7.7 Summary & Keep the Curiosity Alive

Key Points

  • Matter is made of extremely small particles (constituent particles), too small to see, with interparticle spaces between them and interparticle attractions holding them together.
  • The strength of the attractions — set by the particles’ heat (thermal) energy — decides the state: strongest in solids, weaker in liquids, negligible in gases.
  • Solids have a fixed shape and volume (particles packed, only vibrating). Liquids have a fixed volume but no fixed shape (particles move in a limited space). Gases have neither (particles move freely). Liquids and gases are fluids.
  • Melting point = solid → liquid; boiling point = liquid → gas. Evaporation is slow surface vaporisation at any temperature.
  • Interparticle spacing: gas > liquid > solid. So gases are compressible; liquids and solids practically are not.
  • Particles are in constant motion; their spreading and mixing is diffusion, which speeds up with temperature.
  • The real particles of matter are atoms and molecules.

A Closing Challenge: Keep the Curiosity Alive

Think It Through

  1. Solid or liquid? Grains of rice and rice flour take the shape of any jar. Are they solids or liquids? Explain.
  2. The salty sea. Why does ocean water taste salty even though you can’t see the salt?
  3. Spilt milk. Milk spreads across a table while the glass tumbler keeps its shape. Explain both, in terms of particles.
  4. Remove the particles. If you could take all the constituent particles out of a chair, what would be left — (i) nothing changes, (ii) a lighter chair, or (iii) nothing of the chair at all? Justify.
  5. Draw it. Sketch the particle arrangement as ice melts into water and then boils into vapour — showing spacing and movement at each stage.

That completes our first look at what matter is made of. Next, we sort all the world’s substances into a tidy scheme — elements, compounds and mixtures — and meet the symbols chemists use to name them.