Chemistry – Acids, bases and salts - Preparation of salts | e-Consult

Dehydration is the process of removing water molecules from a hydrated salt. This is achieved by heating the hydrated salt. When a hydrated salt is heated, the water molecules gain kinetic energy and eventually escape from the crystal lattice as water vapour (steam). This results in a change in the appearance of the salt. Initially, the salt may appear damp or clumpy due to the presence of water. As water is removed, the salt becomes drier, potentially forming a solid mass or a powder, depending on the extent of dehydration.

For example, magnesium sulfate heptahydrate (MgSO₄ · 7H₂O) will dehydrate upon heating. Initially, the solid will be damp. As heat is applied, water will be driven off, and the solid will gradually become drier. Eventually, the dehydration process will be complete, and the resulting solid will be anhydrous magnesium sulfate (MgSO₄), which will be a white powder.

To determine the value of 'x' in MgSO₄ · xH₂O, you could use a chemical test. A known mass of the hydrated magnesium sulfate would be heated to a constant mass. The mass of water lost during dehydration would be calculated by subtracting the final mass of the anhydrous MgSO₄ from the initial mass of the hydrated salt. The mass of water lost divided by the molar mass of water (18 g/mol) would give you the value of 'x'. This process would be repeated with multiple samples to ensure accuracy.