Solids, Liquids and Gases: Changes of State

The three common states of matter are solids, liquids, and gases. A change in state occurs when matter changes from one state to another due to heat or pressure.
Solid to liquid transition is called melting. The temperature at which a substance melts is known as its melting point. At this point the solid’s particles gain energy to overcome the forces holding them together.
Liquid to gas transition is called vaporisation or evaporation. The temperature at which a substance changes from a liquid state to a gas state is known as its boiling point. Here the liquid’s particles gain enough energy to entirely overcome the forces of attraction between them.
Going from a solid state directly to a gaseous state (skipping the liquid state) is known as sublimation. Examples of substances that exhibit this behaviour include dry ice (solid CO2) and iodine.
The reverse processes also occur. Gas to liquid transition is known as condensation. During condensation, gas particles lose energy and start coming closer together, forming a liquid.
Similarly, liquid to solid change is called freezing or solidification. A substance’s freezing point is the temperature at which it changes from a liquid to a solid state.
A gas to solid transition (without passing through a liquid state) is known as deposition. A substance’s deposition point is at the temperature and pressure where the substance will change from a gas phase to a solid phase.
Changes of state involve energy. Energy is required to overcome the forces of attraction between particles in the process of drawing them apart (melting and evaporation). Releasing this energy (during freezing or condensation) causes the particles to come back together.
Note that a substance’s melting and freezing points are the same, as well as its boiling and condensation points. It’s only how we look at the process (heat is either absorbed or released) that makes the difference.
During a change in state, the temperature of the substance remains constant. This is because the thermal energy is spent in overcoming the intermolecular forces rather than increasing the kinetic energy.
It’s important to remember that not all substances act the same when undergoing changes of state. The points at which different substances will change states depends on the strengths of the forces between their particles.