Simple differences between mixtures and compounds
4 marks · NormalExplain how you could use melting point to distinguish between pure salt and a mixture of salt and sand. (2 marks)
Predict how the boiling point of a mixture of water and alcohol compares to the pure substances, and explain your answer. (2 marks)
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(a) A pure salt will have a sharp, fixed melting point; a salt and sand mixture will melt over a range of temperatures. · must show State that pure substances melt at a single temperature, Mixtures melt over a range
(b) The mixture will have a boiling point between that of pure water and pure alcohol, not a sharp value. · must show Mixture boils over a range, Boiling point not constant
Common mistakes: Describing melting point changes instead of range for mixtures; Saying mixture has same boiling point as components
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Marking points
M1 (a) · 1 mark · States pure salt has a sharp/fixed melting point
A1 (a) · 1 mark · Describes mixture melts over a range of temperatures
M2 (b) · 1 mark · States mixture boils over a range or has a boiling point between pure water and alcohol
A2 (b) · 1 mark · Explains that this is because it is not a pure substance/each component has different boiling points
Worked solution
a) Pure salt melts at one temperature (its melting point). A mixture of salt and sand melts over a range of temperatures because the two components have different melting points.
b) The boiling point of a water and alcohol mixture is not constant; it lies between the boiling points of pure water and pure alcohol. The mixture will boil over a range of temperatures.