Melting and Boiling Points

Melting and boiling points are the temperatures where a substance changes state. These changes happen at fixed temperatures if the pressure stays the same.

Key ideas

• Melting point: the temperature where a solid becomes a liquid.
• Boiling point: the temperature where a liquid becomes a gas, forming bubbles throughout the liquid.
• At the melting or boiling point, the temperature stays constant while the change of state happens, even if energy is being added.

Particle view (why temperature stays constant)

In a solid, particles are held in fixed positions by forces. Heating gives them energy. At the melting point, extra energy is used to loosen these forces so particles can move, not to raise temperature. In a liquid, heating makes particles move faster. At the boiling point, energy is used to separate particles into a gas, forming bubbles. Temperature stays the same until the change finishes.

Water at standard pressure

• Melting (freezing) point of water: $0\,^{\circ}\text{C}$
• Boiling (condensing) point of water: $100\,^{\circ}\text{C}$
• These values apply at standard atmospheric pressure (about 1 atm, sea level).

Boiling versus evaporation

• Boiling: happens at a fixed temperature (the boiling point), bubbles form throughout the liquid.
• Evaporation: happens at any temperature, only from the surface. Faster when temperature is higher, the surface area is larger, or air moves over the surface.
• Evaporation causes cooling because the more energetic particles escape first.

Heating curve (no graph needed)

If you heat ice, the temperature rises to $0\,^{\circ}\text{C}$, stays flat while melting, rises again as liquid water warms, then stays flat at $100\,^{\circ}\text{C}$ while boiling. Flat sections mean energy is changing state, not temperature.

Pressure and impurities

• Lower air pressure lowers the boiling point (water boils below $100\,^{\circ}\text{C}$ on a mountain).
• Impurities (like salt) usually lower the melting point and raise the boiling point.

Reverse changes

• Freezing happens at the same temperature as melting.
• Condensation happens at the same temperature as boiling.
• These release energy to the surroundings.