How Matter Changes State: Melting, Boiling and More

Melting, freezing, boiling (vaporisation), condensation, and sublimation are processes where matter changes from one state to another, each involving either absorption or release of heat. Melting (fusion) is when a solid turns into a liquid by absorbing heat, such as ice melting into water. Freezing (solidification) is when a liquid changes to a solid by releasing heat, like water freezing into ice. Boiling (vaporisation) is when a liquid becomes a gas by absorbing heat, as when water boils to form steam. Condensation (liquefaction) is when a gas turns into a liquid by releasing heat, such as water vapour forming dew. Sublimation is when a solid changes directly into a gas without becoming liquid, absorbing heat in the process; for example, camphor or naphthalene balls disappearing over time. In each case, the direction of heat flow determines whether the process absorbs or releases energy. Students often confuse condensation with freezing, but condensation forms a liquid, not a solid.

Heating or cooling changes the state of matter by affecting the intermolecular space (distance between particles) and the intermolecular force of attraction (the force holding particles together). When a substance is heated, its particles gain energy, move faster, and the intermolecular space increases, weakening the force of attraction. This can cause a solid to melt into a liquid or a liquid to boil into a gas. Cooling removes energy, so particles move slower, come closer, and the force of attraction strengthens, leading to condensation (gas to liquid) or freezing (liquid to solid). For example, when butter is heated in a pan, it melts because the heat energy overcomes the forces holding its particles in a solid structure. On a cold morning, water vapour in the air loses heat, the particles slow down and come closer, forming dew drops by condensation. A common misconception is that only temperature changes matter’s state, but it is actually the change in energy affecting particle movement and attraction.

Heat affects matter in three main ways: it can cause a change of state, thermal expansion, or a chemical change. When heat is supplied, a substance may change its state, like ice melting to water (solid to liquid) or water boiling to steam (liquid to gas). Thermal expansion is when solids, liquids, or gases expand on heating because their particles move further apart. For example, railway tracks have small gaps to allow for expansion on hot days, and the mercury in a thermometer rises as it gets warmer. Chemical change occurs when heat causes a new substance to form, such as when wood burns to form ash and smoke. In real life, heating milk causes it to boil (change of state), a metal lid loosens on a jar when heated (thermal expansion), and cooking rice transforms it chemically. Some students think all changes caused by heat are reversible, but chemical changes like burning are not.

A change is reversible if the original substance can be recovered by reversing the process, and irreversible if it cannot. For example, melting of ice is reversible because the water can be frozen back into ice by cooling. Burning of paper is irreversible because the paper turns into ash and smoke, which cannot be changed back into paper. Boiling water to form steam is reversible, as steam can be cooled to become water again. Cooking an egg is irreversible, as the cooked egg cannot be returned to its raw state. The key is whether the change only affects the physical state (usually reversible) or creates a new substance (usually irreversible). Many students incorrectly classify all changes involving heat as irreversible, but only chemical changes like burning or cooking are truly irreversible.