Sec 1 Science Particulate Nature of Matter notes

The particulate nature of matter is a model. It says every substance is made of very small particles. The particles are always moving, and the motion is random. We draw them as circles so we can talk about arrangement. The circles are not the real size or colour of the particles.

The model is useful because it explains states of matter, diffusion, and expansion. It can be improved. Scientists accepted it because evidence, including the irregular jiggling of tiny grains in a liquid, fitted the idea that invisible particles were colliding with them. You only need the attitude: a model is kept when it explains evidence, and it can be revised.

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration. Concentration here means how many solute particles there are in a given volume. A drop of ink in still water slowly colours the whole beaker, even if nobody stirs, because particles are already moving.

• Diffusion is faster in a gas than in a liquid, because gas particles move faster and have more space.
• A higher temperature makes particles move faster, so diffusion is faster.
• It is net movement. Particles move both ways; more move from the crowded side to the less crowded side.

When a solid, liquid or gas is heated, the particles gain energy and move faster. In general they move further apart, so the substance expands and its volume increases. Because the volume is larger and the mass is the same, the density decreases. On cooling, particles move more slowly and closer together, so the substance contracts.

Mass is conserved. Expansion does not add particles, and contraction does not destroy them. A tightly capped flask of air that is warmed has the same mass afterwards. If the flask is open, air can escape and the mass of what is left in the flask can fall — that is gas leaving, not particles disappearing.

• Melting: particles in a solid gain enough energy to leave their fixed positions and slide past each other. The substance becomes a liquid.
• Boiling: particles throughout the liquid gain enough energy to move far apart and become a gas. Bubbles of gas form inside the liquid.
• Freezing and condensing are the reverse changes. The particles lose energy.

The particles themselves are the same before and after. Ice, water and steam are the same substance in three states. That is why a change of state is a physical change, which links back to the idea of a physical property.