The most common method of preparing insoluble salts is by

Analyze the question: It asks for the usual method of preparing insoluble salts, not separating them.

Concept: The method for making a salt depends on its solubility:

• Soluble salts are made by neutralization (acid + base/alkali), acid + metal, or acid + carbonate.
• Insoluble salts are made by double decomposition, also called precipitation. Two soluble salt solutions are mixed, and their ions swap partners to form one insoluble product (the precipitate) and one soluble product.

Example: Preparing barium sulphate:

BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq)

The white precipitate of BaSO₄ forms immediately because it is insoluble. It is then separated by filtration, washed, and dried.

Checking the options:

• A. Filtration: This only separates the precipitate after it forms. It is not the preparation reaction itself.
• B. Decomposition: This is the breakdown of one compound into simpler substances, usually by heat. It is not a general way to make insoluble salts.
• C. Neutralization: This is used for soluble salts, such as NaCl from HCl + NaOH.
• D. Double decomposition: This is the correct method, because it produces the insoluble salt as a precipitate.

Answer: D. double decomposition

Exam tip: Remember "Insoluble = Double decomposition (precipitation)". Filtration is only the follow-up step. If the question says "preparing a soluble salt", think neutralization, or the acid with metal or carbonate.