Solid

A solid is a state of matter in which the particles (atoms, ions or molecules) are held tightly together in fixed positions. A solid has a definite shape and a definite volume. You can pick it up and carry it. It does not pour like a liquid or float away like a gas. From the pencil in your hand to the mountains beneath your feet, solids are the framework of the everyday world.

  • ShapeFixedStays the same no matter the container
  • VolumeFixedDoes not change with pressure
  • ParticlesPacked closelyVibrate in place but do not move past each other
  • Two main kindsCrystalline + amorphousPatterned or jumbled
  • Hardest natural solidDiamondA type of carbon
  • Densest natural solidOsmium22.6 g/cm3

What makes a solid solid?

In a solid, the particles are arranged closely together and held in fixed positions by strong attractive forces. They still vibrate (in place), but they cannot wander around the way particles in a liquid or gas can.

This locked-in arrangement is what gives solids their fixed shape and resistance to squashing. A solid block of steel does not collapse under its own weight or change shape if you blow on it.

Two main kinds of solid

Solids come in two main types, based on how their particles are arranged:

  • Crystalline: particles arranged in a repeating 3D pattern called a lattice. Examples: salt, sugar, ice, diamond, quartz, most metals.
  • Amorphous: particles jumbled with no long-range pattern. Examples: glass, plastic, wax, rubber, charcoal.

You can often see the difference. Crystalline solids tend to break along clean flat surfaces (think of how a salt crystal cracks). Amorphous solids tend to shatter into curved or irregular shapes (like glass).

Fact Glass is technically an amorphous solid, but it can be described as a very stiff liquid. Its molecules are jumbled like a liquid but barely move because the glass is so cold (in chemical terms). Some museum glass has thicker bottoms than tops, but this is because old glass was hand-made unevenly, not (as you might have heard) because it is slowly flowing downwards. Glass at room temperature is far too stiff to flow visibly even after centuries.

How solids form

Solids form when a liquid (or sometimes a gas) cools enough that the particles attractions overcome their motion. The particles "lock together" into the solid state. This is called freezing or solidifying.

How fast the cooling happens affects the structure of the solid:

  • Slow cooling lets particles settle into neat lattice patterns: large crystals form. Beautiful gem crystals are the result of very slow cooling deep underground.
  • Fast cooling traps particles before they can find their best positions: glass and other amorphous solids form. Lava cooled quickly under water becomes glassy black obsidian; the same lava cooled slowly forms crystals of granite.

Properties of solids

  • Hardness: how resistant a solid is to being scratched. Diamond is the hardest natural substance; talc is one of the softest.
  • Density: mass per unit volume. Osmium is the densest natural solid at 22.6 g/cm3; some aerogels are less than 1 per cent the density of water.
  • Melting point: the temperature at which the solid turns to liquid. Tungsten melts at 3,422 C; helium does not freeze at all under normal pressure.
  • Strength: how much force the solid can take before breaking.
  • Elasticity: whether the solid returns to shape after being squashed or stretched. Rubber bands are highly elastic; clay is not.
  • Conductivity: how well it conducts heat and electricity. Copper and silver are excellent conductors; wood and plastic are insulators.
Did you know? If you held a thumbnail-sized piece of neutron-star matter, it would weigh as much as Mount Everest. Inside a neutron star, atoms have been squashed so hard by gravity that protons and electrons have merged into pure neutrons, packed almost shoulder-to-shoulder. A teaspoon of the stuff would weigh a few billion tonnes on Earth.

Different types of solid materials

  • Metals: shiny, conduct electricity and heat, can be hammered into shape. Iron, copper, gold, aluminium.
  • Ceramics: hard, brittle, do not conduct electricity. Pottery, bricks, china, glass.
  • Polymers: long molecules linked in chains. Plastics, rubber, DNA, proteins.
  • Composites: mixtures of different solids that combine their properties. Concrete (cement + gravel), carbon fibre (carbon fibres + plastic), wood (cellulose fibres + lignin).
  • Crystals: highly ordered solids. Salt, sugar, diamond, quartz, snowflakes.

Solids change state when heated

Heat a solid enough and the particles start vibrating so hard that they break free of their fixed positions. The solid melts into a liquid. Different solids melt at very different temperatures:

  • Mercury: melts at -39 C (it is the only metal that is liquid at room temperature)
  • Ice: melts at 0 C
  • Tin: melts at 232 C
  • Iron: melts at 1,538 C
  • Tungsten: melts at 3,422 C (used for old-fashioned light bulb filaments)

A few solids skip the liquid step and turn straight into gas when heated. This is called sublimation. Dry ice (frozen CO2), iodine crystals and naphthalene mothballs all sublime.

Try this Look at solids around you and decide whether they are crystalline or amorphous. Sugar grains under a magnifying glass have flat, geometric facets: crystalline. A piece of broken glass has curved, wavy edges: amorphous. A salt crystal is cubic: crystalline. A lump of chocolate, hardened wax, plastic and butter are all amorphous. Snowflakes are crystalline (always six-sided). Find one of each in your house.
Deeper dive: why some solids are stronger than others

The strength of a solid depends on the strength of the bonds between its particles. Different bond types give very different materials.

Metallic bonds are responsible for the strength and toughness of metals. The atoms share a "sea" of electrons that flows around the whole lattice. This lets metal atoms slide past each other without the bonds breaking, which is why metals can be hammered into shape (this is called malleability) and stretched into wires (ductility).

Covalent bonds in a continuous network make some of the hardest substances. Diamond is pure carbon arranged in a strong 3D network of covalent bonds. Quartz (silicon dioxide) is similar. These materials are very hard but brittle: they shatter rather than bend, because the network cannot slip without breaking many bonds at once.

Ionic bonds hold together solids like salt and many minerals. Ionic crystals are usually hard but brittle, easily shattered along certain planes (which is why salt crystals always crack into clean cubes).

Molecular solids (like sugar, ice, candle wax) are held together by weak attractions between whole molecules. They tend to be soft and have low melting points.

Modern materials science blends these ideas to design new solids with specific properties. Steel is iron with a tiny amount of carbon added: the carbon atoms wedge between iron atoms and stop them sliding, making steel much harder than pure iron. Carbon fibre is made of long, strong covalent chains aligned in one direction, then bonded together with plastic to make a lightweight composite. Engineers are now designing materials atom by atom to be tough, light or transparent in ways nature never tried.

For more, see liquid and phase changes.