to measure each plant from the soil to the top, in cm, and to write the results in a table with a heading for each column.
Exercise 3F Fair tests with plants
Method. Think about the definition of a fair test.
- Read the definition. A fair test changes only one thing.one thing
- Write the number. So the number of things that change is 1.1
- Write the answer. 1
Check. A fair test changes only one thing. ✓
Method. Check which thing changes in the light test.
- Read A. The place changes, so A is the thing that changes.A changes
- Read B. The amount of water is kept the same in both plants.B stays the same
- Read C. The amount of light changes too, so C is not the same.C changes
- Write the answer. B
Check. Water is kept the same in a fair test. ✓
Method. Think about how many things change in a fair test.
- Read the sentence. A fair test changes only one thing.one
- Write the answer. one
Check. Only one thing changes. ✓
Method. Think about what a bar chart can show about the plants.
- Read the sentence. A bar chart can show how much each plant grew.grew
- Write the answer. grew
Check. The bars show the growth. ✓
Method. Think about what changes in a growing plant that you can measure.
- Think about the plant. A growing plant gets taller.taller
- Name the measurement. The measurement is the height of the plant.height
- Write the answer. height
Check. Measure the height of each plant from the soil to the top. ✓
Method. Think about a tool that measures length in cm.
- Think about the job. A height is a length, so we need a tool that measures length.length
- Name the tool. A ruler measures length in centimetres.ruler
- Write the answer. ruler
Check. A ruler measures height in cm. ✓
Method. Look for the taller bar.
- Compare the bars. The window bar is taller than the cupboard bar.taller bar
- Name the plant. So the window plant grew more.window
- Write the answer. window
Check. The window bar is the taller bar. ✓
Method. Read the height of the window bar.
- Find the bar. The window bar is labelled as the window plant.window bar
- Read the value. The label on the bar says 15 cm.15 cm
- Write the answer. 15 cm
Check. The label on the bar shows the growth. ✓
Method. Add the pots in the light and the pots in the dark.
- Read the two numbers. There are 3 pots in the light and 4 in the dark.3 and 4
- Add them together. The total is the number of pots in both places.3 + 4 = 7
- Write the answer. 7 pots
Check. 4 + 3 = 7, so the answer is right. ✓
Method. Check what was kept the same in the test.
- Read the test. Both plants had the same amount of water, so water was kept the same.same water
- Decide. Water was not the thing that changed, so Ruby’s conclusion is not fair. The answer is no.not fair
- Write the answer. no
Check. The only thing that changed was the light. ✓
Method. Think about what each plant can do with light.
- Think about the window plant. It gets light, so its leaves can make food.light, food
- Predict. So the window plant is predicted to grow taller, and this can be checked by measuring.predict taller
- Write the answer. Accept any reason that says the window plant can make more food from light, and the cupboard plant cannot.
Check. A prediction can be checked with a fair test. ✓
Method. Think about one plant that may grow unusually.
- Think about one plant. One plant may grow differently from the others for a reason that has nothing to do with light.one plant
- Give the reason. Repeating the test with more plants shows whether the results are the same each time.repeat checks
- Write the answer. Accept any answer that says repeating the test checks the results, so that one unusual plant does not mislead the class.
Check. Repeats make the results more reliable. ✓