Reactions of acids with metals

When a suitable metal reacts with an acid, it replaces hydrogen from the acid. The products are a salt and hydrogen gas. Bubbles of gas may be seen as the reaction takes place.

The salt formed depends on both reactants: the metal determines the first part of the salt’s name, and the acid determines the second part. For example, hydrochloric acid forms a chloride salt, while sulfuric acid forms a sulfate salt.

Magnesium reacts with hydrochloric acid to form magnesium chloride and hydrogen:

Here, the salt is magnesium chloride: its name reflects the magnesium metal and the hydrochloric acid. If magnesium reacted with sulfuric acid instead, the salt formed would be magnesium sulfate.

Reactions of acids with carbonates

When an acid reacts with a carbonate, the products are a salt, water and carbon dioxide gas. The gas escapes from the reaction mixture, usually causing visible fizzing.

For example, hydrochloric acid reacts with calcium carbonate to form calcium chloride, water and carbon dioxide:

The release of carbon dioxide distinguishes an acid–carbonate reaction from an acid reacting with a metal or a non-carbonate base. Acids commonly produce hydrogen when they react with metals; reactions with non-carbonate bases form a salt and water without producing carbon dioxide.

Bases and alkalis

A base is a substance that neutralises an acid. Many metal oxides and metal hydroxides are bases. Some bases dissolve in water, while others do not.

An alkali is a base that is soluble in water. When an alkali dissolves, the resulting aqueous solution contains hydroxide ions, $$\ce{OH- (aq)}$$. The symbol $$(aq)$$ means that the ions are dissolved in water.

For example, sodium hydroxide is a base that dissolves in water, producing a solution containing hydroxide ions, so it is an alkali. Copper(II) oxide is also a base, but it does not dissolve in water, so it is not an alkali.

Every alkali is a base, but not every base is an alkali.

Acid–base neutralisation

An acid reacts with a base to form a salt and water. This reaction is called neutralisation. An alkali is a base that dissolves in water.

In aqueous solution, an acid contains hydrogen ions, $$\ce{H+}(aq)$$, and an alkali contains hydroxide ions, $$\ce{OH-}(aq)$$. When the solutions react, the hydrogen ions and hydroxide ions combine to form water:

This ionic equation shows how the alkali neutralises the acid: hydrogen ions and hydroxide ions are removed from the solution as they form water.

The other ions form the salt. For example, hydrochloric acid reacts with sodium hydroxide to form sodium chloride and water. The sodium chloride can remain dissolved in the solution.

Reactions of bases with ammonium salts

A base reacts with an ammonium salt to release ammonia. The ammonium ions in the salt react with the base to form ammonia and water. The remaining ions form a salt.

For example, ammonium chloride reacts with sodium hydroxide to produce ammonia, water and sodium chloride:

In this reaction, ammonium ions react with hydroxide ions from the base:

The remaining sodium and chloride ions make up the salt sodium chloride.

The release of ammonia distinguishes this reaction from the neutralisation of a base by an acid, which produces a salt and water.

Indicator colours in acids and alkalis

An indicator is a substance that has different colours in acidic and alkaline conditions. The colour it shows can therefore help identify whether a solution is an acid or an alkali.

Different indicators have different colour responses:

• Litmus is red in an acid and blue in an alkali.
• Thymolphthalein is colourless in an acid and blue in an alkali.
• Methyl orange is red in an acid and yellow in an alkali.

To interpret an indicator’s colour, you must know which indicator was used. For example, a yellow result indicates an alkali when the indicator is methyl orange.

Universal indicator paper and pH

Universal indicator paper changes colour when it is placed in a solution. To estimate the solution’s pH, compare the paper’s colour with the pH colour scale supplied with it. The closest colour gives an approximate pH value.

The colour changes across the pH scale:

• Acidic solutions have a pH below 7. Their colours progress from red through orange to yellow as pH increases towards 7.
• Neutral solutions have a pH of 7 and give a green colour.
• Alkaline solutions have a pH above 7. Their colours progress from blue to purple as pH increases.

pH and hydrogen ion concentration

pH is related to the concentration of hydrogen ions, $$\ce{H+}$$, in a solution. A lower pH means a higher hydrogen ion concentration and greater acidity. A higher pH means a lower hydrogen ion concentration and greater alkalinity.