Higher and foundation tier

Neutralisation using an insoluble base

You can neutralise an acid using an alkali or a base. The equation for a neutralisation reaction can be written as:

antacids relieve the pain of acid indigestion Bases are substances which can neutralise an acid. They react with acids to produce a salt and water. This is identical to the reaction of alkalis with acids. But remember alkalis are solutions, they are formed when a base dissolves in water. Bases on the other hand are mostly solids.

Common bases are metal oxides, metal carbonates and metal hydroxides. Indigestion tablets for example contain metal carbonates, commonly 0.5g of calcium carbonate (chalk). Calcium carbonate is an excellent base and will quickly neutralise an excess acid present in the stomach and relieve the symptons of acid indigestion. Metal carbonates such as calcium carbonate react with acids according to the equation shown in the box below. The carbon dioxide gas which is given off is the reason why some people may feel slightly bloated after taking an indigestion tablet.

equation to show a metal carbonate reacting with acid

Some bases will dissolve in water, however most are insoluble and will not dissolve in water. If a base dissolves in water it will form an alkali, that is a solution which contains an excess of hydroxide ions, (OH-).

Acids, Bases and neutralisation

Most bases will not dissolve in water to form an alkali, they are insoluble. However they will dissolve in acid, especially if it is hot. In the example below the insoluble base copper oxide, is slowly added to a beaker containing warm sulfuric acid. The copper oxide dissolves in the acid but is insoluble in water, so the basic copper oxide is slowly added to the acid and stirred until it no longer dissolves, this means all the acid has been neutralised. The mixture is then filtered and finally the solution is heated to evaporate the water leaving the solid salt. The experimental procedure is outlined in the diagram. An equation for the reaction is :

acid(aq) + base(s) salt (aq) + water(l)
sulfuric acid(aq) + copper oxide(s) copper sulfate(aq) + water(l)
H2SO4(aq) + CuO(aq) CuSO4(aq) + H2O(l)

Neutralising sulfuric acid with the insoluble base copper oxide

using a base to neutralise an acid

The blue copper sulfate solution is heated in a water bath or you could use an electric heater, and this leaves a white solid in the evaporating basin, this white solid is anhydrous copper sulfate (anhydrous simply means without water). Note: Copper sulfate crystals if dry are white, but if they are left in an open room the crystals absorb water from the air and turn blue, this form of copper sulfate is called hydrated copper sulfate. If the blue hydrated copper sulfate crystals are heated the water evaporated and you are left with colourless (white) anhydrous copper sulfate crystals.

Practice questions

Check your understanding - Questions on neutralisation