Elements, Compounds and Mixtures
5 marks · EasySome students are studying how melting and boiling points differ between pure substances and mixtures, and how to separate mixtures.
State what is meant by a pure substance in terms of melting and boiling points.
Water is mixed with some sugar. Describe what happens to its melting point and boiling point, compared to pure water.
Name one method that could be used to separate the sugar from the water and briefly describe how it works.
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(a) A pure substance melts and boils at fixed (sharp) temperatures. · must show fixed melting and boiling points
(b) The melting point decreases and the boiling point increases compared to pure water. · must show lowered melting point, raised boiling point
(c) Crystallisation: evaporate the water so that sugar forms crystals which can be collected. · must show crystallisation named, method: evaporation, formation, and collection of sugar crystals
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Marking points
B1 (a) · 1 mark · Correctly states fixed melting/boiling point for a pure substance.
B2 (b) · 1 mark · States melting point is lowered by adding sugar.
B3 (b) · 1 mark · States boiling point is raised by adding sugar.
M1 (c) · 1 mark · Names crystallisation as the method for separation.
M2 (c) · 1 mark · Describes evaporation of water and collection of crystals.
Worked solution
a) A pure substance melts and boils at specific, fixed temperatures.
b) The melting point is decreased (water freezes at a lower temperature), and the boiling point is increased compared to pure water.
c) Crystallisation is used: the water is evaporated until sugar starts to crystallise, and then the sugar crystals are collected.