Density and arrangement of particles in different states

What density means

density = mass ÷ volume

Mass is how much stuff there is. Volume is how much space it takes up. Density is the mass in each unit of volume, so the more closely packed the particles, the higher the density.

Think of it as stuff per box of space.

What you need to know

• Everything is made of tiny particles. How they are arranged and how they move depends on whether it is a solid, a liquid or a gas.
• In a solid, the particles are touching and sit in a regular pattern.
• In a liquid, the particles are still touching, but they are randomly arranged and move around and over each other.
• In a gas, the particles are far from each other, randomly arranged, and move very quickly in random directions.
• Have a goA classmate says a liquid's particles are 'touching, in a neat regular pattern, like soldiers on parade'. They are confidently half right. Which part is wrong?

  'In a neat regular pattern'. Liquid particles do touch, but they are randomly arranged.

  Touching is true for both solids and liquids. The regular pattern is what the solid has and the liquid has lost.

• The spaces between gas particles hold nothing at all. That is empty space.
• A gas can be compressed (made to take up less space) because of those spaces. A liquid cannot, because its particles already touch.
• Have a goFinish the sentence: a gas can be compressed because it has ______ between its particles, and a liquid can't because its particles are already ______.

  Spaces (empty space); touching.

  Compressing means making something take up less space, and that only works if there is empty space left to squeeze out.

• Density is the mass of a substance per unit volume: density = mass ÷ volume.
• The more closely packed the particles, the more mass in each unit of volume, so the higher the density.
• Have a goTwo boxes are exactly the same size. Box P holds 30 g of a substance and box Q holds 5 g of the same substance. Which box is more densely packed, and what does that say about its particles?

  Box P. More mass in the same volume means its particles are packed more closely.

  Volume is the same, so the box with more mass has more mass in each unit of volume, which is a higher density.

• Solids and liquids both have touching particles, so their densities are similar and high. A solid is usually a little denser than its liquid.
• A gas has its particles far apart, so the same mass takes up a much larger volume. Its density is much lower.

The big picture

Solids, liquids and gases are made of the same sort of tiny particles, arranged and moving in different ways. Solid and liquid particles touch, so both have a similar, high density. Gas particles are far apart with empty space between them, so the same mass takes up much more room and the density is much lower.

⚠ Watch out

Saying the gaps in a gas are filled with air, or that a liquid is much less dense than a solid. The gaps hold nothing at all, and solids and liquids both have touching particles, so their densities are similar.

Memory hook

Touching means crowded. Crowded means dense. A gas is mostly nothing, so it is light.

Check yourself

Without looking back: describe the particles in a solid, a liquid and a gas in one line each. Then say which two states have similar densities and why.