Acids and bases

Bases are substances that can react with acids and neutralise them. Alkalis are bases that are soluble in water. The pH scale measures how acidic or alkaline a substance is. Substances with a pH lower than 7 are acidic, those with a pH of 7 are neutral and those with a pH greater than 7 are alkaline.

Acids and alkalis

The pH scale

The chemical properties of many solutions enable them to be divided into three categories - acids, alkalis and neutral solutions. The strength of the acidity or alkalinity is expressed by the pH scale.

  • solutions with a pH less than 7 are acidic
  • solutions with a pH of 7 are neutral
  • solutions with a pH greater than 7 are alkaline.

If universal indicator is added to a solution it changes to a colour that shows the pH of the solution.

Diagram of the PH scale

Diagram of pH scale and universal indicator colours

These are some examples of common substances and their pH values.

The pH scale

pHexample substance
strong acidic0battery acid
1stomach acid
2lemon juice
4acid rain
5black coffee
neutral7pure water
8sea water
9baking powder
10milk of magnesia
12soapy water
strongly alkaline14drain cleaner

Bases and acids

Bases are substances that can react with acids and neutralise them. Bases such as metal oxides and metal hydroxides react with acids to form neutral products.

Examples of bases include:

  • copper(II) oxide
  • zinc hydroxide.

An alkali is a soluble base, a base that can dissolve in water. For example, copper(II) oxide is a base because it can neutralise acids but, because it does not dissolve in water, it is not an alkali.

Examples of alkalis include:

  • sodium hydroxide
  • potassium hydroxide.

All alkalis are bases.


When an alkali is added to an acid the pH of the mixture rises. This is because the alkali reacts with the acid to form neutral products. The reverse situation also happens too: when an acid is added to an alkali the pH of the mixture falls. This is because the acid reacts with the alkali to form neutral products.

A reaction in which acidity or alkalinity is removed is called neutralisation. A neutralisation involving an acid and a base (or alkali) always produces salt and water.

acid + base → salt + water

Hydrogen ions and pH

In all solution, all acids contain hydrogen ions, H+. The greater the concentration of these hydrogen ions, the lower the pH.

Naming salts

The name of the salt produced in a neutralisation reaction can be predicted. The first part of the name is ‘ammonium’ if the base used is ammonia. Otherwise, it is the name of the metal in the base. The second part of the name comes from the acid used:

  • chloride, if hydrochloric acid is used
  • nitrate, if nitric acid is used
  • sulfate, if sulfuric acid is used
  • phosphate, if phosphoric acid is used.

The table shows some examples.


acid+basesalt + water
hydrochloric acid+copper oxidecopper chloride + water
sulfuric acid+ sodium hydroxidesodium sulfate + water
nitric acid+calcium hydroxidecalcium nitrate + water
phosphoric acid+iron(III) oxideiron(III) phosphate + water

Carbonates and acids

Carbonates also neutralise acids. As well as a salt and water, carbon dioxide is also produced. The name of the salt can be predicted in just the same way.

For example:

hydrochloric acid + potassium carbonate → potassium chloride + water + carbon dioxide

Neutralisation equations- Higher tier

Ions in solution

  • acids in solution contain hydrogen ions, H+.

  • alkalis in solution contain hydroxide ions, OH-.

Neutralisation can be written as an ionic equation:

H+ + OH-Equilibrium symbolH2O

Neuralisation equations

You need to be able to write balanced symbol equations for neutralisation reactions between acids and bases, and between acids and carbonates. The equations will involve these acids:

  • hydrochloric acid, HCl
  • nitric acid, HNO3
  • sulfuric acid, H2SO4.

The equations will involve these bases:

  • ammonia, NH3- in solution, this is NH4OH
  • potassium hydroxide, KOH
  • sodium hydroxide, NaOH
  • copper(II) oxide, CuO.

They will involve these carbonates:

  • sodium carbonate, Na2CO3
  • calcium carbonate, CaCO3.

For example:

sulfuric acid + potassium hydroxide → potassium sulfate + water

H2SO4 + 2KOH → K2SO4 + 2H2O

nitric acid + calcium carbonate → calcium nitrate + water + carbon dioxide

2HNO3 + CaCO3 → Ca(NO3)2 + H2O + CO2

hydrochloric acid + ammonium hydroxide → ammonium chloride + water

HCl + NH4OH → NH4Cl + H2

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