P1 Matter
Kinetic theory and states of matter
All matter is made of particles (atoms or molecules) in constant, random motion. The kinetic theory explains the properties of solids, liquids and gases in terms of particle behaviour.
| State | Particle arrangement | Movement | Forces between particles |
|---|---|---|---|
| Solid | Regular lattice, very close | Vibrate about fixed positions | Strong |
| Liquid | Random, close | Flow past each other | Moderate |
| Gas | Random, widely spaced | Move rapidly in all directions | Negligible |
Changes of state are physical changes (reversible). During melting and boiling, temperature stays constant while energy is used to break intermolecular bonds — this is latent heat.
Density
Density (ρ) is the mass per unit volume:
ρ = m / V
Units: kg/m³ (SI) or g/cm³ (common). 1 g/cm³ = 1000 kg/m³.
PAG P1.1 (OCR required practical): Measure the density of regular and irregular solids and liquids.
- Regular solid: measure dimensions with a ruler/vernier calliper, calculate V; measure m on a balance.
- Irregular solid: measure displaced volume of water using a displacement can or measuring cylinder.
- Liquid: measure mass of known volume using a measuring cylinder and balance.
Internal energy
Internal energy is the total kinetic energy + potential energy of all particles in a substance.
- Heating a substance increases its internal energy — particles move faster (higher KE) and, for changes of state, intermolecular bonds are broken (higher PE).
- Internal energy increases when temperature rises; during a change of state, temperature is constant but internal energy still increases as potential energy increases.
Specific heat capacity
Q = mcΔθ
where Q = energy transferred (J), m = mass (kg), c = specific heat capacity (J/kg°C), Δθ = temperature change (°C).
Water: c = 4200 J/kg°C — very high, which is why oceans moderate climate.
PAG P1.2: Measure specific heat capacity using a joulemeter/ammeter-voltmeter and a heating coil in a block of metal.
Specific latent heat
Q = mL
where L = specific latent heat (J/kg). No temperature change occurs during this process.
- Specific latent heat of fusion (melting/solidifying)
- Specific latent heat of vaporisation (boiling/condensing) — always larger because more bonds are broken
Gas pressure and the gas laws
Pressure in a gas arises from particle collisions with container walls.
p = F / A (pressure = force ÷ area, Pa)
For a fixed mass of gas at constant temperature (Boyle's law):
p₁V₁ = p₂V₂ (pressure × volume = constant)
Increasing temperature at constant volume increases pressure (particles move faster → more frequent, harder collisions).
For a fixed mass of gas:
p₁/T₁ = p₂/T₂ (at constant V)
Temperature must be in Kelvin: T(K) = θ(°C) + 273.
Combined gas law (Higher tier):
p₁V₁/T₁ = p₂V₂/T₂
OCR examiner tips (J259/01 and J259/02)
- Always state the equation before substituting. The mark scheme awards M1 for stating the formula.
- Density questions often give volume in cm³ but want kg/m³ — convert carefully.
- PAG questions ask you to "describe" — name the equipment AND state what is measured AND how it is calculated.
- For gas laws, temperature in Celsius is a common error trap — always convert to Kelvin.
⚠Common mistakes
- Density unit confusion: answer in g/cm³ when kg/m³ required, or vice versa.
- Forgetting Kelvin: using °C in gas-law calculations.
- Latent heat vs specific heat: during a change of state, use Q = mL, not Q = mcΔθ (Δθ = 0).
- Pressure definition: students write force, not force per unit area.
- Internal energy: confusing it with temperature — internal energy includes potential energy of bonds.
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