How particles explain solids, liquids, gases and changes of state
Matter is made of tiny particles that have spaces between them, are always moving and attract each other. You can see evidence of this when a drop of ink spreads through water or the smell of incense fills a room; this spreading is called diffusion, and it is fastest in gases. In solids, particles are tightly packed and vibrate in place, so solids have a fixed shape and volume. In liquids, particles slide past each other, and in gases they move freely and fill any container.
Matter changes state when temperature or pressure changes. The melting point of ice is 0 °C and the boiling point of water at normal pressure is 100 °C. During melting and boiling, temperature stays constant because the heat supplied, called latent heat, is used to overcome the attraction between particles. Some substances, like camphor and dry ice, change directly from solid to gas, which is sublimation. Evaporation happens at the surface below boiling point and causes cooling. Temperatures are converted using K = °C + 273.
Why it matters
Evaporation explains why sweating cools the body, why water stays cool in earthen pots and why wet clothes dry faster on windy days. Gases are compressed into LPG and CNG cylinders for easy transport. Steam at 100 °C causes worse burns than boiling water because of its latent heat. These particle ideas support later chemistry and physics.
Key terms
- Diffusion
- The spreading of particles from a region of higher concentration to lower concentration.
- Latent heat
- Heat absorbed or released during a change of state without a change in temperature.
- Sublimation
- The change of a solid directly into a gas without becoming a liquid.
- Evaporation
- The change of a liquid into vapour at its surface at any temperature below its boiling point.