States of Matter: A Particle Model

Matter can exist in three states: solid, liquid, and gas. Each state demonstrates different characteristics due to the behaviour of its particles.
Particles in a solid are closely packed in a regular pattern and have a limited ability to move, which results in solids having a definite shape and volume.
Particles in a liquid have slightly more energy than those in a solid. They are closely packed like in a solid, but in an irregular pattern allowing them to slide past each other. Therefore, liquids have a definite volume but can change shape according to the container they are in.
The particles in a gas are spread out and have lots of energy, which allows them to move around freely and randomly. Gases don’t have a definite shape or volume, and will instead fill any container they are placed in.
The change from one state of matter to another is a physical process. For example, melting is the change in state from solid to liquid, while boiling or evaporation is the change from liquid to gas.
The particle model can explain these state changes. As you provide heat (energy), the particles gain more energy and begin moving more frenetically. This additional movement can break the bonds in a solid causing it to melt into a liquid, or in liquid causing it to evaporate into a gas.
Conversely, freezing is the change from liquid to solid, and condensation is the change from gas to liquid. In these processes, particles lose energy and become less mobile, forming bonds to create a liquid or solid state.
Temperature is a major factor affecting these state-changes. Each substance has specific temperatures at which it changes state (its melting and boiling points). For instance, water freezes at 0°C (becoming ice) and boils at 100°C (becoming steam).
Theory of gas pressure: gas pressure is due to the collisions of the gas particles with the walls of their container. If the gas is heated, its particles gain more kinetic energy and hit the walls more often and with greater force, thus increasing the pressure.
The particle model also explains density. In broad terms, density depends on how much space there is between particles. A substance will be denser if its particles are more tightly packed.
Limitations of particle model: Not all properties can be explained using the simple particle model. For example, it does not fully explain why different substances have different states at room temperature or why some substances can’t change states at all.

Remember to use diagrams to help visualise these concepts. They are useful in explaining how particles behave in different states and during state changes.