Physical ScienceMatter Interactions and EnergyYear 4 Science

Temperature: measuring how hot or cold

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What this lesson covers

Temperature tells us how hot or cold something is
Thermometers measure temperature in degrees Celsius (°C)
Reading a thermometer scale, including what each small mark is worth
Predicting a temperature, then measuring it
Why two readings can differ, and how to make readings more accurate

Key idea

What temperature is

Temperature is how hot or cold something is.
Your skin is not a good judge. It compares with whatever it touched last.
So we measure temperature with a thermometer.
The unit is degrees Celsius, written °C.

Key idea

How a thermometer works

A liquid thermometer has a thin glass tube with coloured liquid inside and a bulb at the bottom.
When the liquid warms up, it takes up more space and climbs the tube. When it cools, it shrinks back down.
A digital thermometer shows the number on a screen.
Some temperatures to know:

Thing	About
Water with ice in it	0 °C
A warm classroom	20 °C
Your body	37 °C
Pure water boiling at sea level	100 °C

Key idea

Try it: match the temperature

Key idea

Reading the scale

Find the top of the liquid.
Look at the numbered marks just above and just below it.
Work out what each small mark is worth: count the gaps between two numbers and share the difference between them.
Keep your eye level with the top of the liquid, not above or below it.
Always write the unit, °C, after the number.

Worked example

Example: what does it say?

The thermometer has a number every 10 °C, with 5 small gaps between numbers.
The liquid stops 2 marks above 20. What is the temperature?

A thermometer with numbers 10, 20, 30 and 40, five small gaps between each pair of numbers, and the red liquid stopping two marks above 20, at 24 °C

10 °C ÷ 5 gaps = 2 °C for each markwhat a mark is worth
2 marks × 2 °C = 4 °Chow far above 20
20 °C + 4 °C = 24 °C

Worked example

Example: predict, then measure

Room 9 predicted the temperature of four cups of water, then measured each one.
Which prediction was furthest from the measurement?

Cup	Prediction	Measured
Water with ice	0 °C	2 °C
Cold tap water	10 °C	14 °C
Left on the bench	20 °C	19 °C
Hot tap water	60 °C	48 °C

Ice: 2 − 0 = 2 °C out. Cold tap: 14 − 10 = 4 °C out. Bench: 20 − 19 = 1 °C out
Hot tap: 60 − 48 = 12 °C out
The hot tap water prediction was furthest out, by 12 °C

Key idea

How accurate is it?

Accurate means close to the true value.
Two thermometers in the same cup can read a little differently.
Readings go wrong if you read too soon, hold the bulb in your fingers, look from above or below, or lift the thermometer out of the water to read it.
To check a result, measure more than once, or use two thermometers, and compare.

Worked example

Example: spot the odd reading

Four groups measured the same jug of water.
Which reading is likely to be wrong, and what is the best value for the jug?

Group	Reading
A	21 °C
B	22 °C
C	21 °C
D	30 °C

A, B and C are within 1 °C of each other
D is 8 °C higher than the others
D's reading is likely wrong. Maybe the bulb was held in warm fingers. The jug is about 21 °C

Key idea

Try it: spot the odd reading

Common mistakes

Every small mark on a thermometer is 1 °C.
Count the gaps between two numbers first; on many thermometers each mark is 2 °C or 5 °C.
You can read the thermometer as soon as it goes in.
Wait until the liquid stops moving, then read it while the bulb is still in the water.
If the water feels warm to my hand, it is warm.
Your hand compares with what it touched before; only a thermometer gives a number you can trust.
The answer is 24.
Always write the unit: 24 °C.

Try it at home

Can you trust your hands?

You need: 3 bowls, cold tap water, ice cubes, warm tap water, a towel, a kitchen thermometer, if you have one

Fill the left bowl with cold tap water and ice. Fill the right bowl with warm tap water. Fill the middle bowl with a mix of the two.
Put your left hand in the cold bowl and your right hand in the warm bowl. Count slowly to 30.
Now put both hands into the middle bowl at the same time.
Say how the water feels to each hand. If you have a thermometer, measure the middle bowl.
Look for this: the middle bowl feels warm to one hand and cold to the other, even though it is one temperature. That is why we use a thermometer.

Safety: Use warm water that feels comfortable, never hot, and ask an adult to test it first so nobody gets scalded.

Your turn

What unit do we usually use for temperature?

A thermometer has a number every 10 °C and 10 small gaps between numbers. What is each mark worth?

Each mark is 2 °C. The liquid stops 3 marks above 30. What is the temperature?

You predicted 15 °C and measured 19 °C. How far out was your prediction?

Four readings of one cup are 18 °C, 19 °C, 18 °C and 26 °C. Which is likely to be wrong?

Why should the bulb stay in the water while you read the thermometer?

Show answer

1. Degrees Celsius, °C. 2. 1 °C, because 10 ÷ 10 = 1. 3. 36 °C, because 3 × 2 = 6 and 30 + 6 = 36. 4. 4 °C, because 19 − 15 = 4. 5. 26 °C, because it is far from the other three. 6. Out of the water the liquid starts to change straight away, so the reading would be wrong.

Summary

Recap

Temperature is how hot or cold something is, measured in °C with a thermometer.
Work out what each small mark is worth before you read the scale.
Read at eye level, after the liquid stops moving, with the bulb still in.
Predict first, then measure, and see how far out you were.
Measure more than once and compare to check your results are accurate.

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