What causes water to change state at different temperatures?

Water changes state because of changes in the movement (kinetic energy) of its particles, which depends on temperature.

• Heating: Adds energy, particles move faster, break free, and spread out (solid → liquid → gas).
• Cooling: Removes energy, particles slow down, move closer, and lock together (gas → liquid → solid).

Heating (Melting, Boiling)

• Ice to Liquid (Melting): When ice is heated, the particles vibrate faster. At 0°C, they gain enough energy to break free from their fixed spots. This allows them to slide past each other, turning into liquid water.
• Liquid to Gas (Boiling): If liquid water is heated more, the particles move much faster. At 100°C (under normal pressure), they have enough energy to completely escape the forces holding them together. They spread apart and become gas (steam).

Cooling (Freezing, Condensation)

• Gas to Liquid (Condensation): When steam cools, particles lose energy and slow down. As they get slower, the forces between them pull them together, turning gas back into liquid water.
• Liquid to Solid (Freezing): If water cools further, the particles slow even more. At 0°C, they no longer have enough energy to move freely. They lock into an ordered structure, forming solid ice.