Water of Crystallisation

Many ionic crystals have water molecules withing their structure. We call these hydrated salts. We can drive the water off by heating the salts strongly, creating anhydrous salts (salts with no water inside their crystal structure).

This can be easily seen in substances such as copper (II) sulfate:

Hydrated Copper(II) Sulfate is a blue crystal.

A crucible is placed on a pipe-clay triangle to heat the hydrated copper(II) sulfate strongly.

Anhydrous copper (II) sulfate is a finer powder and is white (left).

However, not all hydrated salts will change colour when dehydrated. Instead, we record the mass of the hydrated salt, then heat it strongly, then record the mass again. If the mass has changed, we can infer that water of crystallisation has been removed by the heating.

The change in mass is the water removed. As we know the molar mass of water (18.0 g mol^-1), we can then calculate the amount of water removed. Once we calculate the amount of the anhydrous salt we have left, we can calculate the formula of the hydrated salt:

EXAMPLE

5.96g of hydrated BaCl₂ was heated strongly for 5 minutes, weighed (5.08g), then heated again for another 2 minutes and reweighed (5.08g).

As the ratio is 1:2, the formula is BaCl₂.2H₂O

In the table, we are always trying to get the ionic crystal to be "1" in the ratio. The value in the ratio for the water molecules should be rounded to the nearest whole number.