Changes of State

Definition of Changes of State

Changes of state are physical changes where a substance transforms between solid, liquid, and gas forms without changing its chemical identity. These changes are reversible, meaning the substance can return to its original state by reversing the process.

The three main states of matter involved are:

• Solid: Particles are tightly packed in a fixed, orderly arrangement.
• Liquid: Particles are close together but can move past each other.
• Gas: Particles are far apart and move freely.

Because these are physical changes, no new substances are formed, and the process can be reversed by changing temperature or pressure.

Processes of State Changes

The main processes involved in changes of state are:

• Melting: Solid to liquid. Occurs when a solid is heated to its melting point.
• Freezing: Liquid to solid. Occurs when a liquid is cooled to its freezing point.
• Boiling: Liquid to gas. Happens when a liquid is heated to its boiling point and bubbles of gas form throughout the liquid.
• Condensation: Gas to liquid. Occurs when a gas is cooled below its boiling point.
• Sublimation: Solid to gas without passing through the liquid state (e.g., dry ice).
• Deposition: Gas to solid without passing through the liquid state (reverse of sublimation).

Each of these processes involves a change in the arrangement and movement of particles, but the substance remains chemically the same.

Energy and Changes of State

Energy is either absorbed or released during changes of state:

• Energy is absorbed when a substance changes from solid to liquid or liquid to gas (melting and boiling). This energy breaks the forces holding particles together.
• Energy is released when a substance changes from gas to liquid or liquid to solid (condensation and freezing). This energy is given out as particles form bonds.

During a change of state, the temperature of the substance remains constant even though energy is being transferred. This is because the energy goes into changing the state, not the temperature.

Latent heat is the amount of energy needed to change the state of 1 kilogram of a substance without changing its temperature.

There are two types of latent heat:

• Latent heat of fusion: Energy required to change 1 kg of a solid to liquid or vice versa.
• Latent heat of vaporisation: Energy required to change 1 kg of a liquid to gas or vice versa.

The formula to calculate energy transferred during a change of state is:

$$E = m \times L$$

where:

• $E$ = energy transferred (joules, J)
• $m$ = mass of the substance (kilograms, kg)
• $L$ = specific latent heat (J/kg)

For instance, if 2 kg of ice melts, and the latent heat of fusion for ice is 334,000 J/kg, the energy required is:

$$E = 2 \times 334,000 = 668,000 \text{ J}$$

Particle Arrangement and Movement

Changes of state involve changes in how particles are arranged and how they move:

• In solids: Particles are closely packed in a regular pattern and vibrate in fixed positions. The forces between particles are strong, holding them tightly together.
• In liquids: Particles are still close but arranged randomly. They move around each other, allowing liquids to flow. The forces between particles are weaker than in solids.
• In gases: Particles are far apart and move quickly in all directions. The forces between particles are very weak or negligible.

When a substance changes state, the forces between particles either break or form:

• Melting and boiling break forces between particles.
• Freezing and condensation form forces between particles.

Particles move faster as energy is added, increasing temperature until the change of state occurs.

For example, when heating ice, particles vibrate faster until they have enough energy to break free from their fixed positions, turning into liquid water.

Quick calculation example: Calculate the energy needed to melt 1 kg of ice if the latent heat of fusion is 334,000 J/kg.

Using $E = m \times L$: $E = 1 \times 334,000 = 334,000 \text{ J}$