Principles of Chemistry
8 marks · NormalA student wants to investigate the solubility of a compound and check whether a white powder she found is a pure substance or a mixture.
Outline an experiment to measure the solubility of a solid in water at 40°C. State the key steps and the measurements you would record.
The student measures the melting point of the powder. She finds that it melts over a range from 76°C to 83°C. Explain what this result suggests about the nature of the powder.
Describe how melting point measurements can be used to distinguish between a pure substance and a mixture. Give an example with data.
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) — · must show Weigh solvent and solute, Add solute to water at 40°C until saturated, Filter and evaporate water to weigh remaining solid, Calculate solubility from masses
(b) — · must show Melting over a range indicates mixture or impurity
(c) — · must show Pure substances melt at a sharp, constant temperature, Mixtures melt over a range, Example with temperatures (e.g. pure X melts at 100°C, mixture at 85–96°C)
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Marking points
M1 (a) · 1 mark · States the solution is saturated by adding solid until no more dissolves at 40°C.
M2 (a) · 1 mark · Mentions filtering out undissolved solid and weighing solution.
M3 (a) · 1 mark · States water is evaporated and residue (dissolved solid) is weighed.
A1 (b) · 1 mark · Correctly states melting over a range means mixture or impurity.
M4 (b) · 1 mark · Explains that pure substances have a sharp melting point, mixtures do not.
A2 (c) · 1 mark · Describes sharp melting point for pure substances, range for mixtures.
M5 (c) · 1 mark · Gives a data example (e.g. pure melts at 120°C, mixture at 110–116°C).
M6 (c) · 1 mark · Explains usefulness for identifying purity (e.g. in chemical preparation).
Worked solution
a. Weigh a clean, dry evaporating basin. Prepare water at 40°C in a boiling tube. Add the solid in small amounts and stir until no more dissolves, forming a saturated solution. Filter the solution to remove any undissolved particles. Measure the mass of the solution plus basin. Then evaporate all water and reweigh to find the mass of solid that was dissolved. Calculate solubility using the known mass of water for the solution.
b. Melting over a range of 76–83°C suggests that the sample is not pure—a pure substance should have a sharp, fixed melting point. This indicates the powder is a mixture or contains impurities.
c. Pure substances melt at a precise temperature, e.g., pure benzoic acid melts at 122°C, while a mixture melts over a wider range, e.g., an impure sample melts from 110°C to 119°C. Therefore, measuring the melting point helps to tell whether a substance is pure or a mixture.