MediumChemistryHigh School (9–10)GeneralMixtures and Solutions › Separation of MixturesConceptualBoard Exam
A sample of alum has some sand and some common salt in it…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A sample of alum has some sand and some common salt in it. Describe how to get pure alum crystals from it, and say where the sand and the salt end up.
Need a reminder? The key ideaShow
Crystallisation purifies a solid: dissolve it in hot water, filter while hot, cool slowly, and filter off the crystals. Dissolved impurities stay in the liquid.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. Dissolve. Stir the sample into a little hot water, so the alum and the salt dissolve.
Step 2. Filter while hot. The sand stays on the filter paper.
Step 3. Cool slowly. As the solution cools, alum's solubility falls a lot, and pure alum crystals form. The small amount of salt stays dissolved.
Step 4. Collect. Filter off the crystals, rinse them with a little cold water, and dry them.
Answer: Dissolve in hot water, filter hot (sand removed), cool to crystallise the alum, filter off the crystals; the salt stays in the liquid.
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