📖 What's the chemistry?
(a) Water of crystallisation
Hydrated magnesium sulfate (Epsom salt) is
MgSO₄·xH₂O — each mole of MgSO₄ is combined with x moles of
water of crystallisation. Heating drives the water off as
vapour, leaving anhydrous MgSO₄. Weighing before and after tells you the mass of
water lost, and moles = mass ÷ Mr turns both masses into a mole ratio:
x = mol H₂O ÷ mol MgSO₄ = 7.
(b) Why heat "to constant mass" matters
If the salt is heated too gently or too briefly, some water of crystallisation stays
behind — the mass after heating is too high, so the water lost (and x) comes out too small.
(c) The QA logic
- Test 1: acidified silver nitrate gives a white precipitate → chloride
(AgCl). The green solution turns blue as the chloride is removed.
- Test 2: with excess ammonia, Cu²⁺ first gives a light blue precipitate that
dissolves to a dark blue solution (the filtrate). Fe³⁺ gives a
reddish-brown precipitate, insoluble in excess (the residue). AgCl also dissolves
in ammonia, so it doesn't confuse the residue.
- Test 3: barium nitrate: no observable change → no sulfate present.
- Test 4: excess sodium hydroxide precipitates both hydroxides — the mixed
solid looks murky green on the paper; every cation is removed, so the filtrate is
colourless.
Examiner tips (from the real report)
- Write "no observable change" when nothing happens — leaving a blank loses the mark.
- Never describe a solution as "clear" — the word is "colourless".
- Don't swap residue (stays on the paper) and filtrate (runs through).
- Record masses to the precision of the balance (2 d.p.) — don't round.