states of matter
Water is the everyday teacher here: it shows up as ice you can hold, water you can pour, and steam you cannot grasp at all. These are the three familiar states of matter — solid, liquid, and gas — and the same substance can move between them simply by heating or cooling. Nothing new is added; the particles are just arranged and moving differently.
The states differ in how tightly the particles are bound and how freely they move. In a solid, particles are locked in place and merely vibrate, so it keeps its shape and volume. In a liquid, they stay close but can slide past one another, so it flows and takes the shape of its container while keeping its volume. In a gas, they fly apart and roam freely, so a gas fills whatever space it is given. A fourth state, plasma, appears at very high energies when atoms are torn into charged pieces.
Which state a substance is in depends on temperature and pressure, the constant tug-of-war between thermal motion pulling particles apart and attractive forces drawing them together. Understanding these states, and the transitions between them, is one of the first jobs of physical chemistry and the gateway to studying gases, liquids, and solids in their own right.
Leave an ice cube on the counter and it puddles into water, then dries into invisible vapour — one substance, H₂O, passing through all three states as the room warms it.
Same molecules, three states — only their motion and spacing change.
State and phase are closely related but not identical: state refers to the broad categories solid, liquid, gas, while phase is more precise, since a single solid substance can exist in several distinct crystalline phases.