States, Classes and Properties of Matter

Solid, liquid, and gas differ in how much particles move versus how strongly they hold onto each other. In a solid, attractions win: particles vibrate in fixed spots. In a liquid, particles have enough energy to slide past each other but still stay close. In a gas, particles move fast enough to break free of those attractions almost entirely, so they spread out and fill whatever space they are in. A gas is not just a liquid with extra room between the same slow particles — the particles themselves are moving far faster, and the wide spacing is the result of that speed, not the cause of the phase change.

A heating curve plots temperature against energy added. While a substance is melting or boiling, the temperature holds flat, because the added energy is going into breaking the bonds or attractions holding particles together, not into speeding the particles up. Once a phase change finishes, all the added energy goes back into raising temperature, so the graph climbs again. This is also why ice is unusual: water molecules form hydrogen bonds, and each molecule locks into place with four neighbors, building an open lattice that holds molecules farther apart than they are in the liquid. That makes solid water less dense than liquid water, so ice floats — the reverse of almost every other substance, where the solid sinks in its own liquid.

Density ties mass and volume together as a single number, and it connects directly to how submerged objects behave: an object denser than a fluid sinks, one less dense floats. Pushing pressure far above normal on a phase diagram raises the boiling point and can erase the usual boundary between liquid and gas, producing a supercritical fluid with properties of both.