Know the distinguishing properties of solids, liquids and gases

2.1.1 State of Matter – Solids, Liquids and Gases

Learning objective

Students will be able to identify and explain the distinguishing properties of solids, liquids and gases as required by Cambridge IGCSE 0625 (2.1 Kinetic particle model of matter – States of matter).

Key terminology (first appearance highlighted)

• State of matter: the form in which a substance exists – solid, liquid or gas.
• Particle: the smallest constituent (atom, molecule or ion) that makes up matter.
• Definite shape: a shape that does not change when the substance is transferred to a different container.
• Definite volume: a volume that does not change when the substance is transferred to a different container.
• Compressibility: the ability of a substance to decrease in volume when pressure is applied.
• Diffusion: the spreading of particles from an area of higher concentration to an area of lower concentration.
• Brownian motion: the random, jittery movement of tiny particles suspended in a liquid or gas, providing experimental evidence for the kinetic particle model.
• Absolute zero: the lowest possible temperature (‑273 °C or 0 K) at which the average kinetic energy of particles is minimal.
• Pressure (of a gas): the result of countless particle collisions with the walls of a container.

Syllabus checklist – distinguishing properties

Why temperature matters – kinetic‑energy link

In the kinetic particle model, temperature is a measure of the average kinetic energy of the particles. For an ideal gas:

\(\displaystyle \frac12 m\overline{v^{2}} = \frac32 k_{B}T\)

where m is the particle mass, \(\overline{v^{2}}\) the mean square speed, \(k_{B}\) the Boltzmann constant and T the absolute temperature (K). As T approaches 0 K (absolute zero) the average kinetic energy approaches zero and particle motion becomes minimal.

Brownian motion – experimental evidence

When microscopic solid particles (e.g., dust) are suspended in a liquid, they are observed to move erratically. This Brownian motion results from incessant collisions with the rapidly moving liquid molecules and confirms that particles are always in motion, even in the liquid state.

Pressure of a gas – particle‑collision explanation

Gas pressure arises from the continual impact of moving particles on the walls of their container. The more frequent and more energetic the collisions (i.e., the higher the temperature or the greater the number of particles), the higher the pressure.

Particle‑diagram illustration

Properties of each state (expanded)

Solids

• Shape: Definite – an ice cube keeps its shape.
• Volume: Definite – the cube occupies a fixed volume.
• Particle arrangement: Ordered lattice; each particle has a fixed position.
• Particle motion: Vibrations about fixed points.
• Compressibility: Very low – a metal block cannot be noticeably compressed.
• Diffusion: Negligible – a marble does not spread out on its own.
• Density: Generally high; note the exception that ice is less dense than liquid water (important exam trap).

Liquids

• Shape: Takes the shape of the container – water fills a glass.
• Volume: Definite – the same amount of water occupies the same volume in any container.
• Particle arrangement: Disordered but close‑packed; no long‑range order.
• Particle motion: Translational (sliding past one another) and rotational.
• Compressibility: Low – a bottle of oil can be squeezed only slightly.
• Diffusion: Moderate – a drop of ink spreads slowly through water.
• Density: Usually lower than the corresponding solid but higher than the gas.

Gases

• Shape: Takes the shape of the container – air inside a balloon.
• Volume: Takes the volume of the container – helium expands to fill a large balloon.
• Particle arrangement: Very dispersed; collisions between particles are infrequent.
• Particle motion: Random, high‑speed translational motion in all directions.
• Compressibility: High – a gas can be compressed into a small cylinder (e.g., a scuba‑tank).
• Diffusion: Rapid – perfume spreads quickly across a room.
• Density: Very low compared with solids and liquids (air ≈ 1 kg m⁻³, water ≈ 1000 kg m⁻³).

Comparison table (quick reference)

Ideal‑gas equation – where it belongs in the syllabus

The equation \(pV = nRT\) links pressure, volume, temperature and amount of substance. It is introduced in 2.1.3 Gases and the absolute scale of temperature. In the context of 2.1.1 it may be mentioned as “extended content” to show how the kinetic model leads to a quantitative relationship.