Particle Model of Matter
States of matter, density, internal energy, and changes of state.
In this topic
States of Matter and Density
The three states of matter:
Solid: particles vibrate in fixed positions, strong forces hold them in a regular arrangement. Fixed shape and volume.
Liquid: particles close together but free to move past each other. Random arrangement. Fixed volume but takes shape of container.
Gas: particles far apart, move randomly at high speed. No fixed shape or volume. Can be compressed.
Density = mass / volume (ρ = m/V) Unit: kg/m³ or g/cm³
To find density of a regular solid: measure dimensions, calculate volume, weigh on balance. To find density of an irregular solid: use displacement method with water.
Key Points
- Particles have more energy in gas > liquid > solid
- Changes of state are physical changes (reversible)
- Melting/boiling = gaining energy; freezing/condensing = releasing energy
- During a change of state, temperature stays constant
Example Questions
3A block of metal has mass 540 g and dimensions 10cm × 3cm × 2cm. Calculate its density in g/cm³.
Volume = 10 × 3 × 2 = 60 cm³. Density = 540/60 = 9 g/cm³
[3 marks]
3Explain, in terms of particles, what happens when a liquid boils.
As the liquid is heated, particles gain kinetic energy and move faster. At the boiling point, particles have enough energy to overcome the intermolecular forces holding them together. They escape from the surface and throughout the liquid, forming a gas. The particles are now far apart and move randomly at high speed.
[3 marks]
Pressure in Gases
Gas pressure is caused by particles colliding with the walls of their container.
When temperature increases, gas particles move faster and collide with the walls more frequently and with more force, increasing pressure.
For a fixed mass of gas at constant temperature: pV = constant (Boyle's Law) p₁V₁ = p₂V₂
Increasing the volume decreases the pressure (and vice versa) because the same number of particles has more/less space.
Key Points
- Pressure (Pa) = Force (N) / Area (m²)
- Gas pressure increases with temperature (at constant volume)
- Gas pressure decreases with increasing volume (at constant temperature)
- 1 atmosphere ≈ 101,325 Pa
Example Questions
2A gas has a volume of 500 cm³ at a pressure of 100 kPa. The volume is reduced to 200 cm³ at constant temperature. Calculate the new pressure.
p₁V₁ = p₂V₂ → 100 × 500 = p₂ × 200 → p₂ = 50000/200 = 250 kPa
[2 marks]