Preparation of salts Revision Notes

Soluble salts are ionic compounds that dissolve in water. They can be prepared by reacting acids with different substances that neutralise the acid, forming the salt and water or other products. The choice of reactant affects the preparation method and purification steps. This note focuses on four methods: using alkalis via titration, excess metals, excess insoluble bases, and excess insoluble carbonates.

Each method involves reacting the acid with the chosen reactant until neutralisation is complete, then removing any unreacted substances and evaporating the solution to crystallise the salt. Understanding these steps is essential for producing pure salts suitable for further use or analysis.

• Soluble salts dissolve in water and can be isolated by evaporation.
• Neutralisation reactions between acids and bases or metals produce salts.
• Purification involves removing unreacted materials and crystallising the salt.

When preparing a soluble salt from an acid and an alkali (a soluble base), titration is the preferred method because it allows precise measurement of the exact amount of alkali needed to neutralise the acid.

The procedure involves adding the alkali gradually to a fixed volume of acid while stirring, using an indicator to detect the neutralisation point (end point). At this point, the acid and alkali have reacted completely, forming a neutral solution of the salt.

After titration, the salt solution is gently heated to evaporate water until crystals begin to form, then left to crystallise fully. This produces pure salt crystals free from excess acid or alkali.

• Use a burette to add alkali to acid slowly.
• Add indicator (e.g., methyl orange) to detect neutralisation.
• Stop titration at the exact neutralisation point.
• Heat solution to evaporate water and crystallise salt.

Some metals react with acids to produce soluble salts and hydrogen gas. To prepare a salt, add excess metal to the acid to ensure all the acid reacts.

The reaction continues until no more bubbles of hydrogen gas are produced, indicating the acid is fully neutralised.

The mixture is then filtered to remove the unreacted metal. The filtrate contains the soluble salt solution, which is heated to evaporate water and crystallise the salt.

• Add excess metal (e.g., magnesium) to acid (e.g., hydrochloric acid).
• Wait until hydrogen gas evolution stops.
• Filter to remove unreacted metal.
• Evaporate solution to crystallise salt.

Insoluble bases such as metal oxides or hydroxides react with acids to form soluble salts and water.

Add excess insoluble base to the acid and stir until no more solid dissolves, indicating all acid has reacted.

Filter the mixture to remove the excess insoluble base. The filtrate contains the salt solution, which is then evaporated to crystallise the salt.

• Add excess insoluble base (e.g., copper oxide) to acid.
• Stir until no more solid dissolves.
• Filter to remove excess base.
• Heat filtrate to crystallise salt.

Insoluble carbonates react with acids to produce soluble salts, carbon dioxide gas, and water.

Add excess insoluble carbonate to the acid and stir until bubbling (CO₂ release) stops, showing acid is neutralised.

Filter to remove unreacted carbonate. The filtrate contains the salt solution, which is evaporated to crystallise the salt.

• Add excess insoluble carbonate (e.g., calcium carbonate) to acid.
• Stir until CO₂ gas evolution ceases.
• Filter to remove excess carbonate.
• Evaporate solution to obtain salt crystals.

When two aqueous solutions containing soluble salts are mixed, their ions can react to form a new salt. If this new salt is insoluble in water, it will form a solid called a precipitate. This process is called precipitation.

The formation of an insoluble salt occurs because the product salt has very low solubility, so it cannot remain dissolved and separates out as a solid.

For example, mixing aqueous barium chloride (BaCl₂) and sodium sulfate (Na₂SO₄) solutions results in barium sulfate (BaSO₄), which is insoluble and precipitates out, while sodium chloride remains dissolved.

• Mixing two soluble salt solutions can produce an insoluble salt.
• The insoluble salt forms a solid precipitate.
• This is a chemical reaction involving ion exchange in solution.

The precipitation reaction can be represented by a balanced ionic equation showing the exchange of ions.

For example, when barium chloride reacts with sodium sulfate:

Ba²⁺(aq) + 2Cl⁻(aq) + 2Na⁺(aq) + SO₄²⁻(aq) → BaSO₄(s) + 2Na⁺(aq) + 2Cl⁻(aq)

• Ba²⁺ and SO₄²⁻ ions combine to form insoluble BaSO₄ solid.
• Na⁺ and Cl⁻ ions remain dissolved as spectator ions.
• The net ionic equation is Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s).

1. Prepare two separate aqueous solutions of soluble salts containing the ions needed to form the insoluble salt.

2. Mix the two solutions in a beaker or flask. A solid precipitate will form if an insoluble salt is produced.

3. Stir the mixture to allow complete reaction and precipitation of the insoluble salt particles, which appear as a cloudy suspension or solid particles in the liquid.

• Use clean glassware to avoid contamination.
• Mix solutions slowly to control precipitation.
• Observe formation of solid precipitate indicating reaction.

After precipitation, the solid insoluble salt must be separated from the liquid (filtrate).

This is done by filtration using filter paper and a funnel.

The solid collected on the filter paper is washed with distilled water to remove impurities and soluble salts adhering to it, then dried to obtain the pure insoluble salt.

• Filter the mixture to separate solid precipitate.
• Wash the precipitate with distilled water to purify.
• Dry the solid to remove moisture before weighing or further use.

Common examples include: barium sulfate from barium chloride and sodium sulfate, lead(II) chloride from lead nitrate and sodium chloride, and silver chloride from silver nitrate and sodium chloride.

This method is widely used in qualitative analysis to identify ions by the formation of characteristic precipitates.

It is also used to prepare insoluble salts for further study or industrial use.

• Barium sulfate (BaSO₄) precipitate is white and insoluble.
• Lead(II) chloride (PbCl₂) forms a white precipitate.
• Silver chloride (AgCl) forms a white precipitate that darkens in light.