OCR GCSE Chemistry

States of matter

Matter is made of tiny particles, and the particle model uses that idea to explain how solids, liquids and gases behave. In the simplest version each particle is drawn as a small solid sphere, and the states differ in how the particles are arranged and how they move.

Solids and liquids are hard to compress because their particles are already touching. A gas can be compressed because most of it is empty space between the particles, which is also why gases have such low densities. Gas particles fill a container because they are free to travel until they hit a wall, and the collisions with the walls are what produce gas pressure.

Particles in a liquid or gas are always moving, and this is the reason for diffusion. If a drop of coloured solution goes into water, or a scent is released in one corner of a room, the particles spread out from where they are crowded into the space around them without anyone stirring. Diffusion is faster in gases than in liquids because gas particles are further apart and move faster.

Elements are substances made of one type of atom only, and atoms of different elements can join to make compounds. The particles in the model can be single atoms, molecules or ions, depending on the substance.

Changes of state

A change of state is a physical change, and there are six of them.

Heating a substance gives its particles more kinetic energy, so they vibrate or move faster. During melting and boiling the temperature stays constant even though energy is still being supplied, because the energy is going into overcoming the forces of attraction between the particles and not into making them move faster. The temperature at which a pure solid melts is its melting point, and the temperature at which a pure liquid boils is its boiling point. When the change is reversed, by cooling, the same amount of energy is released as the forces re-form.

Evaporation happens at any temperature below the boiling point, at the surface of a liquid, when the fastest particles have enough energy to escape. Boiling happens throughout the liquid, at the boiling point, when bubbles of vapour form inside it.

Physical and chemical changes

In a physical change no new substance is made. The particles are the same before and after, and only their arrangement or energy changes, so the change is easily reversed. Melting, dissolving and boiling are examples. Dissolving salt in water breaks up the salt lattice without changing the salt ions themselves, and the salt can be recovered by evaporating the water.

In a chemical change the particles themselves change. Bonds in the reactants break and new bonds form, so atoms are rearranged into different substances. Products usually cannot be turned back into the reactants by simply reversing the temperature, and there is often evidence such as a colour change, a gas being given off, a precipitate forming or a temperature change. The total number of atoms of each element stays the same, so mass is conserved.

The model explains the difference in this way. A physical change only affects how the whole particles are arranged and how strongly they attract each other, so the substance is chemically the same afterwards. A chemical change breaks the bonds inside the particles, which is why the properties of the product differ from those of the reactants.

Limitations of the model

The simple model treats particles as hard, inelastic spheres, like a collection of bowling balls, and that leaves out several things.

• It ignores the forces of attraction between the particles, though these are what have to be overcome when a solid melts or a liquid boils and are the reason substances have different melting and boiling points.
• It shows every particle as a sphere of the same kind, when real particles differ in size and shape, and can be atoms, molecules or ions.
• It draws the space between particles as though the diagram were to scale, and that is not the case. In a gas the distances are many times the size of the particles.
• It does not explain why different materials have different properties. That needs ideas about the particles themselves and how they are held together, which come from bonding.
• It shows particles in a solid as merely touching, when in a real solid they are held in place by bonds or by forces of attraction.

The model is still used, because it explains changes of state, diffusion, gas pressure and density well, and it can be applied to unfamiliar materials to make predictions. Models are kept while they explain the evidence and changed when they stop doing so.