Changes of State

States of Matter

Matter exists in three main states: solid, liquid, and gas. Each state has distinct properties based on how its particles are arranged and move.

• Solids have particles packed closely together in a regular pattern. The particles vibrate but do not move from their fixed positions. Solids have a definite shape and volume.
• Liquids have particles that are close together but not in a fixed pattern. They can move past each other, allowing liquids to flow. Liquids have a definite volume but take the shape of their container.
• Gases have particles that are far apart and move randomly at high speeds. Gases have neither a definite shape nor a definite volume and will expand to fill their container.

For example, ice is a solid with a fixed shape, water is a liquid that takes the shape of its container, and steam is a gas that spreads out to fill any space.

Changes of State

Matter can change from one state to another by heating or cooling. These changes are called changes of state and include:

• Melting: solid to liquid
• Freezing: liquid to solid
• Boiling: liquid to gas (occurs throughout the liquid at boiling point)
• Condensation: gas to liquid
• Sublimation: solid to gas without passing through the liquid state (e.g., dry ice)

Each change of state happens at a specific temperature called the melting point or boiling point, depending on the change.

For example, water melts at 06C and boils at 1006C at normal atmospheric pressure.

Particle Model and Energy

The particle model explains changes of state by considering the energy of particles and the forces between them.

• When a substance is heated, its particles gain energy and move faster.
• During melting and boiling, particles gain enough energy to overcome the forces holding them together (intermolecular forces between particles).
• During freezing and condensation, particles lose energy, slow down, and form forces, changing to a more ordered state.

For example, when ice melts, particles gain energy, vibrate more, and break free from fixed positions to become liquid water.

During boiling, particles gain enough energy to break free from the liquid and become gas, moving freely and rapidly.

Example: When ice at 06C melts to water at 06C, the temperature stays constant during the melting because the energy is used to break forces, not to raise temperature.

Latent Heat

During a change of state, energy is absorbed or released without changing the temperature. This energy is called latent heat. For melting and boiling, energy is used to break intermolecular forces between particles. For freezing and condensation, energy is released as these forces form.

Each substance has a specific amount of energy required for these changes, called the specific latent heat.

Practical Investigations

You can observe and measure changes of state in the lab by heating or cooling substances and recording temperature changes.

• Heat a solid and record its temperature at regular intervals as it melts.
• Heat a liquid and record temperature as it boils.
• Plot a graph of temperature against time to identify flat sections where the substance changes state.

The flat sections on the graph show where the temperature remains constant during melting or boiling, indicating a change of state.

For example, when heating ice, the temperature rises until 06C, then stays constant while the ice melts, then rises again once fully melted.