A road-maintenance depot examines salty sediment swept from a winter gritting store. The aim is to recover sodium chloride crystals from the material, which...
Question 1 Report
A road-maintenance depot examines salty sediment swept from a winter gritting store. The aim is to recover sodium chloride crystals from the material, which also contains sand, small stones and dark soil. A student adds water to the sample and stirs it at room temperature. The sodium chloride forms a solution, but the sand remains insoluble. Table 1 records the masses used and obtained during one trial. The recovered crystals are intended for a chloride test, so they should be as dry as possible.
| quantity | value |
|---|---|
| mass of dry sediment | 50.0 g |
| volume of water added | 100 cm3 |
| mass of dry crystals recovered | 8.4 g |
Table 1
(a) Name the technique used to remove the sand from the salt solution. [1]
(b) State what passes through the filter paper after this technique has been carried out. [2]
(c) Complete the method for obtaining dry sodium chloride crystals from the filtrate. [3]
(d) Explain why the student should not heat the filtrate strongly until every drop of water has disappeared. [2]
(e) Calculate the percentage by mass of sodium chloride recovered from the original sediment. Show your working. [3]
(f) Suggest a suitable test to compare the purity of the recovered sodium chloride with a pure sample. [2]
(g) State why sodium chloride is spread on icy roads. [1]
Answer Details
(a) Use filtration to remove the insoluble sand. [1]
(b) Sodium chloride solution, meaning water containing dissolved sodium chloride, passes through the filter paper. Sand and other insoluble soil particles do not pass through. [2]
(c)
- Heat the filtrate gently to evaporate some water and concentrate the solution.
- Allow the concentrated solution to cool so sodium chloride crystals form.
- Filter the crystals and dry them between filter papers or in a warm place.
These stages obtain dry crystals without losing material by heating to dryness. [3]
(d) Strong heating can make the solution or crystals spit. This loses sodium chloride and gives an inaccurate recovered mass. [2]
(e) \[\text{percentage recovered}=\frac{8.4\text{ g}}{50.0\text{ g}}\times100=16.8\%\]
The percentage by mass recovered is 16.8%. [3]
(f) Measure and compare the melting temperature or melting range of the recovered sample and pure sodium chloride. Pure sodium chloride has a sharp melting temperature, whereas impurities lower and broaden the melting range. [2]
(g) Sodium chloride lowers the melting temperature of ice, so ice melts at a lower temperature. [1]
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