Background

Several hydrated salts can be dehydrated by subjecting the compound to elevated temperature. In some cases the water of crystallisation is not bound very tightly into the crystal lattice and water can be lost at room temperature. This is known as efflorescence, as the crystals take on a powdery appearance at the surface (like flowers).

Efflorescence of sodium carbonate decahydrate

Na₂CO₃.10H₂O(s) → Na₂CO₃.(10-x)H₂O(s) + xH₂O(g)

If the mass loss of a hydrated compound is measured, the relative number of water molecules per formula unit can be determined, and the chemical formula of the hydrated salt known.

One complication is the need to ensure that all of the water has been driven off. This can be assured by heating to constant mass.

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Requirements

Chemicals

• Hydrated magnesium sulfate, MgSO₄.xH₂O(s)
• Hydrated cobalt(II) chloride: CoCl₂.xH₂O
• Hydrated copper(II) sulfate: CuSO₄.xH₂O
• Hydrated copper(II) chloride: CuCl₂.xH₂O

Apparatus

• Oven set to 200ºC
• Electronic balance
• Small watch glass
• Crucible tongs
• Small heat-proof mat

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Procedure

• Grind a small sample of the hydrated salt into a fine powder using a clean pestle and mortar. (This step may not be necessary if the salt is already a fine powder)
• Onto a small weighed and labelled watch glass, measure out about 2g of the powdered hydrated salt
• Ensure that the salt is spread evenly and reweigh.
• Leave in the pre-heated oven for about five minutes.
• Remove and reweigh using a heat-proof cover on the electronic balance. Repeat the procedure to constant mass.
• Record all data.

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Safety

Ensure that care is taken removing the hot watch glass. Use crucible tongs and heat-proof gloves.

Safety glasses must be worn.

Copper and cobalt salts are toxic and must be disposed of in the waste container.

Do not attempt to wash the hot watch glass with cold water, they may shatter.

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