S2.1.3 Chapter 3
A fair test with plants
How can I test what a plant needs?
Your guess
Pick one before you read on. It is fine to be wrong.
By the end you can
- Plan a test that changes only one thing
- Record plant height in centimetres in a table
Tariq has a question. Do beans grow better in garden soil or in sand?
He needs a fair test. In a fair test you change only one thing. You keep everything else the same.
Tariq changes the soil. One cup gets garden soil. One cup gets sand.
The rest stays the same. Same kind of bean. Same cup. Same water. Same place.
Draw it first
Draw Tariq's two cups
1 Draw two cups the same size. Write cup A and cup B.
2 Fill cup A with garden soil. Fill cup B with sand. Write what is in each.
3 Put three beans in each cup. Write everything else the same.
Now only the soil is different. If the plants grow differently, the soil is the reason.
Look at a test that is not fair. Cup A has garden soil and stands by the window. Cup B has sand and stands in a cupboard.
Two things are different, the soil and the light. If cup A grows better, you cannot tell why.
Tariq plants three beans in each cup, not one. One seed may be weak. A weak seed would spoil the test.
See it three ways
As a picture
In words
Only the soil is different. Everything else is the same in both cups.
Now measure. The height of a plant is how tall it is. Measure from the soil to the tip. Use a ruler marked in centimetres. We write cm.
Measure the tallest seedling in each cup. Measure on the same days. Write each number in a table. A table is a grid of rows and columns.
Try it
Run a fair test with beans
You need: two cups the same size, and a plate or tray, garden soil, sand, or soil from another place, six dry beans, the same kind, water and a spoon, a ruler marked in centimetres, paper and a pen for your table.
Be safe: A grown up makes the holes. Wash your hands after you touch soil or sand. Do not eat the beans.
- A grown up makes two holes in the bottom of each cup.
- Fill one cup with garden soil. Fill the other with sand.
- Push three beans into each cup, about 2 cm deep.
- Pour the same water on each cup, until just damp.
- Then give each cup one spoon of water a day. Keep both in the same place.
- Every two days, measure the tallest seedling in each cup.
- Write the height in your table. Mark 0 if nothing shows yet.
What you will see: The table shows example numbers. Yours will not match them. For some days nothing shows above the soil. Then the seedlings come up. Look at all the rows, not just one. Then say what you see.
You can also draw the numbers as a line graph. Here is cup A from the example table. It shows each height on its day.
A wrong idea many people have
One seed in each cup is enough for a fair test.
Why it feels right: Each cup has one plant. If they differ, it must be the soil.
The test that breaks it: Plant six beans in one cup of soil. Give them all the same water in the same place. After a week, measure each one. The heights are not all the same. One weak seed would mislead you.
What is true: Seeds are not all alike. Use three or more in each cup. Then one weak seed cannot spoil the test.
Key words
- fair test
- a test that changes one thing and keeps the rest the same.
- height
- how tall a plant is, from the soil to the tip.
- table
- a grid of rows and columns for writing numbers.
Back to your guess
The answer is D: Change one thing. Keep the rest the same.
A fair test changes only one thing. Then you know what made the difference.
Check yourself
Try each one before you look. A wrong answer tells you where to go next.
Tariq tests garden soil and sand. What does he change?
See the answer
The kind of soil
A fair test changes one thing. Here it is the soil.
- If you chose B: He uses the same kind of bean in both cups. Only the soil changes.
- If you chose C: Both cups stay in the same place. Only the soil changes.
Which of these must stay the same in Tariq's test?
See the answer
The amount of water
Water, beans, cups and place stay the same. If water changed too, he could not tell what made the difference.
- If you chose A: The soil is the one thing he changes.
- If you chose C: Height is what he measures. It will be different.
Why does Tariq plant three beans in each cup?
See the answer
One seed may be weak and spoil the test.
Seeds are not all alike. With three, one weak seed cannot fool you.
- If you chose A: The number of beans does not change how fast each one grows.
- If you chose B: There is no fixed number. But one seed is too few.
Look at the table. How tall was the seedling in cup A on day 10?
See the answer
15 cm
Find the row for day 10. Then read across to the column A cm. It says 15.
- If you chose B: That is cup A on day 8. Look at the row for day 10.
- If you chose C: That is cup B on day 10. Look in the column for cup A.
Look at the table. How much did cup B grow from day 6 to day 12?
See the answer
12 cm
On day 6, cup B was 3 cm. On day 12, it was 15 cm. 15 − 3 = 12.
From an earlier chapter
What does the food store in a bean do?
See the answer
It feeds the tiny plant until its leaves can make food.
The two halves of the bean hold food for the tiny plant.
Diego plants beans in two cups. Cup A has garden soil and gets water every day. Cup B has sand and gets water once a week. Cup A grows taller. Can Diego say the soil made the difference?
See the answer
No. He changed the soil and the water, so he cannot tell.
A fair test changes one thing. Here the soil and the water both differ. The soil may not be the reason.
- If you chose A: Cup A also had more water. Either change could be the reason.
- If you chose B: More beans would not fix this. The trouble is that two things differ, the soil and the water.
Look at the ruler. How tall is the seedling?
See the answer
6 cm
The ruler stands on the soil, so 0 is at the soil. The dashed line from the tip meets the 6.
- If you chose B: You may have counted from the top of the ruler. Start at 0, at the soil.
- If you chose C: You may have counted the marks and called the 0 mark 1. Read the number where the dashed line meets the ruler. It is 6.
Keep it
Copy this into your exercise book.
A fair test changes only one thing. Keep everything else the same. Use a few seeds in each cup. Measure height in cm, soil to tip, on the same days. Write it in a table.
Draw: Two cups. Write what is different and what is the same.