1.4 Melting and freezing
Summary: Imagine ice cubes in your hand, like in Figure 1. What happens? Tags: science oxford-science student-book-7 chemistry Created: 2026-05-19 Last Updated: 2026-05-19
After this topic, you will be able to:
- describe what happens to the particles in a substance when it melts or freezes
- state the factor in the particle model that explains why different substances have different melting points.
Think back
- What is a substance in science?
- Give three properties of a substance in the solid state.
- How do particles move in the liquid state?
Key idea Melting is the change of state from a solid to a liquid. Freezing is the change of state from a liquid to a solid. Melting and freezing are reversible.
Key words melting, freezing, change of state, reversible, conserved, melting point
Imagine ice cubes in your hand, like in Figure 1**. What happens?**
Figure 1Ice melts on warm hands.
Changes of state
When a substance changes from the solid to liquid state, it melts. Melting is the change from solid to liquid. Freezing is the change from liquid to solid.
Melting and freezing are examples of Changes of State. Changes of state are reversible. For example, ice melts to make liquid water, but ice forms again if the water is cooled to 0°C.
A Name the two states involved in freezing.
Explaining melting and freezing
What happens when an ice cube melts? The surroundings (environment around an object) transfer Energy to the ice. This makes its particles vibrate (shake) faster. The particles move away from their places in the pattern (see Figure 2). The particles continue to move around. As more particles leave the pattern, more ice melts.
Figure 2Particles before and after melting.
When a liquid starts to freeze, its particles move more slowly as they transfer energy to their surroundings. The particles get into a pattern and vibrate on the spot. Eventually, all the liquid freezes.
No particles are added or removed when a substance melts or freezes. This means that its mass does not change. Scientists say that mass is conserved when a substance melts.
B Describe how particle movement changes when a substance melts.
What is a melting point?
The temperature at which a substance melts is its melting point. Melting points give us information about the states of substances at different temperatures.
- Below its melting point, a substance is in the solid state.
- Above its melting point, a substance may be in the liquid or gas state. You need more data to work out which.
Look at the data in Table 1. At room temperature (20°C), gallium and gold are below their melting points. At 20°C, gallium and gold are solid. Above 30°C, gallium is liquid. This is why gallium will melt on your hands (see Figure 3). Table 1 The melting points of different substances. Substance
Melting point in °C
gallium
30
gold
1064
oxygen
-219
water
0
Figure 3Gallium is solid at room temperature. It melts on warm hands.
At 20°C, oxygen is above its melting point, so it must be in the liquid or gas state. However, from the data in Table 1, you cannot tell which of these two states it is in.
Gold has a higher melting point than water. This means that the particles in solid gold hold together more strongly than the particles in solid water (ice).
C Write the names of the substances in Table 1 in order of increasing melting point, starting with oxygen.
Stretch zone
- Copy and complete the sentences. The change of state from solid to liquid is freezing/melting. As a substance melts, its particles vibrate slower/faster. The particles start moving around/upwards. The substance is now in the liquid/solid state. The melting point of a substance is the Speed/temperature it melts at.
- Figure 4 shows some particles of a substance in the solid state. Draw a diagram showing the same particles after the substance has melted. Figure 4
Stretch zone
- A substance has a melting point of-7°C. Yanek says the substance is liquid at 20°C. Beth says it could be liquid or gas. Explain who is correct.