1.4 Can Matter Change its State?

Melting & Fusion

When solids are heated, particles gain kinetic energy and vibrate faster. At the melting point, they overcome intermolecular forces and become liquid.

Melting Point of Ice = 273.15 K (0°C)

Latent Heat of Fusion: Energy required to change 1 kg of solid to liquid at melting point without temperature change.

Boiling & Vaporization

Liquid particles gain enough energy to break free from intermolecular forces completely, becoming gas. This happens throughout the liquid (bulk phenomenon).

Boiling Point of Water = 373 K (100°C)

Latent Heat of Vaporization: Energy required to change 1 kg of liquid to gas at boiling point.

Activity 1.12: Observing State Changes

Take 150g of ice in a beaker with a thermometer touching the ice
Heat the beaker on a low flame
Note the temperature when ice starts melting (273 K)
Observe that temperature remains constant during melting
Continue heating water until it starts boiling
Note the boiling temperature (373 K)
Temperature stays constant during boiling despite continuous heating

Key Concepts

Matter exists in three states: solid, liquid, and gas
Temperature remains constant during state changes (latent heat)
Particles at 0°C water have more energy than ice at 0°C
Steam at 100°C has more energy than water at 100°C
Sublimation: Direct change from solid to gas (e.g., dry ice, camphor)
Pressure affects state changes - high pressure can liquefy gases

Sublimation

Some substances change directly from solid to gas without becoming liquid:

Camphor sublimes at room temperature
Dry ice (solid CO₂) sublimes at 1 atm pressure
Iodine crystals sublime when heated gently

Effect of Pressure

Pressure can force particles closer together, changing states: