Describe changes of state in terms of melting, boiling, evaporating, freezing and condensing

In this lesson we’ll explore how matter changes from one state to another. Think of it like a superhero changing costumes – each state has its own powers and rules.

When a solid receives enough heat, its molecules vibrate faster until the bonds that keep them in place break. The solid turns into a liquid. The temperature at which this happens is called the melting point ($T_m$).

• Ice melting into water – like a frozen snowman turning into a puddle.
• Metal melting in a forge – like a steel sword becoming liquid.

Boiling occurs when a liquid’s vapour pressure equals the external pressure. The liquid turns into gas throughout the bulk, not just at the surface. The temperature at which this happens is the boiling point ($T_b$).

• Water boiling in a kettle – like a pot of soup turning into steam.
• Alcohol evaporating in a laboratory – like a clear cloud forming over a beaker.

Evaporation happens at the surface of a liquid when individual molecules gain enough energy to escape into the air. It’s slower than boiling and can occur at any temperature.

• Water drying on a towel – like a sponge slowly losing water.
• Sweat evaporating from skin – like a cool mist forming on a hot day.

When a liquid loses heat, its molecules slow down and arrange into a regular, rigid structure, becoming a solid. The temperature at which this occurs is the freezing point ($T_f$).

• Water turning into ice – like a puddle turning into a frozen pond.
• Chocolate solidifying – like a sweet turning into a hard candy.

Condensation is the reverse of evaporation. A gas loses energy and its molecules come together to form a liquid. It often happens when warm, moist air meets a cooler surface.

• Water droplets on a cold glass – like a cold drink getting a film of water.
• Fog forming in the morning – like a cloud hugging the ground.

1️⃣ Define each process clearly. Use the correct terminology: melting, boiling, evaporation, freezing, condensing.

2️⃣ Relate to the phase diagram. Know how temperature and pressure affect each transition.

3️⃣ Use diagrams. A simple sketch of molecules can illustrate the change.

4️⃣ Give real‑world examples. They help you remember and show understanding.

5️⃣ Remember the equations. For example, $Q = mL_f$ for melting/freezing, $Q = mL_v$ for boiling/condensing.