Melting takes energy in while the temperature stays put

1. Warm a solid and its temperature climbs steadily, because the incoming energy makes the particles vibrate faster about their fixed positions.
2. At the melting point the climbing stops, and the thermometer holds the same reading even though the heater is still on.
3. The energy has not vanished, and it is not making the particles faster: it is being used to break the particles free of their fixed positions by working against the forces holding them there.
4. Because the average kinetic energy of the particles is unchanged, the temperature is unchanged throughout the melting.
5. On a graph of temperature against time this shows as a flat section, lasting until the last of the solid has gone.
6. Once everything is liquid the temperature starts rising again, because the energy can go back into speeding the particles up.
7. For water at standard atmospheric pressure the melting point is $0{}^{\circ }C$.

Boiling turns liquid into gas throughout the liquid

1. The same pattern repeats higher up the graph: the liquid warms steadily until it reaches its boiling point, then the temperature holds still.
2. This time the energy is being used to pull the particles completely apart, so that they escape the liquid entirely rather than merely sliding past each other.
3. Bubbles of gas form all through the liquid, including at the bottom of the pan, and rise to the surface, which is what makes boiling look so different from a liquid quietly drying out.
4. Boiling needs far more energy than melting the same mass, because separating the particles completely is a much bigger job than loosening them.
5. For water at standard atmospheric pressure the boiling point is $100{}^{\circ }C$, and the two fixed points of $0{}^{\circ }C$ and $100{}^{\circ }C$ are worth knowing without looking them up.

Condensation: gas particles slow down and are captured

1. Cool a gas and its particles lose kinetic energy, so they travel more slowly.
2. Slow-moving particles that come close together can no longer break away from one another, because the forces of attraction between them are now strong enough to hold them.
3. The particles gather into a much closer, disordered arrangement, and a liquid forms.
4. The energy the particles had to be given during boiling is released again into the surroundings, which is why steam scalds far worse than water at the same temperature.
5. Condensation happens most readily on a cold surface, because that is where the gas particles lose energy fastest.

Solidification: liquid particles lock into fixed positions

1. Cool a liquid and its particles slow down until they no longer have enough energy to slide past their neighbours.
2. The forces between the particles then pull them into a regular arrangement of fixed positions, and all they can do is vibrate.
3. Energy is given out to the surroundings as this happens, and the temperature stays at the melting point until the last of the liquid has solidified.
4. Melting and solidification happen at the same temperature for a pure substance, so ice melts at $0{}^{\circ }C$ and water solidifies at $0{}^{\circ }C$.