Solids, Liquids & Gases

States of Matter

Matter exists in three main states: solids, liquids, and gases. Each state has distinct properties based on the arrangement and movement of its particles.

• Solids have a fixed shape and a fixed volume. They do not flow and cannot be compressed easily.
• Liquids have a fixed volume but take the shape of their container. They can flow but are not easily compressed.
• Gases have neither a fixed shape nor a fixed volume. They expand to fill their container and can be compressed.

The differences in these properties arise from how particles are arranged and how they move:

• Solids: Particles are tightly packed in a regular pattern. They vibrate about fixed positions but do not move from place to place.
• Liquids: Particles are close together but arranged randomly. They move around each other, allowing liquids to flow.
• Gases: Particles are far apart and move quickly in all directions, colliding with each other and the container walls.

Particle Model

The particle model helps explain the behaviour of solids, liquids, and gases:

• Particles are in constant motion, though the type of motion depends on the state.
• There are spaces between particles; these spaces are smallest in solids and largest in gases.
• Forces of attraction between particles vary: strongest in solids, weaker in liquids, and weakest in gases.
• Energy affects particle movement: more energy means particles move faster and can overcome forces holding them together.

For example, when a solid is heated, its particles vibrate more vigorously until they can break free from their fixed positions and become a liquid.

The particle model does not explain energy changes in detail (covered in other topics), but it gives a useful way to visualise matter at the microscopic level.

Changes of State

Matter can change from one state to another through physical processes called changes of state:

• Melting: Solid to liquid
• Freezing: Liquid to solid
• Boiling: Liquid to gas (occurs throughout the liquid)
• Evaporation: Liquid to gas (occurs at the surface)
• Condensing: Gas to liquid
• Sublimation: Solid to gas without passing through liquid (e.g., dry ice)

During a change of state:

• Energy is either absorbed or released by the substance, known as latent heat.
• The temperature remains constant while the change of state occurs because the energy is used to break or form bonds between particles, not to raise temperature.
• The arrangement of particles changes: for example, in melting, particles move from a fixed, ordered structure to a more random, mobile arrangement.

For instance, when ice melts at 0°C, it absorbs energy to break the forces holding the solid structure, but its temperature stays at 0°C until all ice has melted.

This explains why heating a substance does not always increase its temperature immediately.

Example: If 50 g of ice at 0°C melts completely, the temperature stays at 0°C during melting, despite energy being absorbed.

Density and the Particle Model

Density is a measure of how much mass is contained in a given volume:

$$\text{Density} = \frac{\text{Mass}}{\text{Volume}}$$

The density of a substance depends on how closely its particles are packed:

• Solids generally have the highest density because their particles are tightly packed.
• Liquids have a lower density than solids but higher than gases.
• Gases have the lowest density because their particles are far apart.

For example, iron (a solid) is much denser than air (a gas) because iron’s particles are packed closely, while air particles are spread out.

Density can change when a substance changes state because the volume changes while the mass stays the same.

Example: When water freezes to ice, its density decreases because ice has a more open structure, so ice floats on water.

Examples of Calculations

Example: Calculate the density of a solid block with a mass of 240 g and a volume of 80 cm³.

Using the formula:

$$\text{Density} = \frac{\text{Mass}}{\text{Volume}} = \frac{240\, \text{g}}{80\, \text{cm}^3} = 3\, \text{g/cm}^3$$