Density of materials: the different states of matter

The idea

The three states of matter are solid, liquid and gas. In a solid, particles are closely packed in a regular arrangement and vibrate about fixed positions, so the solid has a definite shape and volume. In a liquid, particles are close but randomly arranged and can slide past each other, so the liquid has a definite volume but takes the shape of its container. In a gas, particles are far apart, move quickly and randomly, and fill any container, so a gas has neither definite shape nor definite volume and is compressible. Heating transfers energy to the particles, increasing their kinetic energy; at melting and boiling points the arrangement changes. Density changes between states because the same mass occupies a different volume, as density = mass ÷ volume.

What earns the marks

• Names the three states of matter as solid, liquid and gas and links each to its particle arrangement.
• Describes a solid as having particles in a regular close-packed arrangement that vibrate about fixed positions, giving fixed shape and volume.
• Describes a liquid as having particles close together but able to move and slide, giving fixed volume but variable shape.
• Describes a gas as having particles far apart moving rapidly and randomly, giving no fixed shape or volume and allowing compression.
• Explains changes of state such as melting, boiling, condensing and freezing in terms of energy transfer and changes in particle motion and spacing.
• Relates the states to density by stating that density = mass ÷ volume and that the same mass occupies a larger volume in the gas state.

Common mistakes and how to put them right

• Thinking that particles in a liquid are far apart like a gas.

  Put it right by stating that liquid particles remain close together but are not in fixed positions.

• Believing that heating destroys particles or changes their size.

  Put it right by explaining that heating increases particle kinetic energy and spacing, not particle size.

• Assuming that a gas has no mass because it is invisible.

  Put it right by stating that gases have mass and exert pressure through particle collisions.

Exam tips

• Draw or describe the particle arrangement for each state, as diagrams often gain credit alongside written explanations.
• Use the terms 'definite shape' and 'definite volume' precisely, because these distinguish solids, liquids and gases in mark schemes.
• When explaining a change of state, name the process and state whether energy is transferred in or out.