Solids, Liquids and Gases

All substances exist as matter, which is commonly found in one of three states: solids, liquids, or gases. The state of a substance is determined by the arrangement and energy of its constituent particles (atoms, molecules, or ions), as well as the strength of the forces between them. This topic explores the microscopic properties that define these states and the energy changes involved in transitioning between them.

### The Kinetic Molecular Theory

The Kinetic Molecular Theory is a fundamental model that explains the physical properties of matter. Its core principles are:

The internal energy of a substance is the total energy stored within it, defined as the sum of the random kinetic energies and potential energies of all its particles.

### Properties of the States of Matter

Solids: In a solid, particles are held together by strong intermolecular forces, packing them tightly in a fixed, often regular, three-dimensional arrangement known as a **crystal lattice**. The particles cannot move from their positions but instead **vibrate** about them. This fixed arrangement gives solids a **definite shape** and a **definite volume**. Heating a solid increases the amplitude of these vibrations.

Liquids: In a liquid, particles have more kinetic energy than in a solid. The intermolecular forces are weaker, allowing the particles to break free from the lattice and **slide past one another**. While they are still closely packed, their arrangement is random. This mobility means liquids have **no definite shape** and take the shape of their container. However, the forces are still strong enough to keep the particles together, giving liquids a **definite volume**.

Gases: Gas particles possess very high kinetic energy, which completely overcomes the weak intermolecular forces between them. Consequently, they are far apart and move rapidly and randomly in straight lines until they collide with other particles or the walls of their container. This explains why gases have **no definite shape** and **no definite volume**; they expand to fill any container they occupy. The cumulative effect of these collisions with the container walls results in **pressure**.

### Changes of State and Latent Heat

Adding or removing thermal energy can cause a substance to undergo a change of state. A key characteristic of these transitions is that they occur at a constant temperature.

* Melting: Solid to liquid.

* Freezing: Liquid to solid.

* Boiling/Vaporisation: Liquid to gas at a specific temperature (the boiling point).

* Evaporation: Liquid to gas at the surface, occurring at any temperature.

* Condensation: Gas to liquid.

* Sublimation: Solid directly to gas (e.g., dry ice).

During a change of state, the thermal energy supplied (or removed) does not change the particles' kinetic energy (temperature remains constant). Instead, it changes their potential energy by doing work to overcome (or be overcome by) intermolecular forces. This hidden energy is called latent heat.

* The specific latent heat of fusion (Lf) is the thermal energy required to change 1 kg of a substance from solid to liquid at its melting point. The formula is: Q = m Lf

* The specific latent heat of vaporisation (Lv) is the thermal energy required to change 1 kg of a substance from liquid to gas at its boiling point. The formula is: Q = m Lv

In these formulas, Q is the thermal energy transferred (Joules, J), m is the mass (kg), and L is the specific latent heat (J/kg).

### The Gas Laws

The behaviour of a fixed mass of an ideal gas is described by laws relating its pressure (p), volume (V), and absolute temperature (T). For these laws, temperature must be on the Kelvin scale (K), where K = °C + 273.

* Boyle's Law: At a constant temperature, the pressure of a gas is inversely proportional to its volume (p ∝ 1/V). Formula: p₁V₁ = p₂V₂

* Charles' Law: At a constant pressure, the volume of a gas is directly proportional to its absolute temperature (V ∝ T). Formula: V₁/T₁ = V₂/T₂

These can be combined into the Combined Gas Law, a powerful tool for calculations involving changes in a gas's state: (p₁V₁)/T₁ = (p₂V₂)/T₂.