Learning focus
Develop precise chemical language, connect equations and practical observations, interpret periodic trends and apply salt-preparation rules to unfamiliar examples.
Concentrating the solution
Heat the pure salt solution gently to evaporate some water. The aim is a hot concentrated or nearly saturated solution, not complete dryness. Strong heating can cause spitting or decompose some hydrated salts.
Testing concentration
A traditional test is to place a drop on a cool surface and look for crystals as it cools. In examination answers, state heat to concentrate or to the crystallisation point, then allow to cool.
Crystal formation
As the solution cools, solubility often decreases and crystals form. Slow cooling generally produces larger, better-shaped crystals. The remaining liquid is the mother liquor.
Collecting crystals
Filter the crystals, wash them with a small amount of cold distilled water and dry between filter papers or in a suitable warm place. Excess washing water would dissolve product and reduce yield.
Practical or data skill
Concentrate a safe salt solution, allow it to cool undisturbed, then filter and dry the crystals.
Examination tip
Never write evaporate to dryness when hydrated crystals are required.
Review questions and suggested answers
Question 1
Why is the solution cooled?
Suggested answer
To reduce solubility and allow crystals to form.
Question 2
Why use cold wash water?
Suggested answer
To remove soluble impurities while minimising product dissolution.
Question 3
What is mother liquor?
Suggested answer
The solution remaining after crystals form.
