States of Matter & Separation Techniques

The Three States of Matter

Everything around you is made of tiny particles. Whether a substance is a solid, a liquid or a gas depends on how those particles are arranged, how much they move, and how much energy they have. This idea is called the kinetic particle theory ("kinetic" means moving).

Key terms Kinetic particle theory — the model that all matter is made of tiny particles in constant motion.

State of matter — solid, liquid or gas; determined by particle arrangement, spacing and energy.

The table below summarises the three states.

Changes of State

When a substance is heated or cooled, energy is added or removed and it can change from one state to another. Each change has a name.

Melting — solid → liquid (heating)
Freezing — liquid → solid (cooling)
Boiling / evaporating — liquid → gas (heating)
Condensation — gas → liquid (cooling)
Sublimation — solid → gas directly, without melting (heating)
Deposition — gas → solid directly (cooling)

When a solid melts, energy supplied is used to overcome the forces of attraction between particles, allowing them to move past one another. When a liquid boils, even more energy separates the particles completely.

Watch out During melting and boiling the temperature stays constant even though heating continues. The energy is being used to break forces between particles, not to raise the temperature. Don't write that "heat disappears."

Heating Curves

A heating curve shows how temperature changes as a solid is heated steadily into a gas. The flat sections are where state changes happen.

The first flat section is at the melting point, the second at the boiling point. A cooling curve is the mirror image, with flat sections at freezing and condensation.

Exam tip A pure substance melts and boils at one sharp temperature, so its plateaus are flat. A mixture melts/boils over a range of temperatures — this is a common way exams test purity.

Diffusion

Diffusion is the spreading of particles from a region of high concentration to a region of low concentration, caused by their random movement. It happens in liquids and gases because their particles are free to move. A drop of ink spreading through water, or perfume filling a room, are everyday examples.

Key terms Diffusion — the net movement of particles from a high to a low concentration due to random particle motion.

Two factors change how fast a gas diffuses:

1. Temperature — higher temperature gives particles more kinetic energy, so they move faster and diffuse faster.
2. Mass of the particles — at the same temperature, lighter particles move faster, so gases with a smaller relative molecular mass diffuse faster.

Worked example In the classic ammonia/hydrogen chloride experiment, cotton wool soaked in ammonia ($NH_3$, $M_r = 17$) is placed at one end of a tube and hydrogen chloride ($HCl$, $M_r = 36.5$) at the other.

A white ring of ammonium chloride forms nearer the HCl end.

This is because $NH_3$ is lighter, so it diffuses faster and travels further before they meet.

Elements, Compounds and Mixtures

All substances can be classified by what particles they contain.

A pure substance contains only one element or one compound, with nothing else mixed in. A mixture can be separated by physical methods because its parts are not chemically joined; a compound can only be split by chemical reactions.

Real world The air you breathe is a mixture of gases (mainly nitrogen and oxygen). Because they are not bonded, they can be separated by fractional distillation of liquefied air in industry to obtain pure oxygen and nitrogen.

Separation Techniques

The right method depends on the substances you are separating and their properties (solubility, boiling point, particle size).

Filtration separates an insoluble solid from a liquid. The mixture is poured through filter paper. The solid (the residue) stays in the paper; the liquid (the filtrate) passes through. Example: separating sand from water.

Crystallisation obtains a soluble solid from its solution. The solution is heated to evaporate some water, making it saturated, then left to cool slowly so pure crystals form. Example: getting copper sulfate crystals from solution.

Simple distillation separates a solvent from a solution (e.g. pure water from salt water). The solution is boiled, the vapour passes into a condenser where it cools back to a liquid (the distillate), and the dissolved solid is left behind.

Fractional distillation separates two or more miscible liquids with different boiling points (e.g. ethanol and water, or crude oil). A fractionating column packed with glass beads gives a temperature gradient: the liquid with the lowest boiling point reaches the top and evaporates over first.

Paper chromatography separates a mixture of soluble coloured substances, such as the dyes in ink. A spot of the mixture is placed on a pencil baseline near the bottom of chromatography paper, which is dipped in a solvent. As the solvent rises, each substance travels a different distance depending on how soluble it is and how strongly it is attracted to the paper.

Exam tip Always draw the baseline in pencil, not ink — pencil is insoluble and won't run up the paper. Make sure the solvent level starts below the spots.

The $R_f$ value identifies a substance by comparing how far it travelled with how far the solvent travelled:

$$R_f = \frac{\text{distance moved by spot}}{\text{distance moved by solvent}}$$

An $R_f$ value is always between 0 and 1 and has no units. Under the same conditions a substance always gives the same $R_f$, so it can be used to identify it.

Choosing the Right Method

Use this quick guide to pick a separation technique:

Exam tip Read the question for clues: "insoluble" points to filtration; "obtain the crystals" points to crystallisation; "collect the water" points to distillation. Match the property of the substance to the method.