Energy Transforming Matter — States and Heating
States of Matter and the Particle Model
All matter is made of tiny particles. The particle model helps explain how the same substance can exist as a solid, liquid or gas depending on how much energy its particles have.
In a solid, particles are held tightly in a regular lattice by strong forces of attraction. They vibrate but cannot move from place to place, which gives solids a fixed shape and volume. In a liquid, particles are still close together but the forces between them are weaker — they can slide past each other, so the liquid takes the shape of its container while keeping a definite volume. In a gas, particles are far apart and move rapidly in all directions, exerting pressure as they collide with the walls of their container.
Solid: particles vibrate in fixed positions — definite shape and volume. Liquid: particles slide past each other — definite volume, no fixed shape. Gas: particles move freely and rapidly — no fixed shape or volume, exerts pressure on its container.
Internal Energy
The internal energy of a substance is the sum of all the kinetic energy of its moving particles and all the potential energy stored in the forces between those particles. When you supply energy to a substance by heating it, the internal energy increases — the particles move faster or their arrangement changes, or both.
The temperature of a substance is a measure of the average kinetic energy of its particles. Raising the temperature means the particles are, on average, moving faster. However, during a change of state, temperature stays constant even though energy is still being supplied. This is because the energy goes into overcoming the forces between particles rather than speeding them up.
Specific Heat Capacity
Different materials need different amounts of energy to achieve the same temperature rise. Specific heat capacity (symbol \(c\)) is the quantity of energy needed to raise the temperature of 1 kg of a substance by 1°C. A high specific heat capacity means the substance stores a large amount of energy per kilogram for each degree of temperature rise.
The relationship between energy, mass, specific heat capacity and temperature change is:
$$\Delta E = m \times c \times \Delta T$$
Where \(\Delta E\) is the change in internal energy in joules (J), \(m\) is the mass in kilograms (kg), \(c\) is the specific heat capacity in J/kg°C, and \(\Delta T\) is the temperature change in °C.
Worked example: An electric heater warms 2.0 kg of water from 20°C to 80°C. The specific heat capacity of water is 4200 J/kg°C. Calculate the energy transferred.
\(\Delta T = 80 - 20 = 60\)°C
\(\Delta E = 2.0 \times 4200 \times 60 = 504\,000\) J = 504 kJ
Always state the unit in your final answer. Check that all values are in SI units before substituting.
Water has a specific heat capacity of 4200 J/kg°C — one of the highest of any common material. This means the sea absorbs large amounts of energy in summer and releases it slowly in winter, moderating coastal climates compared to inland areas.
Latent Heat
When a substance changes state, energy must be supplied (to melt or boil) or released (to freeze or condense) even though the temperature stays constant. This hidden energy is called latent heat. The word “latent” means hidden — the energy is used to break or form bonds between particles rather than raising the kinetic energy of particles.
There are two types: the specific latent heat of fusion (solid ↔ liquid) and the specific latent heat of vaporisation (liquid ↔ gas). The latent heat of vaporisation is always larger than the latent heat of fusion for the same substance, because changing from liquid to gas requires all the intermolecular forces to be overcome, not just weakened.
The energy for a change of state is calculated using:
$$E = m \times L$$
Where \(E\) is energy in joules, \(m\) is mass in kilograms, and \(L\) is the specific latent heat in J/kg.
Worked example: Calculate the energy needed to melt 0.5 kg of ice. The specific latent heat of fusion of water is 334 000 J/kg.
\(E = 0.5 \times 334\,000 = 167\,000\) J = 167 kJ
To measure specific heat capacity in school, a known mass of material is heated electrically. Record the current, voltage, time and temperature rise: \(\Delta E = V \times I \times t\), then \(c = \Delta E \div (m \times \Delta T)\). Insulate the sample to reduce heat losses to the surroundings.
Energy for a change of state: \(E = mL\). During a change of state temperature remains constant — all energy goes into breaking or forming bonds between particles, not into changing their kinetic energy. Latent heat of vaporisation is always greater than latent heat of fusion.
Density and Changes of State
Density is mass divided by volume: \(\rho = m / V\). When a substance changes state, the mass is conserved but the volume usually changes — so the density changes too. Gases have much lower density than liquids because the particles are far apart. An important exception is water: ice is less dense than liquid water because the particles in ice form a structure that takes up slightly more space than liquid water particles, which is why ice floats.