BelleMum, the milk is too cold for me to drink early in the morning. Can I not drink it?
MumDon't worry, Belle. Let me put it in a basin of hot water to warm it up a little.
Heat
Temperature, heat flow, conductors of heat, and what heat does to things. Read a little, then try it yourself. Your progress is saved on this device.
I am feeling so sweaty and hot. Can someone switch on the electric fan, please?
Oh dear! The soup is too hot to drink.
You may want to drink the soup later when it has cooled down.
MumDraw the curtains, Belle. The sunrays are entering the room. The room will become hot.
BelleSure, Mum.
Have you ever wondered what causes objects to become hotter or colder? When objects become hotter or colder, we say that their temperatures change. Scientists say that heat causes temperature to change. Understanding how heat affects temperature will help us to use heat meaningfully in our everyday life.
What I Am Learning
- What is temperature?
- How is temperature measured?
- What causes temperature to change?
- What is the difference between temperature and heat?
Part 1 · Textbook pp. 122–124
What is temperature?
In our everyday life, we often have to describe how hot or cold an object is.
💻 Let's Inquire
- AB: Activity 6.1
- SLS: Temperature
When you have a fever, the doctor needs to know how hot your body is so that the right medication can be given to you.
When you bake a cake, you need to know how hot the oven should be so that your cake can be baked well.
Hi, Belle! What are you doing?
BelleI want to find out how hot or cold each cup of water is.
So what have you found out?
BelleWell, the water in cup 1 is the hottest.
BelleThe water in cup 1 is hotter than the water in cup 2.
BelleThe water in cup 3 is colder than the water in cup 2.
BelleThe water in cup 4 is colder than the water in cup 3 and cup 2. The water in cup 1 is the hottest and the water in cup 4 is the coldest.
Argh! This is confusing. There must be a better way to describe how hot or cold something is than to use words like 'hottest', 'coldest', 'hotter' and 'colder'.
Be careful when handling hot objects.
To describe how hot or cold an object is, scientists use numbers to measure the degree of hotness of an object. The measure of how hot or cold an object is, is known as temperature. We can measure temperature in degree Celsius (°C).
BelleOh! I see! So when we measure temperature, we use numbers, instead of words, to order how hot or cold something is.
| Cup 1 | Cup 2 | Cup 3 | Cup 4 |
|---|---|---|---|
| 50°C | 40°C | 28°C | 10°C |
A hotter object has a higher temperature than an object that is less hot. A greater number represents a higher temperature, while a smaller number represents a lower temperature.
🌏 Science Around Us
Besides numbers, colours can also indicate the degree of hotness of different objects. A thermal camera can do just that!
A thermal camera makes use of colours to indicate different temperatures. In some places such as airports and hospitals, thermal cameras are used to detect people with a fever without having to take the temperature of each person with a thermometer.
A person with a higher temperature than others appears red in the thermal image captured by the thermal camera.
Key idea
Temperature is a measure of how hot or cold something is.
Answer the quick check to finish this part.
Part 2 · Textbook pp. 125–127
How is temperature measured?
Our fingers may be able to tell us very quickly if something is hot or cold. However, our sense of touch may not be able to tell us exactly how hot or cold an object is.
Scientists invented an instrument to measure an object's temperature accurately. We call this instrument a thermometer. There are many types of thermometers, such as the digital thermometer, the infrared thermometer and the laboratory thermometer. Let's find out more about how these thermometers can be used.
A digital thermometer
The digital thermometer is used to tell us our body temperature.
An infrared thermometer
The infrared thermometer is used to tell us the temperatures of the surfaces of solids and liquids.
A laboratory thermometer
The laboratory thermometer is commonly used to tell us the temperatures of liquids and gases. It cannot be used to measure the temperature of solids directly.
To use a laboratory thermometer to measure temperature, make sure that the bulb of the thermometer is fully covered by the liquid or gas. That is why a laboratory thermometer is not suitable to measure the temperature of a solid directly.
🛠 Let's Think and Do
Using apparatus and equipment to gather data
1StepImmerse the bulb of the thermometer fully in the liquid.
2StepWait for the red liquid in the thermometer to stop rising or falling before you read the temperature.
3StepYou may notice that the surface of the red liquid is curved. Take the reading from the bottom of the curved surface by placing your eye at the same level as the bottom of the curved surface.
You can also use a data logger to measure temperatures when conducting investigations. A data logger is a digital device that displays the temperature on the screen when it is connected to a temperature sensor.
Key idea
Temperature can be measured using a thermometer in degree Celsius (°C).
Answer the quick check to finish this part.
Part 3 · Textbook pp. 128–129
What causes temperature to change?
MumBelle, the cold milk is now warm. You may drink it.
BelleThank you, Mum. I wonder why the temperature of the cold milk increased when the bottle was put in the basin of hot water.
When you place a bottle of cold milk into a basin of hot water, you will find that the hot water becomes less hot while the cold milk becomes less cold after some time. We say that the temperature of the hot water decreases and the temperature of the cold milk increases.
Have you ever wondered what causes the temperature of the cold milk to increase and the temperature of the hot water to decrease when the two are in contact with each other?
Scientists explain that the temperature of an object changes because of heat. What is heat? Like light, heat is a form of energy. Heat can cause changes in temperature. When an object gains heat, its temperature increases. When an object loses heat, its temperature decreases.
Having learnt that heat causes a change in temperature, let us now try to explain how the temperature of the bottle of cold milk increases when placed in a basin of hot water.
Temperature of the milk in the bottle increases from 10°C to 30°C.
Since the temperature of the milk increases, the milk must have gained heat.
Temperature of the water in the basin decreases from 60°C to 45°C.
Since the temperature of the water decreases, the water must have lost heat.
Since the bottle of cold milk and the hot water are in contact with each other, the heat gained by the cold milk must have come from the heat lost by the hot water.
The arrows show the direction of heat flow from the hot water to the cold milk.
We can also say that when the bottle of cold milk is placed in the basin of hot water, heat flows from the hot water to the cold milk. When the bottle of cold milk gains heat, its temperature increases. When the hot water loses heat, its temperature decreases.
💻 Let's Inquire
- AB: Activity 6.2
- SLS: Heat Flow
Key idea
Heat is a form of energy. It causes temperature to change. When an object loses heat, its temperature decreases. When an object gains heat, its temperature increases.
Answer the quick check to finish this part.
Part 4 · Textbook pp. 130–132
What is the difference between temperature and heat?
I am feeling so hot after a run. I must have a lot of heat.
What do you mean by that? Do you mean your temperature is now higher than usual?
Is there a difference?
I think so. Let's ask Mr Bala to explain.
Mr BalaThere are differences between temperature and heat.
| Temperature | Heat |
|---|---|
| is a property of an object. | is a form of energy. |
| is a measure of how hot or cold an object is. | causes the temperature of an object to change. |
💻 Let's Inquire
- AB: Activity 6.3
- SLS: Difference Between Heat and Temperature
Mr BalaTemperature is what we measure, while heat is the reason why temperature changes.
Observation
A beaker of hot water left on the table for some time feels less hot. The graph shows that the temperature of the hot water decreases over time.
Explanation
The temperature of the hot water decreases because the hot water loses heat to its surroundings.
Observation
A beaker of cold water left on the table for some time feels less cold. The graph shows that the temperature of the cold water increases over time.
Explanation
The temperature of the cold water increases because the cold water gains heat from its surroundings.
Key idea
Temperature is not the same as heat. Temperature is a property of an object, while heat gained or lost is the reason why the temperature of an object changes.
Answer the quick check to finish this part.
BelleMum, I need to go out now. The glass of hot water is too hot for me to drink.
MumI think you can add a few cubes of ice to the glass of hot water.
BelleHow does that cool down the hot water?
Part 5: How does heat flow?Ouch! My spoon is so hot! Devi, how are you able to stir your drink with your spoon? Isn't it hot?
DeviHmm, no.
Hmm... why does my metal spoon feel hotter than your wooden spoon when both cups contain hot drinks?
Part 8: How fast does heat flow in different materials?Hi, everyone! I'm so sorry. The canned drinks I bought from the supermarket on my way here are no longer cold.
Of course! The weather is so hot. You should have used a cooler bag to keep the canned drinks cold.
How does a cooler bag help?
Part 8: Science Around UsMum, the lid of the jam jar is too tight. I can't open it.
MumWhy don't you try placing the lid in hot water for a few minutes? That will make it easier to open.
How does the hot water help to open the lid?
Part 9: Expansion and contractionYou have learnt in the previous chapter that the temperature of an object changes when the object gains or loses heat. However, have you thought about how heat flows? What affects how quickly or slowly heat is gained or lost in different materials? What other effects does heat gain or heat loss have on objects? By understanding more about heat and its effects, we can make use of it to improve our daily life.
What I Am Learning
- How does heat flow?
- What is a source of heat?
- How fast does heat flow in different materials?
- What are the effects of heat gain or heat loss?
Part 5 · Textbook pp. 136–137
How does heat flow?
In the previous chapter, you learnt that when a bottle of cold milk is placed in a basin of hot water, the temperature of the cold milk increases while the temperature of the hot water decreases.
💻 Let's Inquire
- AB: Activity 7.1 (Part A)
- SLS: Heat Flow
The changes in temperature can be explained by the heat flow from the hot water to the cold milk. In fact, heat always flows from an object with a higher temperature to an object with a lower temperature. We can use this concept of heat to explain many of the changes in temperature happening around us.
| Example | Observation after some time | Explanation of observation |
|---|---|---|
| A pot of water placed on the stove | The metal pot and the water become hotter. |
The flame on the stove has a higher temperature than the metal pot and the water. Therefore, heat will flow from the flame to the pot of water. Since the water in the pot gains heat, its temperature increases. |
| A cold metal spoon placed in a cup of hot coffee | The metal spoon feels hot. |
The coffee has a higher temperature than the metal spoon. Therefore, heat will flow from the hot coffee to the cold metal spoon. Since the metal spoon gains heat, its temperature increases. |
| A cup of hot tea left on a table | The hot tea cools down. |
The hot tea has a higher temperature than its surroundings. Therefore, heat will flow from the hot tea to its surroundings. Since the hot tea loses heat, its temperature decreases. Similarly, since the table and the air around the tea gain heat, their temperatures increase. |
Answer the quick check to finish this part.
Part 6 · Textbook pp. 138–142
Investigating heat flow with line graphs
We can conduct an investigation to find out more about how temperatures of objects change when two objects of different temperatures are in contact with each other. We will use a line graph to examine the changes. Before we look at the investigation, let us learn about the line graph.
🛠 Let's Think and Do
Analysing and interpreting data
Use of line graphs
When conducting an investigation, we often collect a set of data over a period of time. For example, we can take the temperature of water every 5 minutes for a period of 15 minutes.
We can use a table or a graph to present the data collected.
| Time (minutes) | Temperature (°C) |
|---|---|
| 0 | 100 |
| 5 | 80 |
| 10 | 68 |
| 15 | 55 |
Data presented in a table
We use the data from the table to draw a graph.
Data presented in a graph
Let us find out how we can read the graph.
Using a data logger
Let us now use a data logger connected to temperature sensors to look at how temperatures of objects change over time. A data logger shows data on a graph.
We place a metal cup containing water at 50°C into a plastic box containing water at 20°C.
We place temperature sensors into the water in the metal cup and in the plastic box.
Our Observation
The temperature of the water in the metal cup decreases from 50°C to 35°C in 15 minutes.
The temperature of the water in the plastic box increases from 20°C to 35°C in 15 minutes.
The temperatures of the water in the metal cup and in the plastic box remain constant at 35°C after 15 minutes.
Our Explanation
Area A
The temperatures of the water in the metal cup and in the plastic box are different at the start. Therefore, heat will flow from the hotter water to the colder water.
Since the hot water loses heat, its temperature decreases. Since the cold water gains heat, its temperature increases.
Area B
When the temperatures of the water in the metal cup and in the plastic box become the same, there will be no more heat flow between them.
Since the water in the metal cup and in the plastic box no longer gains or loses heat, their temperatures remain unchanged.
The experiment on the previous page shows that heat will flow between objects only when there is a difference in temperature between them. Therefore, heat flows from an object with a higher temperature to an object with a lower temperature until they have the same temperature.
Heat flow within the same object
Do you know why you must never touch the end of a metal rod even though it is not in direct contact with fire?
This is because heat flow happens not only between different objects but also within the same object.
When one end of the metal rod is in direct contact with fire, it gains heat and becomes hotter than other parts. Since the temperature of this end is higher than the temperature of the other parts of the metal rod, heat will flow from this end to the parts with a lower temperature.
Key idea
Heat flows from an object or an area with a higher temperature to an object or an area with a lower temperature until they have the same temperature.
🔎 Let's Explore
Look around you. What are some examples of heat flow?
Try it: name one thing that is hotter than you and one thing that is colder than you. Which way does heat flow when you touch each of them?
Answer the quick check to finish this part.
Part 7 · Textbook pp. 143–144
What is a source of heat?
You have learnt that heat will always flow from an object with a higher temperature to an object with a lower temperature. An object that has a higher temperature than its surroundings is a source of heat.
💻 Let's Inquire
- AB: Activity 7.1 (Part B)
- SLS: Heat
Mr BalaLet's try to identify the source of heat in each of the following examples.
A pot of water placed on a stove
Since the flame on the stove has a higher temperature than the pot of water, heat will flow from the flame to the water in the pot. Therefore, the flame is the source of heat.A cold metal spoon placed in a cup of hot coffee
Since the hot coffee has a higher temperature than the cold metal spoon, heat will flow from the hot coffee to the cold metal spoon. Therefore, the hot coffee is the source of heat.A cup of hot tea left on a table
Since the hot tea has a higher temperature than the table and the air surrounding it, heat will flow from the hot tea to its surroundings. Therefore, the hot tea is the source of heat.There are also natural sources of heat around us. Here are some natural sources of heat.
The Sun
The Sun is Earth's major natural source of heat. Heat from the Sun travels to Earth to keep the planet warm. The amount of heat we receive from the Sun depends on the time of the day and the month.
The Earth
The temperature beneath the Earth's surface is very high. This natural energy from the Earth is used to heat water found underground. This water can be used for soaking in hot springs and for cooking.
Key idea
An object that has a higher temperature than its surroundings is a source of heat.
Answer the quick check to finish this part.
Part 8 · Textbook pp. 145–149
How fast does heat flow in different materials?
If you put a metal spoon and a wooden spoon into the same cup of hot water, the metal spoon will become hot more quickly than the wooden spoon.
You will also notice that hot water in a polystyrene cup takes a longer time to cool down than hot water in a metal cup. Have you ever wondered if the material of an object affects how fast heat flows?
💻 Let's Inquire
- AB: Activity 7.2
Let us look at an experiment to understand how different materials affect how fast heat flows through them.
Mr BalaLet's put four rods of different materials into the container of hot water. Each of the rods is coated with solid wax at its end. The solid wax will melt when the rod becomes hot.
After some time...
Mr BalaOn which rod did the wax melt the slowest?
Wood! Very little wax had melted.
That's true! What does this tell us about how fast heat flows through wood compared to the rest of the materials?
From the experiment,
| we observe that | The order of melting of wax on the rod: Metal (most wax melted) → Glass → Plastic → Wood (least wax melted) |
|---|---|
| we infer that | Since most wax on the metal rod has melted, heat flows the fastest through the metal rod. Since least wax on the wooden rod has melted in the same amount of time, heat flows the slowest through the wooden rod. Metal (heat flows the fastest) → Glass → Plastic → Wood (heat flows the slowest) |
Materials that allow heat to flow through them quickly are called good conductors of heat. Examples of good conductors of heat are metals.
Materials that do not allow heat to flow through them quickly are called poor conductors of heat. Examples of poor conductors of heat are glass, plastic and wood.
💻 Let's Inquire
- SLS: Good and Poor Conductors of Heat
Good and poor conductors of heat can be useful in our daily life. Good conductors of heat are often used to make cooking utensils such as pots, pans and kettles. Heat can pass through metal containers to the food or water quickly. The base of an electric iron is also made of metal so that heat can be transferred quickly from the metal to the fabric.
🧩 Let's Connect
In the theme of Diversity, you have learnt about the properties of different materials. These properties allow a material to be used in certain ways.
Examples of good conductors and poor conductors of heat
While good conductors of heat are very useful in our daily life, they can also cause problems at times. Imagine if the handles of metal pots or electric irons were made of metal. They would be too hot for us to hold once they are heated.
Therefore, you will find that the handles of many cooking utensils are made of poor conductors of heat, such as plastic or wood. Since poor conductors of heat do not allow heat to pass through them quickly, these parts do not get hot quickly.
💻 Let's Inquire
- SLS: Uses of Good and Poor Conductors of Heat
- SLS: Handling Hot Objects Safely
🌏 Science Around Us
Did you know that air is a poor conductor of heat?
Wool has more trapped air between its fibres than cotton. Therefore, a woollen sweater does not allow heat to flow from your body to the surroundings quickly and can keep you warmer than a cotton sweater.
Similarly, polystyrene boxes have plenty of air trapped in them. Therefore, they are poor conductors of heat as well. That is why polystyrene is used to make cooler boxes so that things kept inside them can remain cold for a longer time.
Key idea
Materials that allow heat to flow through them quickly are called good conductors of heat.Materials that do not allow heat to flow through them quickly are called poor conductors of heat.
Answer the quick check to finish this part.
Part 9 · Textbook pp. 150–153
What are the effects of heat gain or heat loss?
You have learnt that heat causes a change in temperature. There are other effects of heat on objects. Heat can also result in the expansion or contraction of objects and in changes in the state of matter.
💻 Let's Inquire
- AB: Activity 7.3
Expansion and contraction
Heat can cause objects to expand or contract. When an object gains heat, it will expand. The object will increase in volume.
Before being heated, the metal ball can pass through the metal ring.
The metal ball is heated by the flame.
After being heated, the metal ball cannot pass through the metal ring.
Be careful when handling hot objects.
Mr BalaWhen the metal ball is heated, it gains heat from the flame. Since the metal ball cannot pass through the metal ring only after being heated, the metal ball must have expanded. This shows that metal expands when heated.
When an object loses heat, it will contract. The object will decrease in volume.
The hot metal ball is placed in a cup of cold water.
The metal ball can pass through the metal ring again.
Mr BalaWhen placed in cold water, the metal ball loses heat to the cold water. Since the metal ball can go through the metal ring after it is placed into cold water, the heat lost by the ball must have caused it to contract.
When the test tube containing coloured water is placed in a container of hot water, the coloured water level in the glass tube rises.
Be careful when handling hot objects.
Mr BalaWhen the coloured water in the test tube is heated, the coloured water gains heat. When the coloured water gains heat, it expands.
When the source of heat is removed from the test tube, the water level in the glass tube slowly drops back to the initial level.
Mr BalaWhen the source of heat is removed, the hot coloured water loses heat to its surroundings. When water loses heat, it contracts.
When the flask with the balloon is placed on a tray of hot water, the balloon inflates. When the source of heat is removed, the balloon deflates.
Be careful when handling hot objects.
Mr BalaWhen the flask of air is heated, the air inside it gains heat. When the air gains heat, it expands. When the flask of air cools down, the air inside it loses heat. When the air loses heat, it contracts.
🌏 Science Around Us
Expansion and contraction of objects due to heat can be useful in our daily life. If you have a metal jar lid that is too tight to open, you simply need to heat up the lid using some hot water. The expansion of the metal lid will make it easier to open the lid.
However, expansion and contraction of objects due to heat can also cause problems in our daily life. Repeated expansion and contraction of a road can cause it to crack if there is no space for it to expand. To solve the problem, engineers have created gaps in the road to allow for expansion of the road on hot days.
💻 Let's Inquire
- SLS: Expansion and Contraction
Answer the quick check to finish this part.
Part 10 · Textbook p. 154
Changes in the state of matter
Heat can result in a change in the state of matter.
🧩 Let's Connect
In the theme of Cycles, you have learnt about three states of matter.
When left on the table, ice will gain heat and melt to become water. When heated on a stove, water will gain heat and eventually turn into steam.
On the other hand, when allowed to cool, steam will lose heat and change into water. When placed in a freezer, water will lose heat to the surroundings and eventually turn into ice.
Key idea
Heat can cause a change in temperature.Heat can cause objects to expand or contract. When an object gains heat, it will expand. When an object loses heat, it will contract.
Heat can cause a change in the state of matter.
💻 Let's Inquire
- AB: Activity 7.4
Answer the quick check to finish this part.
Part 11 · Textbook pp. 155–157
Let's Apply
These are the three questions from the end of the chapter. Choose an answer to see whether you are right and why.
Your score
0 / 3
Answer all three questions.
Part 12 · Textbook pp. 133, 158–159
What I Have Learnt
Chapter 6 · Heat
- Temperature is a measure of how hot or cold something is.
- Temperature can be measured using a thermometer in degree Celsius (°C).
- Heat is a form of energy. It causes temperature to change. When an object loses heat, its temperature decreases. When an object gains heat, its temperature increases.
- Temperature is not the same as heat. Temperature is a property of an object, while heat gained or lost is the reason why the temperature of an object changes.
Chapter 7 · Effects of Heat
- Heat flows from an object or an area with a higher temperature to an object or an area with a lower temperature until they have the same temperature.
- An object that has a higher temperature than its surroundings is a source of heat.
- Materials that allow heat to flow through them quickly are called good conductors of heat. Materials that do not allow heat to flow through them quickly are called poor conductors of heat.
- Heat can cause:
a. a change in temperature.
b. objects to expand or contract. When an object gains heat, it will expand. When an object loses heat, it will contract.
c. a change in the state of matter.
In the topic of Heat, you have learnt:
Keep going!
Finish every part to complete the topic.