Simple differences between mixtures and compounds
4 marks · NormalDescribe the melting point behaviour of a pure solid and a mixture during heating. (2 marks)
A student finds that her sample of a white solid melts over the range 150°C to 163°C. What does this suggest about the purity of the sample? (2 marks)
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(a) Pure solids melt sharply at a fixed temperature; mixtures melt over a range of temperatures. · must show Fixed melting point for pure solid, Range for mixture
(b) The sample is likely to be a mixture or impure because it melts over a range of temperatures. · must show Relates temperature range to impurity/mixture
Common mistakes: Ignoring temperature range as indicator of mixture; Stating melting point depresses without giving reason
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Marking points
M1 (a) · 1 mark · States that pure solids melt at a fixed (sharp) point
A1 (a) · 1 mark · States that mixtures melt over a range of temperatures
M2 (b) · 1 mark · Recognizes melting over a range indicates a mixture
A2 (b) · 1 mark · Describes sample as impure or a mixture due to the range
Worked solution
a) Pure solids have a sharp, fixed melting point. Mixtures melt over a range of temperatures as the different substances melt separately.
b) Since the solid melted over a range of 150°C to 163°C, this suggests it is not pure and likely a mixture or contains impurities.