Hydrous and Anhydrous Substance | Chemistry for GCSE/IGCSE - Year 11 PDF Download

Hydrated & Anhydrous Salts

• When salts are being prepared, some water can be retained within the structure of the salt during the crystallisation process
• This affects the crystal's shape and color
• Salts that contain water within their structure are called hydrated salts
• Anhydrous salts are those that contain no water in their structure
• For instance, consider copper(II) sulfate, which crystallizes to form hydrated copper(II) sulfate, appearing blue
• Upon heating, water is expelled from its structure, resulting in anhydrous copper(II) sulfate, which is white
• The process of removing water from the hydrated salt to form the anhydrous salt is known as dehydration 
• Interestingly, the reaction can be reversed by introducing water to anhydrous copper(II) sulfate:
  hydrated copper(II) sulfate ↔ anhydrous copper(II) sulfate + water

Water of Crystallisation

• Water molecules present in certain salts during crystallization are termed as water of crystallisation.
• A hydrated compound refers to a substance containing water of crystallisation within its structure.
• When representing the chemical formula of hydrated compounds, the water of crystallisation is denoted by a dot. For instance, hydrated copper(II) sulfate is written as CuSO₄ ∙ 5H₂O.
• The formula indicates the quantity of water molecules present in one mole of the hydrated salt. For example, hydrated copper(II) sulfate, CuSO₄ ∙ 5H₂O, contains 5 moles of water in 1 mole of the hydrated salt.
• An anhydrous compound is a substance that lacks water of crystallisation. An example is anhydrous copper(II) sulfate, CuSO₄.
• The conversion between anhydrous and hydrated compounds is reversible. Heating the hydrated salt leads to the formation of the anhydrous salt, and vice versa.

Hydrated Compounds

• Hydrated copper(II) sulfate, CuSO₄ ∙ 5H₂O, contains 5 moles of water in 1 mole of the hydrated salt.
• The formula illustrates the number of moles of water present within one mole of the hydrated salt.

Anhydrous Compounds

• An anhydrous compound, such as anhydrous copper(II) sulfate (CuSO₄) or anhydrous cobalt(II) chloride (CoCl₂), does not contain water of crystallization.

Reversibility of Conversion

The conversion between anhydrous and hydrated compounds is reversible through heating. For instance:

• Anhydrous to hydrated salt: CuSO₄ + 5H₂O → CuSO₄ ∙ 5H₂O
• Hydrated to anhydrous salt (by heating): CuSO₄ ∙ 5H₂O → CuSO₄ + 5H₂O

Diagram showing the dehydration of hydrated copper II) sulfate: