Fig. 1 shows a quality-control flow chart for a company making sodium chloride for saline solution. Rock salt is crushed and mixed with water. Insoluble san...
Question 1 Report
Fig. 1 shows a quality-control flow chart for a company making sodium chloride for saline solution. Rock salt is crushed and mixed with water. Insoluble sand is removed, then water is evaporated from the salt solution to obtain crystals. Samples from the final batch are checked by measuring melting range and by testing whether a solution of the crystals leaves a residue after evaporation. The company needs a pure compound because impurities could affect the medical use of the saline solution.
(a) Name the substance that passes through the filter in Fig. 1. [2]
(b) Give the reason why sand can be separated from sodium chloride solution by filtration. [2]
(c) Use particle ideas to describe what happens to water particles when the salt solution is evaporated. [4]
(d) Suggest two tests or observations that would show the final sodium chloride crystals are pure. [4]
Answer Details
- (a) The substance passing through the filter is sodium chloride solution, also called the filtrate. [2 marks]
- (b) Sand is insoluble in water, so its solid particles do not dissolve. The filter paper traps these particles while the salt solution passes through. [2 marks]
- (c) When heated, water particles gain energy and move faster. They overcome the forces between particles, become water vapour and leave the solution. Sodium chloride remains because it does not evaporate under these conditions. [4 marks]
- (d) Measure the melting point or melting range: pure sodium chloride has a sharp melting point. Dissolve a sample in water and evaporate it: there should be no unexpected solid residue other than sodium chloride. Chromatography giving one spot for a suitable impurity test is also acceptable. Any four valid points. [4 marks]
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