Preparation: A solution of an alkali (e.g., sodium hydroxide, NaOH) is reacted with a solution of an insoluble carbonate (e.g., calcium carbonate, CaCO₃). The reaction produces a soluble salt, water, and carbon dioxide gas. The balanced equation for the reaction between sodium hydroxide and calcium carbonate is:

Separation: The insoluble product (e.g., calcium hydroxide, Ca(OH)₂) is separated from the solution by filtration. This is done using a Buchner funnel and filter paper. The filtrate contains the desired soluble salt (e.g., sodium carbonate, Na₂CO₃). Carbon dioxide gas is evolved during the reaction, which can be observed as bubbles.

Purification: The filtrate is evaporated to dryness. This leaves the solid salt, which may be impure. The salt can be further purified by recrystallization. This involves dissolving the impure salt in a minimum amount of hot water, filtering the hot solution to remove any insoluble impurities, and then allowing the solution to cool slowly. As the solution cools, the pure salt crystallizes out, leaving the impurities behind in the solution. The crystals are then filtered, washed with cold water, and dried.

Water of crystallisation refers to the water molecules that are incorporated into the crystal lattice structure of certain ionic compounds during their formation. These water molecules are held in place by weak electrostatic forces. The 'dot and number' notation (e.g., CuSO₄ · 5H₂O) indicates the number of water molecules associated with each formula unit of the compound. This notation shows that for every one formula unit of copper(II) sulphate (CuSO₄), there are five water molecules present in the crystal structure. Similarly, sodium chloride (NaCl) forms crystals with six water molecules per formula unit (NaCl · 6H₂O). The water molecules are not chemically bonded to the ions but are part of the crystal's structure.

• CuSO₄ · 5H₂O: Each formula unit of copper(II) sulphate is associated with five water molecules.
• NaCl · 6H₂O: Each formula unit of sodium chloride is associated with six water molecules.

The solubility of salts is governed by several general rules. These rules are based on the ionic charges and the lattice energy of the crystal structure. Here's a breakdown of the solubility of the specified salt types:

• Sodium, Potassium and Ammonium Salts: Salts of sodium (Na⁺), potassium (K⁺) and ammonium (NH₄⁺) are generally soluble in water. This is because these ions have relatively low charge densities and the hydration energy of the ions is high, overcoming the lattice energy.
• Nitrates: All nitrates (NO₃^-) are soluble in water. The nitrate ion is a strong, weakly coordinating anion, leading to high solubility.
• Chlorides: Most chlorides (Cl^-) are soluble in water. However, there are exceptions. Specifically, chlorides of silver (AgCl), lead (PbCl₂) and mercury(I) (Hg₂Cl₂) are insoluble.
• Sulfates: Most sulfates (SO₄^2-) are soluble in water. However, sulfates of barium (BaSO₄), calcium (CaSO₄), and lead (PbSO₄) are insoluble. This is due to the high ionic charges of Ba²⁺, Ca²⁺ and Pb²⁺, which result in a high lattice energy.
• Carbonates: Most carbonates (CO₃^2-) are insoluble in water. The exception are carbonates of sodium (Na₂CO₃), potassium (K₂CO₃) and ammonium (NH₄)₂CO₃. These are soluble because the alkali metal cations and ammonium ion have high charge densities, which increases the hydration energy and overcomes the lattice energy.
• Hydroxides: Most hydroxides (OH^-) are insoluble in water. The exceptions are hydroxides of sodium (NaOH), potassium (KOH), ammonium (NH₄OH) and calcium (Ca(OH)₂), which are only partially soluble. The solubility of calcium hydroxide is limited, but it does dissolve to a small extent.