Matter Study Guide

When you heat a system, you are adding energy to it. This added energy can cause two main effects: either the temperature of the system will increase, or the system will undergo a change of state. Sometimes, both can happen in sequence.

Temperature increase occurs when the kinetic energy of the particles within the system increases. The particles vibrate or move faster, and this increased internal energy is observed as a rise in temperature. If you continue to supply heat, the temperature will keep rising until a specific point known as a melting point or boiling point.

At these specific points, instead of the temperature increasing further, the system undergoes a change of state. For example, a solid will melt into a liquid, or a liquid will boil into a gas. During a change of state, the energy supplied is used to overcome the forces holding the particles together, rather than increasing their kinetic energy. This means that the temperature remains constant during the melting and boiling processes, even though heat energy is continuously being added. This energy is referred to as latent heat. Once the entire substance has changed its state, any further heating will again cause its temperature to rise until the next change of state (e.g., boiling if it has just melted).

When substances change state, for example, from solid to liquid (melting) or liquid to gas (boiling), energy is either absorbed from the surroundings or released into them. During these phase changes, the temperature of the substance does not change, even though energy is being added or removed. This is because the supplied energy is not increasing the kinetic energy of the particles, but rather is used to overcome the forces of attraction between particles to change their arrangement. For example, during melting, energy is used to break down the rigid structure of a solid, allowing particles to move more freely as a liquid. Similarly, during boiling, energy is used to separate liquid particles into widely spaced gas particles. The amount of energy required for a change of state for a unit mass of a substance is called its 'specific latent heat'. We can calculate the total energy (E) involved in a change of state using the formula: E = m × L, where 'm' is the mass of the substance in kilograms (kg) and 'L' is the specific latent heat in joules per kilogram (J/kg). Specific latent heat has two forms: specific latent heat of fusion (Lᶠ) for melting/freezing, and specific latent heat of vaporisation (Lᶥ) for boiling/condensing. It is crucial to use the correct specific latent heat value for the specific phase change occurring.