A salt is an ionic compound formed when the hydrogen ions of an acid are replaced by metal ions or ammonium ions. The method chosen to prepare a salt depends on the solubility of the reactants. For soluble salts, two primary laboratory techniques are used: the excess solid method and titration. The IGCSE 0654 Core syllabus requires a descriptive understanding of these procedures and the terminology associated with the water content of the resulting crystals.

This method is used when the starting material (other than the acid) is an insoluble solid. This includes many metals, insoluble bases (metal oxides/hydroxides), and insoluble carbonates.

The Experimental Procedure

1. Reaction: Add the solid reactant to a fixed volume of warm acid while stirring. Continue adding the solid until it is in excess (i.e., no more dissolves and solid remains at the bottom). This ensures all the acid has been neutralised.
2. Filtration: Filter the mixture. The excess solid remains on the filter paper (residue), while the clear salt solution passes through (filtrate).
3. Evaporation: Heat the filtrate in an evaporating dish to remove some of the water until the crystallisation point is reached (tested by dipping a glass rod to see if crystals form on cooling).
4. Crystallisation: Leave the concentrated solution to cool slowly. Large, well-defined crystals will form.
5. Isolation: Filter the crystals, wash them with a small amount of cold distilled water to remove impurities, and pat them dry with filter paper.

This method is required when the base is soluble (an alkali), such as sodium hydroxide. Because both reactants are liquids, you cannot see when the reaction is complete by looking for excess solid.

The Experimental Procedure

1. Measurement: Use a pipette to add a fixed volume of alkali to a conical flask. Add a few drops of an indicator (e.g., methyl orange).
2. Titration: Slowly add acid from a burette into the flask, swirling constantly, until the indicator just changes color (the end-point).
3. Repetition: Note the volume of acid used. Repeat the experiment using the same volumes of acid and alkali but without the indicator, to obtain a pure, colorless salt solution.
4. Crystallisation: Follow the same evaporation and drying steps as the excess solid method.

Many salts incorporate water molecules into their crystal lattice structure during the crystallisation process.

• Hydrated Substance: A substance that is chemically combined with water molecules (known as "water of crystallisation").
• Example: Copper(II) sulfate pentahydrate ($CuSO_4 \cdot 5H_2O$), which forms characteristic blue crystals.
• Anhydrous Substance: A substance that contains no water. This state is often achieved by heating a hydrated salt until the water evaporates.
• Example: Anhydrous copper(II) sulfate ($CuSO_4$), which is a white powder.

The synthesis of salts is a fundamental laboratory skill that combines stoichiometric precision with practical separation techniques. By selecting the correct method based on reactant solubility and utilizing filtration and crystallisation, chemists can produce high-purity ionic compounds for a vast array of industrial and scientific uses.