Particle Model of Matter
States of Matter and the Particle Model
All matter is made up of tiny particles (atoms or molecules) that are constantly moving. The particle model (also called the kinetic theory model) explains the differences between the three states of matter — solids, liquids, and gases — in terms of how the particles are arranged and how they move.
- Solids: particles are closely packed in a regular pattern. They vibrate about fixed positions but do not move from place to place. Strong forces of attraction hold them together. This is why solids have a fixed shape and a fixed volume.
- Liquids: particles are close together but arranged randomly. They can slide past each other, which is why liquids flow and take the shape of their container. They have a fixed volume but no fixed shape.
- Gases: particles are far apart and move rapidly in random directions. The forces between them are very weak. Gases have no fixed shape and no fixed volume — they spread out to fill any container.
In solids, particles vibrate in fixed positions. In liquids, particles move past each other. In gases, particles move randomly at high speed. The strength of forces between particles decreases from solid to liquid to gas.
Density
Density tells you how much mass is packed into a given volume. It is calculated using the equation:
$$\rho = \frac{m}{V}$$
Where \(\rho\) is density in \(\text{kg/m}^3\), \(m\) is mass in kg, and \(V\) is volume in \(\text{m}^3\).
Because particles are packed most closely in solids, solids generally have the highest density. Gases have the lowest density because their particles are spread far apart. For example, the density of iron is about \(7874 \text{ kg/m}^3\), water is \(1000 \text{ kg/m}^3\), and air is only about \(1.2 \text{ kg/m}^3\).
Regular solid: Measure the mass using a balance. Measure length, width, and height using a ruler or vernier callipers, then calculate volume (e.g. \(V = l \times w \times h\) for a cuboid). Use \(\rho = m / V\).
Irregular solid: Measure the mass on a balance. Use a displacement can (eureka can) or a measuring cylinder to find the volume by water displacement. Use \(\rho = m / V\).
Liquid: Place a measuring cylinder on a balance and zero it. Pour in the liquid, then read the mass and volume. Use \(\rho = m / V\).
Changes of State
When a substance changes state — for example, from solid to liquid (melting) or from liquid to gas (boiling) — the mass is conserved. No particles are gained or lost; only the arrangement and energy of the particles change.
The key changes of state are:
- Melting (solid → liquid) and freezing (liquid → solid)
- Boiling/evaporation (liquid → gas) and condensation (gas → liquid)
- Sublimation (solid → gas directly, without passing through the liquid state)
These are physical changes — unlike chemical changes, the original substance can be recovered if the change is reversed. The particles themselves do not change; only their spacing and movement alter.
Internal Energy, Specific Heat Capacity, and Latent Heat
When you heat a substance, the energy stored inside it increases. This is called internal energy. If the substance is not changing state, the added energy raises its temperature — the particles move faster. If the substance is changing state (e.g. melting or boiling), the temperature stays constant while the energy is used to break bonds between particles.
Specific heat capacity (\(c\)) tells you how much energy is needed to raise the temperature of 1 kg of a substance by 1 °C. The equation is:
$$\Delta Q = m \times c \times \Delta \theta$$
Where \(\Delta Q\) is the change in thermal energy (J), \(m\) is mass (kg), \(c\) is specific heat capacity (J/kg °C), and \(\Delta \theta\) is the change in temperature (°C).
Specific latent heat (\(L\)) is the energy needed to change the state of 1 kg of a substance without any change in temperature. The equation is: \(Q = m \times L\).
Place a known mass of water in an insulated beaker. Use an immersion heater connected to a joulemeter to heat the water. Record the temperature rise using a thermometer. Calculate \(c\) using \(c = \frac{\Delta Q}{m \times \Delta \theta}\). The accepted value for water is \(4200 \text{ J/kg} \degree\text{C}\). Results may differ due to energy lost to the surroundings.
Reducing Unwanted Energy Transfer
Thermal insulation reduces unwanted energy transfers from buildings and objects. Methods include using materials with low thermal conductivity (such as foam, fibreglass, or trapped air), double glazing (a layer of air or vacuum between two panes of glass reduces conduction and convection), cavity wall insulation, loft insulation, and reflective surfaces to reduce radiation.
Gas Pressure and Temperature
Gas pressure is caused by gas particles colliding with the walls of their container. The more frequent and forceful these collisions, the higher the pressure. When the temperature of a gas increases, its particles gain kinetic energy and move faster. If the volume is kept constant, the faster-moving particles hit the walls more frequently and with greater force, so the pressure increases.
Absolute zero is the temperature at which particles have the minimum possible kinetic energy and effectively stop moving. It is −273 °C, which equals 0 kelvin (0 K). To convert between Celsius and kelvin, use:
$$\text{temperature in K} = \text{temperature in} \degree\text{C} + 273$$
Absolute zero is −273 °C (0 K). At this temperature, particles have minimum kinetic energy. To convert: K = °C + 273.