Science notes, unit 6

Heat

Textbook Science Year 5 (DLP), pages 145–162

What this chapter covers

You can explain that heat is a form of energy and that temperature is the degree of hotness or coldness of a material (6.1.1)
You can measure temperature with a thermometer, using the correct technique and reading the meniscus level, in degrees Celsius (6.1.2)
You can state the freezing point (0°C) and boiling point (100°C) of water and describe how the temperature changes when water is heated (6.1.3)
You can describe how hot water cools until it reaches room temperature, and say that heat flows from hot to cold (6.1.4)
You can show that solids, liquids and gases expand when they gain heat and contract when they lose heat (6.1.5)
You can explain why expansion and contraction matter, such as gaps on bridges and railway tracks and loose electric cables (6.1.6)

Why does a hot drink become cold? Why do the railway tracks and the electric cables need special care in hot weather? The answers are all about heat. In this unit you will learn what heat and temperature are, how to measure temperature with a thermometer, and how heat makes materials expand and contract.

6.1Heat and temperature

What are heat and temperature?

Heat is a form of energy that is transferred because of a difference in temperature. A material becomes hotter when it gains heat and colder when it loses heat.

Temperature is the degree of hotness or coldness of a material. We measure it with a thermometer. The standard unit is the degree Celsius, with the symbol °C.

When Ana's hot drink becomes cold, the drink loses heat to the surroundings. Its temperature goes down.

Heat and temperature are not the same. Heat is energy. Temperature tells us how hot or cold something is.

Did you know? Besides degree Celsius, some countries use degree Fahrenheit (°F) on their thermometers.

Thermometers

A thermometer has a glass tube with a temperature scale and a liquid inside. There are two kinds you must know.

Thermometer	Liquid inside	Where it is used
Room thermometer	Coloured liquid	Reading the temperature of a room
Laboratory thermometer	Alcohol (coloured) or mercury (silver)	Measuring the temperature of liquids in experiments

A room thermometer with tube, temperature scale and liquid labelled, and two laboratory thermometers with mercury liquid and alcohol liquid, marked with 0, 37 and 100 degrees Celsius — A room thermometer and two laboratory thermometers. Remember three numbers: water freezes at 0 °C, normal body temperature is 37 °C and water boils at 100 °C.

How a thermometer works

The liquid in a thermometer expands (gets bigger) when it gains heat and contracts (gets smaller) when it loses heat.

When the temperature	The liquid	The liquid level
increases	expands	rises
decreases	contracts	falls

Two thermometers in beakers: the liquid level at 30 degrees when the temperature decreases and at 40 degrees when it increases — The liquid level falls when the temperature decreases. It rises when the temperature increases.

The thermometer in a beaker of water shows 30 °C. Ice is added and the water gets colder. Will the liquid level rise or fall? Why?

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It falls, because the liquid in the thermometer loses heat and contracts.

Using a thermometer correctly

Put a beaker of water on a flat surface.
Hold the upper part of the thermometer so that it is upright.
Put the thermometer into the water. The bulb must not touch the bottom of the beaker.
Wait until the liquid level stops moving. Then put your eye level with the meniscus and read the temperature.

Four photos: an empty beaker with water, holding the thermometer upright, putting the thermometer into the beaker, and reading the level with the eye level with the liquid — The four steps for using a thermometer.

The meniscus level is the curved level of the liquid in the glass tube of a thermometer.

A girl reading an alcohol thermometer. Eye too high is wrong, eye level with the meniscus is correct, eye too low is wrong — Alcohol thermometer: only the eye **level with the meniscus** gives the correct reading.

A girl reading a mercury thermometer with her eye level with the meniscus — Mercury thermometer: the eye must also be level with the meniscus.

Do not use a thermometer to stir the water. It can break. Never read it with your eye too high or too low.

Reading the scale

First find the value of one small mark. Between 30 and 40 there are 10 spaces, so each small mark is 1 °C. Count up from the last number and write the unit.

A thermometer scale from 20 to 50 degrees Celsius with the red liquid level at 36 — The liquid level is 6 marks above 30. The reading is **36 °C**.

Example: three beakers

Tap water, hot water and cold water are measured in turn. The bar chart shows tap water 28 °C, hot water 64 °C and cold water 12 °C. Hot water is 64 − 12 = 52 °C hotter than cold water.

Bar chart: tap water 28, hot water 64, cold water 12 degrees Celsius — A bar chart of the temperature of three kinds of water.

Freezing point and boiling point of water

The freezing point of water is 0 °C. The boiling point of water is 100 °C.

When ice is heated, it changes to water and the temperature goes up. The longer we heat the water, the higher its temperature, until it boils at 100 °C.

Mr Thermometer story: ice at 0 degrees, water at 26 degrees after 40 minutes, then heated water boiling at 100 degrees after 15 minutes — In the story, ice is at 0 °C (freezing point). Heated water reaches 100 °C (boiling point).

Heating water in the laboratory. Measure the temperature every five minutes. Stop heating when the water boils, and be careful with the hot water and the Bunsen burner.

Apparatus on a retort stand and tripod: a beaker of ice cubes with a thermometer, then the same beaker of heated water — Measure the temperature of the ice cubes, then of the water as it is heated.

What is the trend of the temperature as water is heated? What is the relationship between the heating time and the temperature?

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The temperature increases. The longer the heating time, the higher the temperature of the water.

Heat flows

Heat always flows from a hotter place to a colder place. Look at the cup of hot tea.

Comic: a boy pours hot tea, stirs it with a metal spoon, finds the spoon hot, and drinks the tea when it is less hot — Heat flows from the hot tea to the spoon, and from the spoon to the boy's hand.

The metal spoon becomes hot because it gains heat from the hot tea.
The tea loses heat to the spoon and to the air, so its temperature decreases.
The temperature keeps falling until it is equal to the room temperature. Then it stays constant.

Line graph: tea cools from 85 degrees to 28 degrees by 25 minutes, then stays at 28 degrees — The tea falls from 85 °C and becomes constant at 28 °C from 25 minutes. The room temperature is 28 °C.

Did you know? People in very cold places make a bonfire. The heat from the fire flows to the cold air and people around it.

Ana puts a bottle of hot milk into a container of tap water before she gives it to her little brother. Why?

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Heat flows from the hot milk to the cooler tap water. The milk loses heat, so its temperature decreases and it becomes less hot.

Ravi's bicycle key was left on the school field in the hot sun. It became too hot to hold. Why?

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The key gained heat from the Sun, so its temperature increased.

Expansion and contraction of materials

Materials expand when they gain heat and contract when they lose heat. This happens to solids, liquids and gases.

Material	Heated or cooled	What happens
Solid (metal ball)	Heated with a Bunsen burner	The ball expands and cannot pass through the ring.
Solid (metal ball)	Put in water to cool	The ball loses heat and contracts, so it passes through the ring again.
Liquid (coloured water)	Flask in hot water	The water expands and its level rises in the glass tube.
Liquid (coloured water)	Flask in ice	The water contracts and its level falls.
Gas (air in a bottle)	Bottle in hot water	The air expands and the balloon gets bigger.
Gas (air in a bottle)	Bottle in ice	The air contracts and the balloon gets smaller.

Activity 1: a metal ball and ring: passing through the ring, heating the ball, trying again, cooling in water, and trying again — Activity 1: the metal ball and ring.

Activity 2: pouring coloured water into a conical flask, fitting a glass tube, and putting the flask in hot water and in ice — Activity 2: the level of coloured water in the glass tube.

Activity 3: a balloon over a plastic bottle, put in hot water and then in ice — Activity 3: the balloon on a plastic bottle.

Everyday examples. Melia poured hot water into a bottle and screwed on the cap. The air inside gained heat and expanded, so the cap popped off. Two glasses were stuck together. Ice cubes were put into the top glass. The glass lost heat and contracted, so the glasses came apart.

Four pictures: hot water poured into a bottle, the cap screwed on, the bottle left, and the cap popping off — Situation 1: the air in the bottle expands and pushes the cap off.

Three pictures: two glasses stuck together, ice cubes put into the top glass, and the glasses separated — Situation 2: the glass contracts when it loses heat.

The metal ball does not become heavier when it is heated. It only becomes bigger. Use a test tube holder for the hot ball.

Why expansion and contraction matter

Builders know that materials change size. They design things with this in mind.

Situation	Why it is designed this way
Gaps between metal connectors on a bridge	Let the bridge expand when the surroundings are hot. This prevents damage to the bridge surface.
Gaps between connectors on a railway track	Let the track expand when the surroundings are hot. This prevents the track from bending.
Electric cables hung loosely between poles	Let the cables contract when the surroundings are cold. This prevents the cables from snapping.

Photo of a road bridge with metal connectors and gaps — A bridge, a railway track and electric cables are all made with expansion and contraction in mind.

Photo of a train on a railway track, with a close-up of the gap between track connectors — A bridge, a railway track and electric cables are all made with expansion and contraction in mind.

Did you know? You can make your own thermometer. Put coloured water in a plastic bottle with a straw. In hot water the level in the straw goes up. In ice it comes down.

Pictures: Science Year 5 (DLP) textbook, pp. 146–158 (thermometers p. 146, liquid level and use steps p. 147, meniscus p. 148, Mr Thermometer p. 150, freezing and boiling point set-up p. 151, tea p. 152, bottle cap and glasses p. 154, activities 1, 2 and 3 pp. 155–157, bridge, railway and cables p. 158). The thermometer scale and graphs are drawn for this site.

Summary

Idea	Remember
Heat	A form of energy transferred because of a temperature difference. Flows from hot to cold.
Temperature	Degree of hotness or coldness. Measured in °C with a thermometer.
Thermometer	Liquid rises when it expands and falls when it contracts. Eye level with the meniscus.
Freezing point and boiling point	Water freezes at 0 °C and boils at 100 °C.
Hot water left in a room	Temperature falls until it equals the room temperature, then stays constant.
Expansion	Solids, liquids and gases expand when they gain heat.
Contraction	Solids, liquids and gases contract when they lose heat.
Uses	Gaps in bridges and railway tracks, and loose electric cables.