States of matter and phase changes

Every state of matter is a different dance of the same particles. In a solid, particles are packed tightly and vibrate in place in a fixed arrangement, which is why solids hold their shape. In a liquid, the particles still touch but slide past each other, so a liquid takes the shape of its container. In a gas, particles fly free with wide space between them, filling the whole container.

The deep idea is that the particles are never still. Even a block of ice at its coldest is a lattice of vibrating molecules. Heating gives the particles more energy: the vibrations grow until the crystal breaks apart, and the solid melts into a liquid. The particles do not get bigger, break apart or turn into something new. They simply move differently. Melting is a change of motion, not a change of material.

Phase changes are named by direction. Solid to liquid is melting; liquid to gas is evaporation or boiling; gas to liquid is condensation; liquid to solid is freezing; solid directly to gas is sublimation, like dry ice; and gas directly to solid is deposition, like frost. Each change is driven by energy entering or leaving the particles, and no new substance is created by any of them.

The steam above a kettle is an invisible gas; the white cloud you see is the tiny droplets formed as the vapor cools and condenses. Water boils at 100 degrees at sea level, but evaporation happens at any temperature: a puddle dries at 20 degrees because high-energy molecules escape from the liquid's surface. Boiling and evaporating both turn liquid to gas, one through the whole liquid and one at the surface, with the same particle story behind both.