Identifying states of matter comes down to how matter behaves in terms of shape, volume, and how its particles are arranged and move. Solid objects keep a definite shape and a definite volume because their particles are tightly packed and mainly vibrate in place. Liquids have a definite volume but take the shape of their container, since their particles can slide past one another and flow. Gases have neither a definite shape nor a definite volume because their particles are far apart and move freely, filling both the space they occupy and the space around them. Other properties like color, density, or melting point aren’t universal indicators of state, since they depend on the material and conditions rather than the state itself. Similarly, mass, texture, chemical reactivity, size, weight, or odor don’t reliably reveal whether matter is a solid, liquid, or gas.

During condensation, gas molecules must slow down enough to be held together by intermolecular forces as they form a liquid. Kinetic energy is tied to how fast the particles move, so as they slow, the average kinetic energy decreases. The energy released when these attractions form leaves the system as heat to the surroundings, so the gas cools while it condenses. The molecules don’t stop moving entirely—they just have less kinetic energy than in the gaseous state.

Fusion is the melting process: a solid absorbs heat and becomes a liquid. At the melting point, the particles gain enough energy to overcome some of the forces holding them in a rigid, fixed arrangement, so they can move past each other and form a liquid. A familiar example is ice turning into water as it warms. So, the phase change where a solid becomes a liquid is fusion. Other transitions involve different changes: liquid turning into gas is vaporization, solid turning into gas is sublimation, and gas turning into solid is deposition (the reverse of sublimation).

When a solvent evaporates, the amount of solute stays the same while the solution’s volume decreases. Concentration is the amount of solute per unit volume, so with less solvent and the same solute, that ratio goes up. So the solute concentration increases as evaporation continues. If the solution becomes saturated, some solute might start to precipitate, but the immediate effect of evaporation is an increasing concentration.