Making Salts

Types of Salts

Salts are ionic compounds formed when the hydrogen ions (H⁺) from an acid are replaced by metal ions or other positive ions. The type of salt formed depends on the acid and the substance it reacts with.

• Soluble salts from acids: These are salts that dissolve in water, such as sodium chloride (NaCl) formed by reacting hydrochloric acid with sodium hydroxide (a base).
• Salts from metals: When certain metals react with acids, they form salts and hydrogen gas. (See Reactions of Acids - Acids with Metals for details.)
• Salts from metal oxides or hydroxides: Metal oxides (like copper oxide) or metal hydroxides (like sodium hydroxide) react with acids to form salts and water.
• Salts from carbonates: Metal carbonates react with acids to form salts, water, and carbon dioxide gas. (See Reactions of Acids - Acids with Carbonates for more.)

For instance, when hydrochloric acid reacts with sodium hydroxide, the neutralisation reaction produces sodium chloride and water:

$$\text{HCl (aq)} + \text{NaOH (aq)} \rightarrow \text{NaCl (aq)} + \text{H}_2\text{O (l)}$$

Methods of Making Salts

There are several ways to make salts depending on the starting materials and the solubility of the salt desired.

Reaction of Acid with Metal Oxide or Metal Hydroxide

Metal oxides and metal hydroxides are bases and react with acids in a neutralisation reaction to form salt and water only. This is a common method to prepare soluble salts.

Example: Copper oxide reacts with sulfuric acid to form copper sulfate and water.

$$\text{CuO (s)} + \text{H}_2\text{SO}_4 \text{(aq)} \rightarrow \text{CuSO}_4 \text{(aq)} + \text{H}_2\text{O (l)}$$

In this reaction, copper oxide (a base) neutralises sulfuric acid to produce copper sulfate (a salt) and water.

Titration Method for Soluble Salts

Titration is used to make a pure, soluble salt when the acid and base are both solutions. It involves carefully adding one solution to another until neutralisation is reached, indicated by a suitable indicator.

Steps:

• Measure a fixed volume of acid into a conical flask.
• Add a few drops of indicator (e.g., methyl orange or phenolphthalein).
• Slowly add the alkali (or base) from a burette until the indicator shows neutralisation.
• Repeat without indicator to get pure salt solution.
• Evaporate and crystallise the solution to obtain pure salt crystals.

Example: Preparing sodium chloride by titrating hydrochloric acid with sodium hydroxide solution.

During titration, the neutralisation reaction is:

$$\text{HCl (aq)} + \text{NaOH (aq)} \rightarrow \text{NaCl (aq)} + \text{H}_2\text{O (l)}$$

Reaction of Acid with Metal

Some metals react directly with acids to form salts and hydrogen gas. This method is less controlled and often produces impure salts, so it is less commonly used for making pure salts.

Reaction of Acid with Carbonates

Metal carbonates react with acids to form salts, water, and carbon dioxide gas. This method is useful for making salts like calcium sulfate.

Preparing Insoluble Salts

Some salts are insoluble in water and cannot be made by neutralisation or titration. Instead, they are made by precipitation reactions.

• Precipitation reactions: Mix two solutions containing ions that react to form an insoluble salt (precipitate).
• The insoluble salt forms as a solid suspended in the solution.
• Filter the mixture to separate the solid precipitate.
• Wash the precipitate with distilled water to remove impurities.
• Dry the precipitate to obtain the pure insoluble salt.

Example: Mixing solutions of silver nitrate and sodium chloride produces silver chloride, an insoluble salt.

$$\text{AgNO}_3 \text{(aq)} + \text{NaCl (aq)} \rightarrow \text{AgCl (s)} + \text{NaNO}_3 \text{(aq)}$$

Silver chloride (AgCl) precipitates as a white solid.

Purification of Salts

After making a salt, it is important to purify it to obtain pure crystals suitable for use or analysis.

• Crystallisation: Evaporate the solvent (usually water) from the salt solution until crystals start to form.
• Allow the solution to cool so crystals grow larger and purer.
• Removing excess reactants: If excess solid base or carbonate was used, filter the mixture before crystallisation to remove unreacted solid.
• Drying crystals: After crystallisation, filter the crystals and dry them between filter paper or in a warm place.

Crystallisation is the key method to obtain pure, dry salt crystals from a solution.