Particle Model and Density

Density is basically how tightly packed particles are in a material - think of it as how much stuff you can cram into a space. You calculate it using the formula: density = mass ÷ volume.

In solids, particles are like people squashed together in a packed lift - they're held close by strong forces and can only vibrate in place. This tight packing usually makes solids the densest state of matter.

Liquids have weaker forces between particles, so they're like people at a crowded party who can move around each other but stay close together. They form irregular arrangements and are typically less dense than solids.

Gases have almost no forces holding particles together - imagine people running freely around a massive field. The particles zip about at high speeds with loads of space between them, making gases much less dense.

Top Tip: When measuring density in practicals, remember that irregular objects need the water displacement method (eureka can), whilst regular objects just need length × width × height calculations.

Changes of State and Internal Energy

Changes of state are physical changes - like when ice melts or water boils - and they're completely reversible. The key ones are melting, freezing, boiling, condensing, and sublimating (when solids go straight to gas, like dry ice).

Here's the crucial bit: during any change of state, mass is always conserved - you don't lose or gain particles, they just rearrange themselves.

Internal energy is all the energy stored by particles in a system - both their movement energy (kinetic) and the energy from their positions (potential). When you heat something, you're pumping energy into these particles.

Specific heat capacity tells you how much energy you need to raise 1kg of a substance by 1°C. The formula is: ΔE = mcΔθ. Different materials need different amounts of energy - water needs loads, which is why it takes ages to heat up.

Specific latent heat is the energy needed to change 1kg of a substance's state without changing temperature. It's why ice at 0°C feels different from water at 0°C - the particles have different energy levels.

Remember: Latent heat of fusion = solid to liquid, latent heat of vaporisation = liquid to gas.

Gas Particles and Pressure

Gas particles are like tiny bouncy balls constantly smashing into everything around them at mental speeds. The faster they move, the hotter the gas feels - temperature is directly linked to how energetic these particles are.

Pressure happens when these speedy particles bash into container walls. More collisions or faster-moving particles both create higher pressure - it's like being pelted with tennis balls versus ping pong balls.

Here's where it gets interesting: if you heat a gas in a fixed container, the particles speed up and hit the walls harder, increasing pressure. But if you give the same gas more space to move around, there are fewer wall collisions, so pressure drops.

This gives us a brilliant relationship: pressure × volume = constant (for a fixed amount of gas at constant temperature). Squash a gas into half the space, and the pressure doubles - that's why balloons pop when squeezed too hard.

When you compress a gas, you're doing work on it, which increases its internal energy and usually its temperature too. This is why a bicycle pump gets warm when you're inflating tyres.

Key Point: Pressure and volume are inversely proportional - when one goes up, the other comes down, assuming everything else stays the same.