Elements, Compounds and Mixtures
5 marks · EasyA chemist dissolves 10 g of sodium chloride in 1.0 litre of pure water.
State whether the resulting solution is a pure substance, a compound, or a mixture. Give a reason.
The boiling point of pure water is 100°C. Adding the salt changes the boiling point to 100.2°C. Calculate the increase in boiling point.
Suggest a separation technique you could use to obtain pure water from this solution. State the process and what happens during it.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) It is a mixture, because it contains more than one substance (salt and water) that are not chemically combined. · must show mixture: substances not chemically joined, reason mentioning presence of both salt and water
(b) 0.2 °C · must show 100.2 - 100 = 0.2°C
(c) Simple distillation. Heat the salt solution so the water boils, then condense the steam to collect pure water; the salt remains behind. · must show simple distillation named, description includes boiling and condensation
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Correctly classifies the mixture as a mixture (not pure substance or compound).
E1 (a) · 1 mark · Gives a valid reason for classification: not chemically combined, made of salt and water.
A1 (b) · 1 mark · Correctly calculates the boiling point increase: 0.2°C.
M1 (c) · 1 mark · Names simple distillation as the separation technique.
M2 (c) · 1 mark · Describes heating, boiling, condensation of water, and salt left behind.
Worked solution
a) The solution is a mixture because salt and water are mixed but not chemically combined.
b) The boiling point increases from 100°C to 100.2°C, so the increase is 0.2°C.
c) Simple distillation is used: the salt solution is heated to boil off water, which is condensed and collected as pure water, leaving salt behind.