States of Matter: Solid, Liquid, Gas, Plasma, Phases

Three ways to move

Everything around you is made of particles that never stop moving. In a solid they vibrate in place, locked in position, so a solid keeps its shape. In a liquid they stay close but slide past each other, so a liquid keeps its volume yet takes the shape of its container. In a gas they are far apart and fly freely, so a gas fills any container it is given. Temperature measures how fast the particles move on average. Drag the temperature in the sandbox and watch the same particles switch from trembling to sliding to flying.

Water's two big numbers

At sea-level pressure, 101.325 kPa, water melts and freezes at 0 °C and boils at 100 °C. Notice something odd while ice melts: the temperature stays at 0 °C until all the ice is gone, because the heat goes into loosening the bonds between molecules rather than speeding them up. Melting 1 kg of ice takes about 334 kJ. Turning 1 kg of boiling water into steam takes about 2260 kJ, nearly seven times more, which is why steam burns are so serious. The picture follows a block of ice being heated and draws the temperature line with its flat steps.

Plasma

Heat a gas hot enough and its atoms start to lose electrons. The result is plasma: a gas of charged ions and free electrons that glows and conducts electricity. Lightning turns the air in its channel into plasma at about 30 000 K. The Sun is a ball of plasma, and so are the stars, which makes plasma the most common state of visible matter in the Universe. On Earth you meet it in neon signs and in the auroras near the poles. It is often called the fourth state of matter. In the picture, atoms lose electrons that fly off as sparks of light while the temperature climbs.

Pressure moves the boiling point

Water boils when its vapour pressure equals the pressure of the air above it. Take away some of that pressure and it boils earlier. On the summit of Mount Everest, where the air pressure is about a third of that at sea level, water boils at about 70 °C, which is hot enough to disappoint a tea drinker. Raise the pressure and the opposite happens. A pressure cooker seals steam inside until the pressure reaches about twice that of the air, so the water boils at about 120 °C and food cooks faster. On the phase diagram a dot marks the state of water for each temperature and pressure; move it and see which region it falls in.

The odd one: ice floats

Most liquids shrink as they freeze. Water does the opposite. Liquid water is densest at about 4 °C, and below that its molecules begin locking into an open hexagonal pattern held by hydrogen bonds. Ice therefore takes up about 9 % more space than the water it came from, with a density of 0.92 g/cm³ against almost 1.00 for liquid water, so ice floats. In winter a lake cools until its surface water reaches 4 °C, the densest water sinks, and ice forms on top like a lid. Under it the water stays liquid and fish survive. In the sandbox, cool the water from 10 °C towards 0 °C and watch the particles open up into a lattice.

States of matter