Pure substances and mixtures: chemical bonding is the dividing line

1. In chemistry, a pure substance is made of a single element or a single compound, so every particle in it is the same.
2. This is stricter than the everyday meaning: shop “pure” orange juice is really a mixture of water, sugars and flavourings, so a chemist would not call it pure.
3. A mixture contains two or more elements or compounds that are mixed but not chemically bonded together.
4. Each substance in a mixture keeps its own chemical properties, because no reaction has joined them.
5. Air (mainly nitrogen and oxygen), seawater (water with dissolved salts) and crude oil are everyday mixtures.
6. Because the parts of a mixture are not held by chemical bonds, they can be pulled apart by physical methods that use far less energy than breaking bonds.
7. A physical separation only changes how substances are arranged and makes no new substances, so no chemical reaction takes place.
8. The best method depends on the state of each part and on properties such as boiling point and solubility.

Filtration: pulling an insoluble solid out of a liquid

1. Filtration separates an insoluble solid from a liquid or from a solution.
2. Pour the mixture into filter paper in a funnel; the liquid drains through while the solid is trapped.
3. The solid left in the paper is the residue; the liquid that passes through is the filtrate.
4. It works because the solid particles are too large to fit through the tiny holes in the paper, while liquid particles pass easily.
5. For example, sand can be filtered out of a mixture of sand and water.

Crystallisation: recovering a soluble solid from its solution

1. Crystallisation gets back a soluble solid that is dissolved in a solvent.
2. Warm the solution gently so the solvent evaporates, leaving fewer solvent particles to keep the solid dissolved.
3. As solvent leaves, the solution becomes saturated (it can hold no more solid) and crystals begin to form.
4. Stop heating once the first crystals appear and let the rest cool slowly, so larger, purer crystals grow.
5. Filter off the crystals and pat them dry between sheets of filter paper.
6. Gentle heating over a water bath protects crystals that would be damaged, and salts that break down when heated strongly.

Distillation: separating liquids by their boiling points

1. Distillation separates a liquid from a solution, or separates two or more liquids that are mixed.
2. Simple distillation separates a solvent from a solution, for example pure water from salt water.
   1. Heat the solution; the liquid boils and turns to vapour, leaving the dissolved solid behind.
   2. The vapour passes through a condenser, where it cools and condenses back to a liquid.
   3. The liquid collected is the distillate, and the dissolved solid stays in the flask.
3. Fractional distillation separates miscible liquids (liquids that mix) that have different boiling points.
   1. A fractionating column stands between the flask and the condenser.
   2. Vapours rise and repeatedly condense and re-evaporate up the column, so the liquid with the lowest boiling point reaches the top first.
   3. Each liquid is collected as the thermometer reaches its boiling point; this is how crude oil and mixtures like ethanol and water are separated.

Chromatography: separating dissolved substances by how far they travel

1. Chromatography separates the dissolved substances in a mixture, such as the coloured dyes in an ink.
2. A spot of the mixture is placed on paper (the stationary phase) and a solvent (the mobile phase) moves through it.
3. Each substance is attracted to the paper and dissolves in the solvent by a different amount.
4. A substance that is more soluble and less attracted to the paper is carried further.
5. The substances spread into separate spots, which shows how many substances were in the mixture.
6. No new substances are made, so chromatography is a physical separation like the others.