Fill in the blanksQuestion 1 · Recall
Loading…
By the end you can use the terms hydrated, anhydrous and water of crystallisation, read formulas like , and calculate the number of water molecules in a hydrate from heating data.
Blue copper(II) sulfate crystals feel completely dry. Yet about 36 % of their mass is water. Heat them and they crumble to a white powder as the water steams off.
That water isn't wet or trapped in puddles. It's built into the crystal itself, in a fixed ratio, and it's called water of crystallisation.
Water of crystallisation
Water molecules that are chemically bonded into the crystal structure of a compound, in a fixed ratio.
Hydrated and anhydrous
A hydrated compound contains water of crystallisation. An anhydrous compound contains no water of crystallisation.
means: in the crystal, there are five water molecules for every unit. The dot means "combined with", not "multiplied by".
So for the formula mass you add: , plus , gives 249.6. (And % water: the hook above.)
| hydrated | colour | anhydrous | colour |
|---|---|---|---|
| blue | white | ||
| pink | blue | ||
| (Epsom salts) | white | white | |
| (washing soda) | white | white |
Heating drives the water off: . Add water to the white powder and it turns blue again, which is why anhydrous copper(II) sulfate is used to test for water.