States of matter

In the morning you scrape ice off the car window, at noon you drink water, and steam rises over the pasta pot. Three times the same substance, three completely different faces. How a substance switches between solid, liquid, and gas is revealed by the particle model.

Solid, liquid, gas

In the solid state the particles sit tightly packed in fixed places and only tremble: that's why ice keeps its shape. In the liquid state they still hold together but can slide past each other: water fits any glass. In the gaseous state the particles race freely and far apart through space: steam fills every corner. And most importantly: in all three states they are the same particles, only their motion and spacing change.

Melting and boiling

If you add heat, the particles jiggle ever more vigorously until they tear free from their places: the substance melts. For water this happens at 0 °C. Keep heating and the particles break away from each other entirely and the substance boils: for water at 100 °C, when the air pressure is normal, as it is down at sea level. On a mountain 3000 metres high the air pressure is lower, and there water already boils at about 90 °C. Cool things down and it all runs backwards: gas condenses into liquid, like on the fogged bathroom mirror, and liquid freezes into a solid, like the puddle on a frosty night.

Evaporating without boiling

Laundry dries even at 20 °C, although the water never boils. That's because not all particles are equally fast: the fastest ones at the surface manage the jump into the gas state even below 100 °C. This is called evaporation. It goes faster the warmer, windier, and drier the air is.