Preparation of Salts

Preparation of salts depends on the physical and chemical properties of the salt. For example, when choosing the best method to prepare a salt you need to consider the following:

• is the salt soluble? If yes, we usually prepare it by neutralisation of acid and carbonate or acid and base. Next ask yourself, is the base or carbonate soluble? If yes, then use titration. Otherwise you otherwise mix the acid with excess base or carbonate.
• if the salt is insoluble, then prepare it by precipitation.

Solubility of salts

We can classify normal salts into two categories, those which are soluble in water and those which are insoluble in water.

Salts which are soluble in cold water:

• nitrates
• all common sodium, potassium and ammonium salts
• chlorides except chlorides of lead, silver and mercury
• sulphates except sulphates of lead, barium and calcium.

Preparing soluble salts

There are basically four methods of preparing soluble salts.

• metal and acid reaction
• carbonate and acid reaction
• alkali and acid reaction
• insoluble base and acid reaction

Acid and metal

We only use this method when preparing salts of less reactive metals because the reaction would be dangerous when dealing with very reactive metals. Preferable metals for this method of salt preparation are:

• magnesium
• aluminium
• zinc
• iron
• tin.

Example: Preparation of zinc sulphate salt

• add a spatula load of powdered zinc to a beaker of dilute sulphuric acid. The zinc reacts and dissolves producing bubbles of hydrogen.
• continue adding zinc until no more zinc dissolves. The presence of undissolved zinc signifies that all acid has reacted.
• filter off the excess undissolved zinc.
• the collected filtrate is a solution of zinc sulphate.
• boil off some of water in the filtrate to produce a saturated solution.
• leave the saturated solution near a window sill for some days to slowly evaporate and crystallise.
• crystals of zinc sulphate will be formed.

The equation for the reaction is:

Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g)

The identity of the gas produced can be confirmed by burning it with a burning splint. It burns with a loud popping sound, showing that it is hydrogen.

Acid and carbonate

We this method of salt preparation with any metal carbonate and any acid, as long as the salt produced is a soluble salt.

Example: Preparation of copper nitrate salt

• add a spatula load of powdered copper carbonate to a beaker of dilute nitric acid. The copper carbonate reacts and dissolves producing bubbles of carbon dioxide.
• continue adding copper carbonate until no more copper carbonate dissolves. The presence of undissolved copper carbonate signifies that all acid has reacted.
• filter off the excess undissolved copper carbonate.
• the collected filtrate is a solution of copper nitrate.
• boil off some of water in the filtrate to produce a saturated solution.
• leave the saturated solution near a window sill for some days to slowly evaporate and crystallise.
• crystals of copper nitrate will be formed.

The equation for the reaction is:

CuCO₃(s) + 2HNO₃(aq) → Cu(NO₃)₂(aq) + H₂O(l) + CO₂(g)

The identity of the gas produced can be confirmed by adding it to limewater. It turns limewater milky, showing that it is carbon dioxide.

Acid and alkali (soluble base)

An alkali is a soluble base. We use this method to prepare salts of very reactive metals, such as potassium and sodium. The reaction between a very reactive metal and acid is very violent and dangerous. This is the reason why we use an alkali (a metal oxide that dissolves in water to produce OH^– ions).

Since for the neutralisation reaction between an acid and a soluble base both reactants are soluble we cannot determine the endpoint by the presence of an excess reactant. We therefore use a volumetric technique called titration. We add the acid is slowly into to a measured volume of the alkali using a burette until the indicator changes colour.

An indicator is a chemical that shows when the alkali has been neutralised completely by the acid. Examples of indicators are phenolphthalein and methyl orange.

After titrating the reactants we note the endpoint and the volume of acid needed to completely neutralise the alkali. We then used the resultant volume of acid to neutralise the alkali, this time without the indicator.

The crystals of the obtained salt can then be obtained by crystallisation.

Acid and insoluble base

We use this method to prepare a salt of an unreactive metal, such as lead or copper. Usually for these unreactive metals it is not possible to use a direct reaction of the metal with an acid because they either do not react with acids or react very slowly with acids.

Example: Preparation of copper sulphate salt

• add a spatula load of powdered copper oxide to a beaker of dilute sulphuric acid. The copper oxide reacts and dissolves producing a blue solution.
• continue adding copper oxide until no more copper oxide dissolves. The presence of undissolved copper oxides signifies that all acid has reacted.
• filter off the excess undissolved copper oxide.
• the collected filtrate is a solution of copper sulphate.
• boil off some of water in the filtrate to produce a saturated solution.
• leave the saturated solution near a window sill for some days to slowly evaporate and crystallise.
• crystals of copper sulphate will be formed.

The equation for the reaction is:

CuO(s) + H₂SO₄(aq) → CuSO₄(aq) + H₂O(l)

Preparation of insoluble salts by precipitation

An insoluble salt, for example barium sulphate, can be made by precipitation. In this method, we mix two soluble solutions of the chosen salts and then filter off the insoluble salt precipitated out of the solution.

Example: Preparation of barium sulphate salt

• To prepare barium sulphate (an insoluble salt), mix barium chloride (a soluble salt) and sodium sulphate (a soluble salt).
• The reaction produces a precipitate of barium sulphate.
• Filter off the precipitate, wash it with distilled water and dry it.

The equation for the reaction is:

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