Molecular Matter

All materials are made of tiny particles too small to see. This model explains the properties of solids, liquids and gases, and helps us understand temperature, pressure and changes of state.

What are particles?

• Atoms are the smallest building blocks of elements (like oxygen or iron).
• Molecules are two or more atoms joined together (like water, H₂O). Some substances are made of single atoms; others of molecules.
• Microscopic particles seen under a microscope in experiments (like smoke or pollen grains) are much larger than atoms or molecules. They help us see evidence for the motion of the tiny molecules.

Particles in solids, liquids and gases

• Solids: particles are packed closely in fixed positions but vibrate. Strong forces hold them. Fixed shape, not easily compressed.
• Liquids: particles are close but can slide past each other. Forces are weaker than in solids. Fixed volume but can flow and take the container’s shape.
• Gases: particles are far apart and move quickly in all directions. Forces are very weak unless particles collide. No fixed shape or volume, easily compressed.

Temperature and particle motion

Temperature shows how fast particles are moving on average. Higher temperature means higher average kinetic energy: $\text{hotter} \Rightarrow \text{faster particles}$. At absolute zero, $-273\,^{\circ}\mathrm{C}$ or $0\,\mathrm{K}$, particles have the least kinetic energy.

Heating makes particles move faster and, in liquids and gases, spread further apart (expansion). Cooling slows them down. Changes of state: melting (solid to liquid), boiling/evaporation (liquid to gas), freezing (liquid to solid), condensation (gas to liquid).

Gas pressure

Gas particles move randomly and collide with container walls. Each collision pushes on the wall, creating pressure. In a fixed volume, higher temperature $\to$ faster collisions $\to$ higher pressure. More particles in the same space also increases pressure.

Evidence: Brownian motion

Under a microscope, tiny smoke or pollen grains in air or water jiggle randomly. This Brownian motion happens because invisible, fast-moving molecules hit the grains from different sides. Warmer conditions make the jiggling faster because molecules move faster.

Diffusion

Diffusion is the spreading out of particles from high to low concentration due to random motion. Smells spreading across a room are diffusion in a gas. Diffusion is faster in gases than in liquids, and faster when warmer.

Common misconceptions

• Particles in solids do move: they vibrate about fixed positions.
• Heating does not make particles bigger; it increases their motion and spacing.
• The spaces between gas particles are mostly empty, not filled with air.
• Not all particles move at the same speed; temperature is an average.
• Compressing a gas pushes particles closer; the particles themselves do not squash.