States of Matter: Solids, Liquids and Gases Explained

The three states of matter are solid, liquid and gas, and the difference between them is how much energy their particles have. In a solid the particles are locked in a regular pattern and can only vibrate. In a liquid they still touch but slide past each other. In a gas they are far apart and fly about in straight lines. Heat a substance and its particles gain energy, until the forces holding them together can no longer keep them in place.

How do particles behave in solids, liquids and gases?

The particle model explains solids, liquids and gases with one idea: the same particles, with more or less energy, held together by more or less of the forces between them.

	Solid	Liquid	Gas
Arrangement	regular lattice, tightly packed	close together, no pattern	far apart, random
Movement	vibrate about fixed positions	slide past each other	fast, in straight lines
Forces between particles	strong	strong enough to keep them touching	almost none
Shape	fixed	takes the shape of the container	fills the whole container
Can it be squashed?	no	hardly	easily

That table explains everyday behaviour. A liquid can be poured because its particles can move past each other, but it can't be squashed because they already touch. A gas can be squashed because most of its volume is empty space. Air pressure is the push of billions of gas particles hitting a surface every second.

The particles themselves never change. An ice crystal, a drop of water and a puff of steam are all made of the same H₂O molecules, with the same size and the same mass. Only their energy and their arrangement are different.

What happens when you heat a solid?

Heating a solid gives its particles more kinetic energy, so they vibrate harder and the temperature rises. At the melting point they have enough energy to break free of their fixed positions, and the solid becomes a liquid. Heat the liquid and its particles move faster still, until at the boiling point they break away from each other completely and escape as a gas.

The names of the changes of state:

• Melting: solid → liquid
• Freezing: liquid → solid
• Boiling or evaporating: liquid → gas
• Condensing: gas → liquid
• Subliming: solid → gas, skipping the liquid. Dry ice (solid carbon dioxide) does this at −78.5 °C.

Evaporation and boiling both turn a liquid into a gas, but they aren't the same. Evaporation happens only at the surface, at any temperature, when the fastest particles escape: that's how a puddle dries. Boiling happens throughout the liquid, at one temperature, with bubbles of gas forming inside it.

What does a heating curve show?

A heating curve is a graph of temperature against time as a substance is heated steadily. For water starting as ice at −30 °C, it has five parts:

1. Ice warms up from −30 °C to 0 °C. The line slopes up.
2. Ice melts at 0 °C. The line is flat, even though heat is still going in.
3. Water warms up from 0 °C to 100 °C. The line slopes up again.
4. Water boils at 100 °C. The line is flat again, and for much longer.
5. Steam warms up above 100 °C.

The sloping parts follow Q = m c ΔT, where c is the specific heat capacity: the energy to warm 1 g by 1 °C. Liquid water's is 4.18 J/g°C, unusually high, which is why the sea warms and cools so slowly. Ice and steam both have about half that, so they warm up faster for the same heating, and their parts of the curve are steeper.

A cooling curve is the same thing run backwards, with flat parts at the boiling point (condensing) and the melting point (freezing).

Why does temperature stay the same when ice melts?

So why does temperature stay the same when ice melts, even with the heater on? Because the energy goes into breaking the forces between the particles, not into speeding them up. Temperature measures how fast the particles move. While ice is melting, every joule is spent pulling molecules out of the lattice, so their speed, and the temperature, stays the same until all the ice has melted.

That hidden energy is called latent heat, and it follows Q = m L:

• The latent heat of fusion of water is 334 J/g: the energy to melt 1 g of ice at 0 °C.
• The latent heat of vaporisation of water is 2260 J/g: the energy to boil 1 g of water at 100 °C.

Boiling takes almost seven times as much energy as melting. Melting only loosens the particles, which still touch; boiling pulls them completely apart. That's why the boiling step on the heating curve is so much longer than the melting step.

Latent heat works in reverse too. Steam at 100 °C scalds far worse than water at 100 °C, because as it condenses on your skin it gives out those 2260 J/g as well. And sweating cools you down because evaporating sweat takes its latent heat from your skin.

Melting and boiling points depend on the forces

Different substances melt and boil at very different temperatures, because the forces between their particles are different strengths.

Substance	Melting point	Boiling point
Oxygen	−218 °C	−183 °C
Ethanol	−114 °C	78 °C
Water	0 °C	100 °C
Sodium chloride	801 °C	1413 °C

Oxygen molecules barely attract each other, so oxygen is a gas at room temperature. Water molecules attract each other strongly (through hydrogen bonds), so water is a liquid. In sodium chloride the particles are ions held by strong attractions in every direction, so it stays solid until 801 °C: see ionic vs covalent bonds for why.

The boiling point also depends on pressure. At the top of Mount Everest the air pressure is about a third of sea level, and water boils at roughly 70 °C, too cool to cook an egg properly. A pressure cooker does the opposite: higher pressure raises the boiling point to about 120 °C, so food cooks faster.

Common mistakes

• "The particles get bigger when they melt." They stay the same size. Only the spaces between them and their movement change.
• "Particles in a solid don't move." They vibrate all the time, except at absolute zero, −273 °C.
• "Temperature keeps rising while it boils." It stays at the boiling point until all the liquid has gone.
• "Evaporation needs 100 °C." Water evaporates at any temperature; it boils at 100 °C.
• Using Q = mcΔT for a change of state. During melting or boiling ΔT is zero. Use Q = mL.

Frequently asked questions

What are the 3 states of matter?

The three states of matter are solid, liquid and gas. Each is the same particles with a different amount of energy: vibrating in a fixed lattice (solid), touching but sliding past each other (liquid), or far apart and moving fast (gas). A fourth state, plasma, forms at extremely high temperatures, when atoms lose their electrons. The Sun and lightning are plasmas.

Is the boiling point of water always 100 °C?

Only at normal sea-level air pressure. Lower pressure makes water boil at a lower temperature, about 70 °C on the top of Mount Everest. Higher pressure raises the boiling point, which is how a pressure cooker reaches about 120 °C. Dissolving salt in water also raises its boiling point slightly.

What is the difference between heat and temperature?

Temperature measures how fast the particles in something are moving on average, in °C or kelvin. Heat is energy transferred because of a temperature difference, in joules. A bath of warm water holds far more heat energy than a cup of boiling water, even though its temperature is lower, because it has so many more particles.

Try it yourself

Open States of Matter and turn the heater up to full. Watch 1 g of ice at −30 °C vibrate, melt from the outside in, slide around as a liquid and boil off the top as steam, while the heating curve draws itself with its two flat steps. Switch on Show forces between particles to see what holds each state together, then drag the heater below zero to run it all backwards. Try ethanol and oxygen too: the same shape of curve at very different temperatures. To see how heat changes chemical reactions rather than states, try Chemical Energetics.