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Acids and bases

What you need to know

Reflections and Exam tips


Acids and bases

pH scale

The pH scale tells us the acidity or alkalinity of a solution.

The scale shows that acids have a pH value of below seven.

Alkalis have pH values greater than seven, and anything with a pH value of exactly seven is neutral.

pH is defined as the negative logarithm of the hydrogen ion concentration pH = – log [H+],  [H+] = 10-pH and provides a measurement of the hydrogen ion concentration, [H+].

An INDICATOR is a dye which changes colour according to whether it is in an acidic, neutral or alkaline solution.

Acids are compounds that produce hydrogen (H+) ions in solution.

Alkalis are substances that produce hydroxide (OH-) ions in solution.


A neutralisation reaction is where an acid reacts with a base to produce a neutral solution of a salt and water.

A salt is a compound made when the hydrogen in an acid is replaced by a metal.

Neutralising sulphuric acid produces salts called SULPHATES. Sulphuric acid (aq) has the formula H2SO4 and contains two H+ ions and one SO42-.

Neutralising nitric acid produces salts called NITRATES. Nitric acid (aq) has the formula HNO3 and contains the ions H+ and NO3-.

Neutralising hydrochloric acid produces salts called CHLORIDES. Hydrochloric acid(aq) has the formula HCl and contains the ions H+ and Cl-.


When two aqueous solutions are mixed, they may react to form a product that is insoluble in water. The solid is called a precipitate and the reaction is called a precipitation reaction.

Insoluble salts

  1. The acid and alkali are mixed forming the salt by precipitation
  2. The mixture of products is filtered, trapping the salt
  3. The salt is rinsed in distilled water, then left to dry.

Soluble salts from bases

  1. Base is added to acid and heated until it dissolves
  2. More base is added until no more will dissolve using up all the acid
  3. The mixture is filtered to remove the excess base
  4. The filtrate is heated to evaporate some of the water. When it cools, salt crystals will form.


What you want to achieve:

Understand the difference between a base and an alkali
Be able to name a salt formed in a reaction
Understand the significance of the pH scale and know what it measures

Be able to define and give an ionic equation for the process of neutralisation

Give ionic equations to show substances acting as acids and alkalis

Be able to explain the difference between weak and strong acids

Be able to explain why certain steps are taken in the synthesis of a salt

Be able to write ionic equations to show the formation of insoluble salts

Be able to write ionic equations to show the formation of ammonium ions and hydroxide ions in water