1bElements, compounds and mixtures

Three words that look similar and mean quite different things. Getting them apart is worth doing properly, because almost everything later in the course assumes you can.

The three, defined

The test that separates a compound from a mixture

Ask one question: are the substances chemically bonded?

• Air contains nitrogen, oxygen and argon. Nothing is bonded to anything, and the proportions vary from place to place. Air is a mixture.
• Carbon dioxide contains carbon bonded to oxygen, always in the ratio 1:2. It is a compound.

There is a second clue that follows from the first. A mixture's composition can be varied — you can have more or less oxygen in a gas mixture. A compound's cannot: change the ratio and you have a different substance.

This is why steel is an alloy, not a compound. The iron and carbon are mixed, not bonded, and the amount of carbon is chosen deliberately to give mild steel or high-carbon steel.

Why mixtures can be separated and compounds cannot

Because a mixture's parts are not bonded, physical methods — filtering, distilling, chromatography — can pull them apart. Separating a compound needs a chemical reaction, because bonds must be broken.

That single fact explains why fractional distillation works on crude oil, and why you cannot distil sodium out of salt.

Purity, and what a melting point tells you

A pure substance melts and boils at fixed temperatures. Pure water melts at 0 °C and boils at 100 °C at normal atmospheric pressure. Boiling point in particular changes with pressure, so the condition matters when the fixed value is the evidence.

A mixture melts or boils over a range. If a sample starts melting at 96 °C and is still melting at 103 °C, it is not pure.

Impurities also lower a melting point. Salt on an icy road works for exactly this reason: the salty water freezes below 0 °C, so the ice melts.

That gives you a physical test for purity that needs no chemicals at all — measure the melting or boiling point and see whether it is sharp.

Five techniques, and the whole skill is choosing the right one. Each works because of a specific difference between the substances being separated.

Filtration versus crystallisation

This is the choice students get wrong most often, and one question settles it: is the solid dissolved?

• Not dissolved — sand in water. Filter it. The sand stays on the paper as the residue, the water passes through as the filtrate.
• Dissolved — salt in water. Filtering does nothing at all; the dissolved salt passes straight through with the water. You need crystallisation.

Simple distillation

Two things happen, and a good answer names both:

1. The solution is heated. The solvent boils off, leaving the dissolved solid behind in the flask.
2. The vapour passes into a condenser, where it is cooled back into a liquid and collected.

Note what the condenser does not do. It does not remove the salt — the salt never leaves the flask. Its only job is cooling.

Distillation and crystallisation recover opposite halves of the same solution: distillation gives you the pure liquid, crystallisation gives you the dissolved solid.

Fractional distillation

Use this when you are separating two liquids, not a liquid from a solid.

A fractionating column sits above the flask and provides a large surface area. As the vapour rises it repeatedly condenses and re-evaporates. The liquid with the higher boiling point condenses lower down and runs back; only the one with the lower boiling point reaches the top and is collected first.

Simple distillation copes when boiling points are far apart. When they are close — a few degrees — the column is what makes separation possible.

Chromatography separates substances that are all dissolved in the same solvent — the case where filtering and distilling are no help.

How it separates

Spots of the mixture are placed on paper, and a solvent rises through them. Every dissolved substance is pulled two ways at once:

• towards the paper, which holds it back;
• into the moving solvent, which carries it up.

A substance that is more soluble and less strongly held travels further. One that clings to the paper travels a shorter distance. That difference is what spreads the mixture out.

Reading a chromatogram

• One spot suggests the substance is pure — it does not prove it, because two substances can share an Rᶠ in one solvent.
• Several spots means it is a mixture, and each spot is one substance in it.
• Two spots at the same height, on the same paper, are probably the same substance. Confirm by running again in a different solvent.

That last point has a condition attached: on the same paper. Distances can only be compared within one chromatogram, run in one solvent.

Rf values

An Rf value turns a distance into a number that can be compared between experiments.

Rf = distance moved by the spot ÷ distance moved by the solvent front

Both distances are measured from the starting line.

Two checks on any answer:

• Rf has no units. It is one distance divided by another.
• Rf is always less than 1. A spot cannot outrun the solvent carrying it. An answer above 1 means the division was done upside down.

Rf values only identify a substance if the solvent and the paper are the same. The same dye can give 0.42 in one solvent and 0.65 in another, and neither value is wrong.

Two practical details that carry marks

Draw the starting line in pencil. Ink would dissolve in the solvent and travel up the paper, adding spots that were never in your sample.

Keep the solvent level below the starting line. If the solvent covers the spots at the start, the dyes wash off into the solvent instead of rising through the paper, and the experiment produces nothing.

Chromatography separates a mixture of coloured substances by how far each travels up a paper as a solvent rises through it.

Apparatus

Chromatography paper, beaker with a lid or watch glass, solvent (water or ethanol), pencil, ruler, capillary tube or dropper.

Method

1. Draw a baseline about 1 cm from the bottom in pencil.
2. Spot the inks on the baseline, keeping each spot small. Let it dry and respot to concentrate the colour.
3. Pour solvent into the beaker so its level is below the baseline.
4. Stand the paper in the solvent and cover the beaker.
5. Let the solvent rise, and remove the paper before the solvent front reaches the top.
6. Mark the solvent front in pencil immediately, then let the paper dry.

Rᶠ values

Rᶠ = distance travelled by the spot ÷ distance travelled by the solvent

Both distances are measured from the baseline, and the spot is measured to its centre.

Rᶠ has no units and is always less than 1 — the spot is carried by the solvent, so it cannot outrun it. An Rᶠ above 1 means something was measured from the wrong place.

The same substance gives the same Rᶠ under the same conditions — same solvent, same type of paper, same temperature. That is why an unknown is identified by running it alongside knowns on the same paper in the same tank, rather than by comparing with an Rᶠ from a book.

Why each step

Pencil, not pen. Ink would dissolve in the solvent and travel up the paper with the samples.

Solvent below the baseline. If the solvent covers the spots they dissolve into the liquid in the beaker instead of moving up the paper, and nothing separates.

Lid on the beaker. It stops the solvent evaporating from the paper, which would slow the rise and give an uneven front.

Stop before the front reaches the top. Once the front runs off the edge you cannot measure how far the solvent went, so no Rᶠ can be calculated.

Colourless substances

A spot you cannot see can still be located — spray with a locating agent or view under UV light — and then measured exactly as above.

Interpreting the result

• One spot — a pure substance.
• Several spots — a mixture, one spot per component.

Two substances that happen to travel the same distance in one solvent can be told apart by running the paper again in a different solvent.