Particles in solids, liquids & gases

Particle theory models matter as small solid spheres. The arrangement, movement and energy of the particles decides the state:

State symbols in equations: (s) solid, (l) liquid, (g) gas, (aq) aqueous (dissolved in water).

Read the particle diagram

Melting, boiling, freezing, condensing

Changing state is a physical change — it is reversible and conserves mass; no new substance is made. Each change has a name:

• Melting (solid → liquid) and freezing (liquid → solid) both happen at the melting point.
• Boiling/evaporating (liquid → gas) and condensing (gas → liquid) both happen at the boiling point.
• Sublimation is solid → gas directly (and deposition is gas → solid).

Name the change of state

Tap the change of state happening in each case.

Reading a heating curve

Heat a solid steadily and plot temperature against time. The graph has two flat plateaus — at each one, energy is going into breaking forces between particles (changing state), not raising the temperature:

Key idea: on a flat section both states are present and the temperature holds steady because the added energy overcomes the forces between particles instead of speeding them up.

Interpret the curve

Limits of the simple particle model

The simple particle model is useful, but it is a simplification. You should be able to give its limitations:

• It shows particles as solid spheres — real particles are not solid balls.
• It shows all particles as the same size, when in reality they differ.
• It treats particles as having no forces between them, yet these forces of attraction are exactly what must be overcome to change state.

What the model misses

Pure vs mixture

In chemistry a pure substance is a single element or compound with nothing else in it. A mixture contains two or more substances not chemically bonded together.

A pure substance has a sharp, fixed melting point and boiling point. A mixture melts and boils over a range of temperatures — that's how melting-point data tests purity.

Common slip: "pure" in chemistry does not mean "clean" or "natural". Pure orange juice is really a mixture; only a single element or compound is chemically pure — and it has a sharp melting point.

Is it pure?

Choosing a technique

Because a mixture isn't chemically bonded, it can be separated by physical methods. Match the method to the mixture:

• Filtration — separates an insoluble solid from a liquid (e.g. sand from water).
• Crystallisation — gets a soluble solid back from its solution by heating to evaporate solvent, then cooling so crystals form.
• Simple distillation — separates a liquid from a solution (e.g. water from salt water) by boiling then condensing.
• Fractional distillation — separates a mixture of liquids with different boiling points, using a fractionating column.
• Paper chromatography — separates a mixture of soluble, coloured substances (e.g. dyes in an ink).

Fractional distillation

The mixture is heated; the liquid with the lowest boiling point vaporises first, rises up the fractionating column, then condenses in the water-cooled condenser and is collected. Higher-boiling liquids stay behind or condense lower down.

Which separation technique?

Tap a mixture on the left, then the technique on the right that separates it.

Chromatography & the R_f value

Paper chromatography separates soluble substances. The paper is the stationary phase; the solvent that rises up it is the mobile phase. More-soluble substances travel further. Each spot's position is described by its R_f value:

Watch out: R_f has no units and is always less than 1 (the spot can't pass the solvent front). Draw the baseline in pencil so it doesn't dissolve, and measure to the spot's centre.

Your turn — calculate R_f

One more R_f

Making water potable

Potable water is water that is safe to drink — it is not the same as pure water, because it still contains dissolved substances. To make ground water or waste water potable:

• Sedimentation — suspended solids settle to the bottom and are removed.
• Filtration — passing the water through filter beds removes finer insoluble solids.
• Sterilisation — microbes are killed, e.g. by chlorination (chlorine gas), ozone or UV light.

Potable ≠ pure: potable water is safe to drink but is a mixture (it has dissolved salts). Distillation would make it pure but needs a lot of energy.

Safe to drink

The key points to know

You've covered the whole of Edexcel Topic 2 — including the Higher-only limitations of the particle model. Press Finish to see your score.