Density and States of Matter

Density is simply how much mass is packed into a given volume - think of it as how "heavy" something feels for its size. You calculate it by dividing mass by volume, and anything less dense than water will float on it.

When substances change from solid to liquid to gas, the mass stays exactly the same because you're not adding or removing particles. What changes is how those particles are arranged and how much energy they have.

In solids, particles are locked in fixed positions and just vibrate on the spot - like people in assigned cinema seats. Liquids have particles that can slide past each other but stay touching, whilst gases have particles flying around freely with loads of space between them. This is why gases are much less dense than solids and liquids.

Remember: The flat sections on temperature-time graphs show you exactly when melting or boiling happens - the temperature stays constant even though you're still adding heat!

Internal Energy and Gas Behaviour

Internal energy is all the energy stored by particles - both their movement energy (kinetic) and their position energy (potential). When you heat something up, you're increasing this internal energy, either making particles move faster or helping them break free from their positions.

Specific latent heat is the energy needed to change 1kg of a substance's state without changing temperature. There are two types: fusion (solid to liquid) and vaporisation (liquid to gas). You can measure this using controlled heating experiments.

Gas pressure happens because particles constantly bash against container walls. Increase the temperature and particles move faster, hitting walls harder and more often. Squeeze the gas into a smaller space and you get more impacts per second on the walls.

Key relationship: For gases at constant temperature, pressure × volume stays constant - this explains why your football feels harder when you squeeze it!

The random motion of smoke particles under a microscope proves that gas molecules are constantly moving in unpredictable directions, which explains all the pressure effects we observe.