CSEC Chemistry Paper 2

Acids, Bases & Salts-Salts cont’d
Have you ever wondered why some salts appear crystalline and beautiful while some look powdery and dull?
The reason is a bit similar to what the doctors say about how your skin looks when you drink plenteous amounts of water versus not drinking much or any at all.
To begin and answer yesterday’s closing and today’s opening question, a hydrated salt is one that has water of crystallisation (attached to its structure) while an anhydrous salt is one that has no water of crystallisation (attached to its structure). [Did you get it right?]
Think of hydrated as “with water,” similar to when we hydrate ourselves and anhydrous as literally “without water.” It makes sense that hydrated salts can be “dehydrated” to give anhydrous salts.
Take hydrated copper (II) sulphate for example (the name you will see on the container from its distributors is copper (II) sulphate pentahydrate)
Formula CuSO4.5H2O. (It is really copper sulphate with 5 units of water attached to each unit of copper (II) sulphate.
In its hydrated form, CuSO4.5H2O is a blue crystalline solid. When heated, this water can be driven off, yielding the anhydrous form, “so-so” copper (II) sulphate, CuSO4. The latter is not as crystalline in its appearance but appears rather off-white/white and powdery.
CuSO4.5H2O(s) –> CuSO4(s) + 5 H2O(g)
Hydrated                Anhydrous            Water driven off as water vapour
The water can be re-added to the CuSO4(s) to give hydrated copper (II) sulphate pentahydrate (the reverse of the above equation)
Contributed by: Kemil Walford

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