What 'pure' really means

Eduqas wants you to tell apart two meanings of the word pure:

• Everyday meaning — something with nothing added, e.g. "pure" milk or "pure" orange juice (which are actually mixtures!).
• Scientific meaning — a pure substance is a single element or a single compound, with nothing else in it.

So, scientifically, pure milk is not pure: it is a mixture of water, fats, proteins and sugars. Distilled water is a pure substance — it is only the compound H₂O.

Watch out: "pure" in chemistry does not mean "clean" or "wholesome". It means only one element or one compound is present.

Pure — or not?

Melting points reveal purity

A pure substance has a sharp, fixed melting point and boiling point — it melts at one definite temperature. A mixture melts (and boils) over a range of temperatures, and the presence of impurities also lowers the melting point.

Specified practical SP1A: determine a melting point, e.g. naphthalene (a pure substance — sharp) compared with candle wax (impure — melts over a range).

Reading a melting point

Three kinds of substance

• Element — made of only one type of atom (e.g. copper, oxygen). Cannot be broken down chemically.
• Compound — two or more types of atom chemically bonded together in fixed proportions (e.g. water H₂O, carbon dioxide CO₂).
• Mixture — two or more elements or compounds not chemically bonded, so they keep their own properties and can be separated by physical means.

Element, compound or mixture?

Tap the correct label for each substance.

Formulations — useful mixtures by design

Many useful materials are formulations: a formulation is a mixture made to a designed composition, with each component present in a carefully measured quantity so the product does its job.

Eduqas exemplifies formulations with: food and drink products, medicines, sunscreens, perfumes and paints. Other everyday examples are fuels, alloys, fertilisers and cleaning products.

Watch out: a formulation is a mixture — not a single pure substance. What makes it a formulation is that its composition is designed (the ratio of ingredients is chosen on purpose).

What is a formulation?

Choosing a separation technique

Because a mixture's parts are not bonded, they can be separated by physical methods. Eduqas requires these four:

• Filtration — separates an insoluble solid from a liquid (e.g. sand from water). The solid is caught by the filter paper as residue.
• Crystallisation — obtains a soluble solid from its solution by evaporating the solvent until crystals form, then cooling and filtering them out.
• Simple distillation — separates a liquid (solvent) from a solution; the liquid evaporates, then condenses and is collected (e.g. pure water from salt water).
• Fractional distillation — separates a mixture of liquids with different boiling points using a fractionating column (e.g. ethanol from water, or crude oil into fractions).

Key choice: insoluble solid + liquid → filtration; dissolved solid you want back → crystallisation; want the liquid back → simple distillation; two miscible liquids → fractional distillation.

Fractional distillation

Two liquids with different boiling points are heated. Vapour rises up the fractionating column; the liquid with the lower boiling point reaches the top first, passes into the condenser and is collected. The higher-boiling liquid condenses lower down and runs back.

Pick the technique

Tap the best separation method for each mixture.

Chromatography & the two phases

Chromatography separates the substances in a mixture and depends on how each one is shared between two phases:

• The stationary phase — the phase that does not move. In paper chromatography this is the paper (in thin-layer chromatography it is a thin layer of silica on a plate).
• The mobile phase — the moving fluid (the solvent) that travels through the stationary phase, carrying the mixture with it.

A substance that is more soluble in the solvent (and less attracted to the paper) travels further. A pure substance gives one spot; a mixture gives several spots — so chromatography also tests purity.

Watch out: draw the baseline in pencil (ink would dissolve and run), and the solvent level must start below the baseline or the spots wash off.

Calculating an R_f value

Each substance is given an R_f value — a ratio used to identify it:

Watch out: R_f has no units and is always less than 1 (the spot can't outrun the solvent). Measure to the centre of the spot.

Your turn — R_f value

Another R_f

Reading a chromatogram

Topic 1 in a nutshell

You've covered the whole of Eduqas Topic 1 — purity, classification, formulations and every required separation method. Press Finish to see your score.