Methods of Separating & Purifying Substances Flashcards

All 25 cards in this deck

What is a pure substance in chemistry?

A single element or compound, containing only one type of substance.

What does 'pure' usually mean in everyday use?

A substance that is natural or has had nothing added to it. e.g. 'pure' orange juice is chemically a mixture.

What does melting point data tell you about a sample that melts over a range of temperatures?

It is impure (a mixture). A pure substance has a sharp, single melting point.

What are the steps to obtain pure water from a salt solution by simple distillation?
e.g. sea water

1. Heat the solution — the water boils/evaporates, leaving the salt behind.
2. The steam (water vapour) passes into the condenser.
3. The vapour condenses to form pure water, which is collected.

What are the steps to obtain crystals of a salt from its solution?
e.g. copper sulfate solution

1. Heat the solution (e.g. using a water bath) to evaporate some water and concentrate it — do not evaporate all the water.
2. Leave to cool so crystals form.
3. Filter off the copper sulfate crystals and leave to dry.

True or false? Filtration can be used to separate a dissolved salt from its solution.

False. Dissolved substances pass straight through the filter paper; filtration only separates an insoluble solid from a liquid.

In paper chromatography, what are the mobile phase and the stationary phase?

Mobile phase = the solvent running through the paper; stationary phase = the paper.

Why do the substances in a mixture move different distances up chromatography paper?

They have different solubilities in the solvent, so they move at different rates over the paper.

What does a chromatogram show for a pure substance?

A single spot. An impure substance (mixture) gives two or more spots.

What is the formula for the $R_f$ value?

$R_f = \frac{\text{distance moved by the spot}}{\text{distance moved by the solvent front}}$
Rearranged: distance moved by spot $= R_f \times$ distance moved by solvent front.

In the ink chromatography core practical, why is the start line drawn in pencil?

Pencil is insoluble in the solvent, so it will not run up the paper and spoil the results (ink would).

Why must the pencil start line be above the level of the solvent in the beaker?

So the ink spots do not dissolve away into the solvent instead of travelling up the paper.

What improvement to simple distillation apparatus increases the amount of distillate collected?

Use a (Liebig) condenser, or surround the collecting test tube with iced water — this cools the vapour more effectively so more of it condenses.

Why is a flammable solvent such as ethanol heated with a water bath rather than a Bunsen burner?

Its vapour is flammable and could catch fire from the naked flame.

What is potable water?

Water that is safe to drink. It may still contain impurities, as long as they are harmless.

Why is waste water filtered when making it potable?

To remove insoluble substances/solids, e.g. sand.

What is the reason for chlorination when making water potable?

To kill bacteria/microorganisms.

True or false? Water used in chemical analysis must not contain any dissolved salts.

True. Dissolved salts could interfere with or react in the analysis, giving incorrect results.

What are the steps to separate two liquids that mix completely, using fractional distillation?

e.g. a mixture of liquid A (boiling point 43 °C) and liquid B (boiling point 103 °C)

1. Heat/boil the mixture.
2. The liquid with the lower boiling point boils off first and rises up the column — A (43 °C).
3. Its vapour is condensed by the condenser and collected; B stays in the flask.

What happens in the sedimentation stage of water treatment?

The water is left to stand so insoluble particles sink to the bottom. A chemical such as aluminium sulfate can be added to make very small particles stick together into larger clumps, which settle faster.

When is fractional distillation needed instead of simple distillation?

When the liquids in a mixture have boiling points close together. Simple distillation only works for getting a liquid out of a solution, or for liquids with very different boiling points.

A spot from a mixture has the same $R_f$ value as a known pure substance run in the same solvent. What does this suggest?

The mixture probably contains that substance.

In paper chromatography, which substance in a mixture ends up furthest up the paper?

The one that is most soluble in the solvent. It travels with the moving solvent for more of the time, so it gets furthest from the start line.

Why is distilling sea water rarely used to make drinking water in the UK?

Boiling large volumes of water needs a lot of energy, so it is expensive. The UK has enough fresh water from rivers, lakes and ground water; distilling sea water is mainly used in dry countries with little fresh water.

In the ink core practical, how does simple distillation help identify the ink's solvent?

The solvent boils off and is collected, and the thermometer shows the temperature it boils at. Comparing this boiling point with known values identifies the solvent, e.g. a reading of about 78 °C suggests ethanol.