When the temperature of a gas increases, the average kinetic energy of the gas particles also increases. Kinetic energy is directly proportional to temperature. This means the particles move faster and, consequently, possess more kinetic energy.

According to kinetic theory, gas particles are assumed to be in constant random motion and occupy a volume. As the particles move faster, they collide with the walls of the container more frequently and with greater force. This increased frequency and force of collisions effectively push the walls of the container outwards.

Therefore, the gas expands to occupy a larger volume to accommodate the faster-moving particles. This expansion is a direct consequence of the increased kinetic energy causing the particles to exert greater pressure on the container walls, leading to a greater overall volume. The volume increase is proportional to the increase in temperature, assuming pressure remains constant.

Solids: In a solid, particles are very closely packed together in a fixed, regular arrangement. This arrangement can be crystalline (repeating pattern) or amorphous (no repeating pattern). The separation between particles is very small. The particles in a solid vibrate about fixed positions, but they do not move past each other. Strong intermolecular forces (e.g., ionic bonds, covalent network, metallic bonds, or strong van der Waals forces) hold the particles in place. This strong attraction is what gives solids their definite shape and volume.

Liquids: In a liquid, particles are still close together, but they are not in a fixed arrangement. The particles are randomly arranged and can move past each other. The separation between particles is greater than in a solid. The particles in a liquid vibrate and slide past each other. Intermolecular forces are weaker than in solids (e.g., van der Waals forces, hydrogen bonding). This allows liquids to flow and take the shape of their container, but they still maintain a definite volume.

Gases: In a gas, particles are widely separated and randomly arranged. The separation between particles is large. The particles move randomly and rapidly in all directions. The particles in a gas have very weak intermolecular forces, allowing them to move freely and fill the entire volume of their container. Gases have neither a definite shape nor a definite volume.

Melting: Melting is the transition from a solid to a liquid state. In a solid, particles are arranged in a fixed, ordered lattice structure and vibrate about their fixed positions. When heat is added, the particles gain kinetic energy and vibrate more vigorously. At the melting point, the particles have enough energy to overcome the intermolecular forces holding them in the lattice, allowing them to move more freely and transition into a liquid. The energy required to melt a substance is called the latent heat of fusion. The arrangement of particles becomes less ordered in the liquid state, with particles able to slide past each other.

Boiling: Boiling is the transition from a liquid to a gaseous state. In a liquid, particles are close together but can move past each other. When heat is added, the particles gain kinetic energy and move faster. At the boiling point, the particles have enough energy to overcome the intermolecular forces holding them in the liquid, allowing them to escape into the gaseous phase. The energy required to boil a substance is called the latent heat of vaporization. The arrangement of particles becomes disordered in the gas state, with particles moving randomly and independently.

Evaporating: Evaporation is the transition from a liquid to a gaseous state that occurs at temperatures below the boiling point. Some particles at the surface of the liquid gain enough kinetic energy to overcome the intermolecular forces and escape into the air. The rate of evaporation depends on factors such as temperature, surface area, and air circulation. The particles that evaporate have sufficient kinetic energy to overcome the intermolecular forces, even if the bulk of the liquid does not have enough energy to boil.

Freezing: Freezing is the transition from a liquid to a solid state. As heat is removed from a liquid, the particles lose kinetic energy and slow down. At the freezing point, the particles lose enough energy to overcome the intermolecular forces and become locked into a fixed, ordered lattice structure. The arrangement of particles becomes more ordered in the solid state, with particles vibrating about fixed positions.

Condensing: Condensing is the transition from a gaseous to a liquid state. As a gas loses energy, the particles lose kinetic energy and move slower. When the particles lose enough energy to overcome the intermolecular forces, they come closer together and form a liquid. The arrangement of particles becomes more ordered in the liquid state.