Explain what the term impure means element and can pure

A This ‘ice hotel’ is made entirely from ice and snow – these are both water in

Did you know?

B Particles in the solid state contain the smallest amount of stored energy; particles solid

in the gas state contain the most.

Exam-style question Extend

Exam-style questions will follow on publication of the sample assessment materials by Edexcel.

The composition (make-up) of a pure substance:

• cannot be changed

Gold is an element and can be pure, but
compounds can also be pure. The sugar we use
at home is a compound called sucrose. It contains

A You can tell this gold bar is very nearly pure because of the ‘999.9’ stamped on it. A number lower than 1000 on this ‘fineness’ scale means it is impure.

B Pure sucrose is always sucrose, no matter how finely it is ground down.

Melting points

When a solid melts, its particles gain enough energy to overcome the weak forces of attraction between them. They move further away from one another and the solid becomes a liquid. The temperature at which this

A pure substance has the same composition in every part of it, and so its physical properties are the same in every part. So, all of a pure substance will melt at the same temperature until all the substance has changed state. The melting point of pure gold is 1063 °C and the melting point of oxygen is −218 °C.

Time (seconds)

D heating curves for a pure substance and a mixture

E1 A piece of gold jewellery is 750 on the fineness scale. Would you expect the jewellery to have a sharp melting temperature? Explain your answer.

• How can crystallisation be used to separate mixtures?

• What are the hazards and risks when separating mixtures by filtration and crystallisation?

They open their mouths and take in water. When they close their mouths, they push out the water through filters. Small animals (such as krill) get stuck in the filters and are swallowed.

Crystallisation

separated by filtration.

B Crystals in the Giant Crystal Cave in Mexico took over 500 000 years to form.

Filtration and crystallisation

3 In the solution mining of salt, give the names of the:

the solution once it is saturated, because hot crystals may spit out. Further			5
			5
i	ii
water vapour			5
			5	4 When is a solution said to
	filter paper
			5	5 Explain why crystals form
				during crystallisation.
			6	6 Explain why the crystals in
	filtrate

During crystallisation, the risks from spitting can be reduced by wearing eye previous page?

protection, removing the Bunsen burner before the solution is completely dry and/or using steam to heat the evaporating basin gently (as above).

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toxic (especially if the vapour is inhaled). Large crystals can be made to help scientists
work out what the compounds are made of. Explain how you would make plant-compound crystals using methanol.

• How can chromatography be used to separate mixtures?

• What are the differences between mixtures and pure substances on a chromatogram?• How do you calculate an Rf value?

are above the level of solvent in the container.

The Rf value is the distance the compound has risen divided by the distance the solvent has risen. Both measurements are made from the starting

Paper chromatography

Paper chromatography can be used to:

• distinguish between pure and impure substances									5
• identify substances by comparing the pattern on the chromatogram with
the patterns formed by known substances
										some sweets.
• identify substances by calculating their Rf values.
									5
									6
										yellow sweets?
										the most soluble dye?
E104 E110 E120 E122 E133								pink

C The chromatogram on the left was done using known substances. The chromatogram on the right shows that the orange and blue sweets contain single dyes.

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Extend

these Rf values to be useful.

• How do simple distillation and fractional distillation differ?

• How would you reduce risks when carrying out a distillation experiment?

6 b Explain why some irons may not work well if you use ordinary tap water.

When mineral water evaporates, only the water turns to a gas (vapour). The solid minerals, which have much higher boiling points, are left behind.

flask and the vapour travels along the

can be used to purify water.

• to make alcoholic drinks such as whisky and vodka

hot vapour rises up the column. At first, the vapour condenses when it hits the cool glass and drips back

D distillation apparatus with a fractionating column

The vacuum flask that we now use to keep drinks hot was originally used

S2 Explain the safety precautions you need to take when carrying out distillation in a laboratory.

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Progression questions

Purifying sea water

Water is separated from dissolved salts using simple distillation. Sea water is heated so that water vapour leaves it quickly. This vapour is then cooled and condensed, forming water without the dissolved salts.

may be used to provide
drinking water in oil-rich
coastal countries. oil in

Water for drinking

In the UK, the raw material for producing drinking water comes from rivers,

• small insoluble particles such as grit and silt
• soluble substances, including salts, pesticides and fertilisers

Only about 2.5% of the Earth’s water is fresh water. Of that, only

gravel

water for
homes and
industry