Higher and foundation tier

Alkalis and bases

indigestion Your stomach contains hydrochloric acid. Hydrochloric acid is a strong acid and even when dilute will have a pH of one. If too much acid is produced in the stomach then you may feel a burning pain, this is called indigestion. Antacids are tablets that are taken to relieve acid indigestion. But what is in the indigestion tablets to cure the indigestion?

Indigestion tablets contain a substance called a base. Bases are substances which will neutralise an acid. Examples of bases include substances such as metal oxides, metal carbonates and metal hydroxides. Most indigestion or antacid tablets contains 500mg (0.5g) of calcium carbonate or chalk. It might seem odd that to cure indigestion you eat a tablet containing chalk! Calcium carbonate or chalk is a harmless substance which in small amounts will not cause any harm when eaten. Calcium carbonate is a base, it will neutralise any excess stomach acid and relieve the pain caused by indigestion.

Base and alkalis

Bases are substances which neutralise an acid. They react with acids to produce a salt and water. Common bases are metal oxides, metal carbonates and metal hydroxides. These substances are all solids. Some bases will dissolve in water, however most are insoluble and will not dissolve in water. If a base dissolves it water it will form an alkali e.g.

Common bases Soluble or Insoluble Can it form an alkali.
copper oxide insoluble no
sodium oxide soluble yes
calcium carbonate insoluble no
sodium hydroxide soluble yes
potassium hydroxide soluble yes
bases form alkalis when they dissolve in water

The table above gives examples of some common bases you may have used in your science lab. Notice that all soluble bases will dissolve in water to form alkalis and insoluble bases cannot form alkalis. Remember all alkalis are solutions. Alkalis are solutions which contain an excess of hydroxide ions, OH- . It is the hydroxide ions which give alkalis their characteristic properties. Alkalis feel soapy to the touch, not that you should touch most alkalis since they are corrosive. All alkalis have a pH above 7.

Weak alkalis such as ammonium hydroxide have a pH around 9 whereas strong alkalis such as lithium hydroxide, sodium hydroxide or potassium hydroxide have high a high pH in the range 12-14. The colour chart for universal indicator is shown below, you can see that for alkalis the colours range from dark green to purple. The higher the pH of the alkaline solution the higher the concentration of hydroxide ions (OH-(aq)) present.

universal colors for acids and alkalis


Alkalis are solutions formed when bases dissolve in water. One of the most common alkalis used in science labs is sodium hydroxide. This is formed when the base sodium oxide dissolves in water:

sodium oxide(s) + water(l) sodium hydroxide(aq)

Similarly with other metal oxide bases, they dissolve in water to form metal hydroxides. Solutions of metal hydroxides are alkalis.
potassium oxide(s) + water(l) potassium hydroxide(aq)
calcium oxide(s) + water(l) calcium hydroxide(aq)
All alkalis are solutions which contain an excess of hydroxide ions, OH-(aq) .

Key Points

Practice questions

Check your understanding - Questions on Alkalis and bases