4.2.4 Soluble Salts

Soluble Salts - Reactions with Metal Oxides and Hydroxides

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Metal oxides and metal hydroxides are compounds that, unlike pure metals, will react with acids to produce salts and water. These reactions are fundamental to understanding how soluble salts are formed.

General Reaction with Acids

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When metal oxides or hydroxides react with acids, the general reactions are:

Metal Oxide+Acid→Salt+Water

Metal Hydroxide+Acid→Salt+Water

These reactions result in the formation of a salt and water, similar to the neutralisation reaction between an acid and an alkali.

Soluble and Insoluble Bases

Soluble Bases (Alkalis): Metal hydroxides that dissolve in water are called alkalis. When these alkalis react with acids, they produce a salt and water, just like their insoluble counterparts.
Insoluble Bases: Metal oxides and hydroxides that do not dissolve in water can still react with acids to form a salt and water.

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Examples of Reactions The identity of the salt produced depends on the specific metal oxide or hydroxide and the acid used in the reaction. Here are two examples:

Reaction with Potassium Hydroxide:

Potassium hydroxide (KOH) is a soluble base (alkali).
When it reacts with nitric acid (HNO₃), it produces potassium nitrate (KNO₃) and water:

\ce KOH(aq)+HNO3(aq)→KNO3(aq)+H2O(l)

Reaction with Magnesium Oxide:

Magnesium oxide (MgO) is an insoluble metal oxide.
When it reacts with nitric acid (HNO₃), it produces magnesium nitrate [Mg(NO₃)₂] and water:

\ce MgO(s)+2HNO3(aq)→Mg(NO3)2(aq)+H2O(l)

In these reactions, the metal from the metal oxide or hydroxide combines with the non-hydrogen part of the acid to form the salt, while the remaining H⁺ ions from the acid combine with the OH⁻ or O²⁻ ions from the metal base to form water.

Neutralisation and Salt Formation

Similar to reactions with pure metals, these reactions also result in the formation of salts. However, in this case, the base provides the metal ion, while the acid provides the anion (the non-hydrogen part). The resulting salts are typically neutral, as the basic metal compounds neutralise the acids.

Practical Applications

These reactions are important for producing various salts, which have numerous industrial and laboratory applications. The ability to form salts through the reaction of metal oxides and hydroxides with acids is essential for understanding many chemical processes, including those used in pharmaceuticals, agriculture, and manufacturing.

In summary, metal oxides and hydroxides react with acids to produce soluble salts and water, with the specific salt formed depending on the metal base and the acid used. These reactions are key to understanding salt production and the broader concept of neutralisation in chemistry.

Soluble Salts Simplified Revision Notes for GCSE AQA Chemistry