1.5 Boiling

Summary: Close your eyes. Imagine water boiling in a pan. What can you hear? What can you see? Boiling is a change of state from liquid to gas. You can use the particle model to explain boiling. 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 boils
• predict the state of a substance at a given temperature from its melting and boiling points, where both are above 0°C
• give the boiling point of a substance from its temperature-time graph.

Think back

1. Give two properties of a substance in the gas state.
2. How are particles arranged in the gas state?
3. What is mass?

Key idea When a substance boils, it changes state from liquid to gas. Boiling happens when a liquid is heated to a certain temperature, called its boiling point.

Key words boiling, boiling point

Close your eyes. Imagine water boiling in a pan. What can you hear? What can you see? Boiling is a change of state from liquid to gas. You can use the particle model to explain boiling.

Explaining boiling

When water boils, bubbles of steam (water in the gas state) form all through the liquid. In the liquid, water particles touch their neighbours. But inside the bubbles, the water particles spread out (see Figure 1).

Figure 1Particles in boiling water (not to scale).

As water boils, the steam bubbles rise to the surface of the liquid. They escape into the air. All the particles still exist, so the total mass of steam and water is the same as the mass of water at the start. The total mass of the substance stays the same. Scientists say that mass is conserved in boiling.

A Use the particle model to explain why mass is conserved (stays the same) in boiling.

What is a boiling point?

Boiling happens if enough Energy is transferred (moved) to the particles. The particles in some substances hold together more strongly than in other substances. This means that different substances need different amounts of energy to boil. Therefore, different substances boil at different temperatures. The temperature a substance boils at is its boiling point.

Measuring a boiling point

You can measure the boiling point of a substance like this:

• Pour the liquid into a beaker (cup).
• Heat the liquid and measure the temperature every minute.
• Plot the results on a graph.

Kemal heated liquid water. He used data to plot a graph (see Figure 2). At first, the temperature increased. At 100°C, the water bubbled strongly. It was boiling. The temperature remained at 100°C. This is the boiling point of water.

Figure 2A temperature-time graph for heating water. Full text alternative to the content

B Explain how the graph shows that the boiling point of water is 100°C.

Using boiling points

You can use data about boiling points to help identify substances.

Lucy has a colourless liquid. Her teacher tells her it could be water, ethanol, or propanol. Lucy finds out the boiling points of these substances. She writes them in Table 1.

Lucy heats her liquid with an electric heater. She measures its temperature every minute. At 78°C, the liquid bubbles strongly. It remains at 78°C for several minutes. Lucy concludes that the liquid is ethanol. Table 1 A The boiling points of water, ethanol, and propanol. Substance

Boiling point in °C

water

100

ethanol

78

propanol

97

If you know the melting point and the boiling point of a substance, you can predict its state at different temperatures. Figure 3 shows that the melting point of silver is 961°C and the boiling point is 2162°C.

Figure 3The melting and boiling points of silver.

At room temperature (20°C), silver is in the solid state. At 961°C, its melting point, silver exists in both the solid and liquid states. Between 961°C and 2162°C, silver is liquid. At 2162°C, its boiling point, silver exists in both the liquid and gas states. Above 2162°C, silver exists in the gas state.

C Predict the state of silver at 1000 °C.

Stretch zone

1. Copy and complete the sentences. When a substance boils, it changes state from liquid/gas to liquid/gas. Bubbles form at the top of/all the way through the liquid. A certain substance boils at any/a certain temperature.
2. Answer the questions. Draw a particle diagram to show five particles of a substance in the liquid state.Draw the same five particles after the substance has boiled.

Stretch zone

1. Use the data to predict the state of oxygen at -200°C. Data for oxygen:melting point = −219°Cboiling point = −183°C