Salt preparations
4 marks · NormalThe table below shows the solubility of different ionic compounds in water:
| Compound | Solubility |
|---|---|
| Potassium nitrate | Soluble |
| Sodium chloride | Soluble |
| Calcium sulfate | Almost insoluble |
| Lead(II) chloride | Insoluble |
| Ammonium carbonate | Soluble |
| Copper(II) hydroxide | Insoluble |
Name one compound from the table that is insoluble in water.
State the general rule for the solubility of nitrates.
Identify a compound from the table that would be suitable for making a salt by precipitation, and explain your choice.
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.
Show answer
(a) Lead(II) chloride OR Copper(II) hydroxide · must show compounds marked insoluble
(b) All nitrates are soluble in water. · must show general rule for nitrates
(c) Lead(II) chloride (or copper(II) hydroxide) is suitable because it is insoluble, so it can be made by precipitation when two suitable soluble solutions are mixed. · must show names insoluble salt, explains precipitation method applies
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Names one insoluble compound from the table (lead(II) chloride or copper(II) hydroxide).
B2 (b) · 1 mark · States that all nitrates are soluble in water.
E1 (c) · 1 mark · Identifies an insoluble compound for precipitation (lead(II) chloride or copper(II) hydroxide).
E2 (c) · 1 mark · Explains that precipitation method is suitable because the salt is insoluble.
Worked solution
a) An example of an insoluble compound is lead(II) chloride (or copper(II) hydroxide).
b) The rule is that all nitrates are soluble in water.
c) Lead(II) chloride is suitable for preparation by precipitation because it is insoluble, so it forms a solid when solutions containing Pb²⁺ and Cl⁻ ions are mixed.