PSLE Science · Primary 5 and 6
Heat and Temperature
Heat is a form of energy, and this topic belongs to the Energy theme in PSLE Science. You need to know the difference between heat and temperature, which way heat flows, what happens to objects when they gain or lose heat, and which materials are good or poor conductors of heat. Many PSLE questions describe an everyday situation, such as a hot drink cooling or a jar lid that will not open, and ask you to explain it. The words you use matter, so this guide shows the phrasing that earns marks.
What the syllabus expects
- Distinguish between heat and temperature, and measure temperature with a thermometer in degrees Celsius
- Explain that heat flows from a hotter place or object to a colder one until both are at the same temperature
- Describe the effects of heat gain and heat loss, including expansion, contraction and changes of state
- Classify materials as good or poor conductors of heat and relate this to their uses
- Explain the results of fair tests on how well materials conduct heat
Heat and temperature are different
Heat is a form of energy. Temperature is a measure of how hot or cold something is. It is measured in degrees Celsius (°C) using a thermometer. Heat and temperature are linked but they are not the same. A bathtub of water at 40 °C contains more heat than a cup of water at 40 °C, because there is much more water, even though the temperature is the same. The main source of heat on Earth is the Sun. When an object gains heat, its temperature usually rises, and when it loses heat, its temperature usually falls. A laboratory thermometer contains a liquid that expands and rises up its narrow tube when it gains heat. Some useful values: ice melts at 0 °C, pure water boils at 100 °C and normal body temperature is about 37 °C.
Heat flows from hotter to colder
Heat always flows from a hotter place or object to a colder one. It keeps flowing until both are at the same temperature. When you hold an ice cube, heat flows from your hand to the ice. Your hand loses heat, so it feels cold, and the ice gains heat and melts. Cold does not flow into your hand. A cup of hot tea left on a table loses heat to the cooler air around it until it reaches room temperature. A cold drink from the fridge gains heat from the warmer air around it until it also reaches room temperature. The bigger the difference in temperature, the faster heat flows, which is why a hot drink cools quickly at first and more slowly as it gets close to room temperature.
Expansion, contraction and changes of state
Most solids, liquids and gases expand when they gain heat and contract when they lose heat. Gaps are left between sections of railway tracks so that the metal has room to expand on hot days without bending. A dented ping-pong ball can be fixed by placing it in hot water: the air inside gains heat and expands, pushing the dent out. A tight metal lid on a glass jar becomes easier to open after it is held under hot water, because the metal lid expands more than the glass. Gaining or losing heat can also change the state of matter. Ice gains heat and melts into water. Water gains heat and boils or evaporates into water vapour. Water vapour loses heat and condenses into water droplets, and water loses heat and freezes into ice.
Conductors and insulators of heat
A good conductor of heat allows heat to flow through it quickly. Metals such as copper, aluminium, iron and steel are good conductors of heat, so cooking pots are made of metal to let heat from the stove reach the food quickly. A poor conductor of heat, also called an insulator, allows heat to flow through it slowly. Plastic, wood, rubber, cloth, styrofoam and air are poor conductors. Pot handles are made of plastic or wood so that they do not become too hot to hold. Trapped air is a very poor conductor, which is why a styrofoam box, full of tiny pockets of air, keeps ice cream cold and keeps hot food warm. It slows down heat flow in both directions. A metal spoon feels colder than a wooden spoon in the same room because the metal conducts heat away from your hand faster.
Worked example
Jia Hui poured 200 ml of water at 80 °C into a metal cup and 200 ml of water at 80 °C into a plastic cup of the same size and shape. After 10 minutes, the water in the metal cup was at 50 °C and the water in the plastic cup was at 65 °C. The room temperature was 30 °C. (a) Calculate the drop in temperature in each cup. (b) Explain the difference. (c) Predict the temperature of the water in both cups after a few hours, and explain.
- For (a), subtract the final temperature from the starting temperature. Metal cup: 80 − 50 = 30 °C. Plastic cup: 80 − 65 = 15 °C.
- For (b), first say which way heat flows: the water is hotter than the surrounding air, so it loses heat to the surroundings through the cup.
- Metal is a good conductor of heat, so heat flowed through the metal cup quickly and the water lost heat faster.
- Plastic is a poor conductor of heat, so heat flowed through the plastic cup slowly and the water lost heat more slowly.
- For (c), heat keeps flowing from the water to the cooler air until they are at the same temperature, so both cups of water will end up at room temperature.
Answer: (a) The metal cup dropped 30 °C and the plastic cup dropped 15 °C. (b) Metal is a better conductor of heat than plastic, so the water in the metal cup lost heat to the surroundings faster. (c) Both will be at 30 °C, room temperature, because heat flows from the water to the air until they are at the same temperature.
Common mistakes
- Saying 'cold flows' into an object. Only heat flows. Something feels cold because heat is flowing out of your hand into it.
- Using heat and temperature as if they mean the same thing. Heat is a form of energy, while temperature tells you how hot or cold something is.
- Saying a styrofoam box 'keeps the cold in'. It is a poor conductor of heat, so it slows down heat flowing in from the warmer surroundings.
- Saying metal objects in a room are colder than wooden ones. They are at the same temperature. Metal only feels colder because it conducts heat away from your hand faster.
- Saying the temperature of water keeps rising while it boils. Boiling water stays at 100 °C while it gains heat and turns into water vapour.
Exam tips
- Use 'gains heat' and 'loses heat', and say where the heat flows from and to, such as 'heat flows from the hot water to the cooler air'.
- For conductor experiments, name what was changed (the material) and what was kept the same (the volume of water, the starting temperature, the size and shape of the container and the time).
- In expansion questions, say which material gains heat and expands, then link that to what you can see happening.
Practice questions
1. Easy: Why are small gaps left between sections of a railway track?
Show answer
On hot days the metal tracks gain heat and expand. The gaps give the metal room to expand, so the tracks do not bend out of shape.
2. Medium: Explain why cooking pots are made of metal but their handles are made of plastic.
Show answer
Metal is a good conductor of heat, so heat flows quickly from the stove through the pot to cook the food. Plastic is a poor conductor of heat, so heat flows through the handle slowly and it stays cool enough to hold safely.
3. Harder: A ping-pong ball with a dent, but no crack, is placed in hot water and the dent pops out. Explain why. Why would this not work if the ball were cracked?
Show answer
The air inside the ball gains heat from the hot water and expands, pushing the dented part of the ball outwards. If the ball were cracked, the expanding air would escape through the crack, so it would not push the dent out.
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