Name and describe the changes of state in terms of energy and particle movement, and explain diffusion using the particle model

What this dot point is asking

The syllabus wants you to name the changes of state, melting, freezing, boiling, condensing, and sublimation, and to describe each in terms of particle movement and energy. You also need to explain why the temperature stays constant during a change of state, and to explain diffusion, the spreading out of particles, using the particle model. The big idea is that heating gives particles energy, and changes of state happen when that energy overcomes the forces between particles.

The answer

The changes of state

Each change has a name and a direction:

• Melting: solid to liquid (on heating).
• Freezing (solidifying): liquid to solid (on cooling).
• Boiling (and evaporating): liquid to gas (on heating).
• Condensing: gas to liquid (on cooling).
• Sublimation: solid straight to gas without melting, as with dry ice.

On heating, particles gain energy and move more, and forces between them are overcome, so the substance moves from solid toward gas. On cooling, particles lose energy and move less, so it moves back toward solid.

Why temperature is constant during a change of state

When a solid is melting, or a liquid is boiling, the temperature stays the same even though heat is still being added. This is because the energy is used to overcome the forces between the particles (to break the fixed arrangement or to separate the particles completely), not to make them move faster. Since the average energy of the particles is not increasing, the temperature does not rise until the change is complete.

Diffusion

Diffusion is the spreading out of particles from where they are crowded together (high concentration) to where there are fewer of them (low concentration), because the particles are always moving randomly. It explains why a smell spreads across a room and why a coloured dye spreads through still water.

What makes diffusion faster

Diffusion is faster when:

• the temperature is higher, because particles move faster, and
• it happens in a gas rather than a liquid, because gas particles are far apart and move more freely.

Lighter particles also diffuse faster than heavier ones at the same temperature.

Examples in context

Example 1. Steam scalding worse than boiling water. Steam at $100\ ^\circ\text{C}$ causes worse burns than water at $100\ ^\circ\text{C}$ because, when steam condenses on the skin, it releases the extra energy that was used to turn the water into a gas. This is the same energy that was absorbed during boiling without raising the temperature, now given back during condensing.

Example 2. A tea bag colouring water. When a tea bag sits in still hot water, the colour spreads out on its own by diffusion as the dissolved particles move randomly from the crowded region near the bag to the rest of the cup. It happens faster in hot water than in cold because the particles move faster.

Try this

Q1. Name the change of state when a gas turns directly into a solid, and the reverse change when a solid turns directly into a gas. [2 marks]

• Cue. The solid-to-gas change is sublimation; both directions are described as sublimation in this syllabus (the gas-to-solid step is the reverse).

Q2. Explain why a puddle of water dries up faster on a warm day than a cold day. [2 marks]

• Cue. On a warm day the water particles have more energy and evaporate (escape to the gas state) more quickly, so the puddle dries faster.

Q3. Two gases are released at opposite ends of a tube. Explain why they eventually mix in the middle. [2 marks]

• Cue. The particles of each gas move randomly in all directions and diffuse, spreading from where they are crowded toward where there are fewer of them, so the gases meet and mix.