CIE IGCSE Chemistry

Revision Notes

8.1.1 The Characteristic Properties of Acids & Bases

Properties of Acids

  • Acids have pH values of below 7, have a sour taste and are corrosive
  • In acidic conditions blue litmus paper turns red and methyl orange indicator turns red
  • Acids are substances that can neutralise a base, forming a salt and water
  • When acids react, they will lose electrons to form positively charged hydrogen ions (H+)
  • The presence of H+ ions is what makes a solution acidic


Example: Hydrochloric Acid

HCl (aq)   →    H+ (aq)    +    Cl- (aq)

Typical reactions of acids

Acids and metals

  • Only metals above hydrogen in the reactivity series will react with dilute acids.
  • When acids react with metals they form a salt and hydrogen gas:
Acid    +    Metal   →    Salt    +    Hydrogen

Examples of reaction between acids and metals:

Acids and metals table, IGCSE & GCSE Chemistry revision notes


Acids with bases (alkalis)

  • Metal oxides and metal hydroxides can act as bases
  • When they react with acid, a neutralisation reaction occurs
  • In all acid-base neutralisation reactions, salt and water are produced
Acid    +    Base   →    Salt    +    Water

Examples of reaction between acids and bases:

Bases-and-acids-table, IGCSE & GCSE Chemistry revision notes


Acids with metal carbonates

  • Acids will react with metal carbonates to form the corresponding metal salt, carbon dioxide and water:
Acid  +  Metal Carbonate → Salt  +  Carbon Dioxide  +  Water

Examples of reaction between acids and carbonates:

Acids with metal carbonates table, IGCSE & GCSE Chemistry revision notes

Properties of Bases

  • Bases have pH values of above 7
  • A base which is water-soluble is referred to as an alkali
  • In basic (alkaline) conditions red litmus paper turns blue and methyl orange indicator turns yellow
  • Bases are substances which can neutralise an acid, forming a salt and water
  • Bases are usually oxides or hydroxides of metals
  • When alkalis react, they gain electrons to form negative hydroxide ions (OH)
  • The presence of the OH ions is what makes the aqueous solution an alkali

Example: Sodium Hydroxide

NaOH (s)   →    Na+ (aq)    +    OH (aq)

Typical reactions of bases

Bases and acids

  • When they react with an acid,a neutralisation reaction occurs
  • Acids and bases react together in a neutralisation reaction and produce a salt and water:
Acid    +    Base   →    Salt    +    Water

Examples of reaction between bases and acids:

Bases-and-acids-table, IGCSE & GCSE Chemistry revision notes


Alkalis and ammonium salts

  • Ammonium salts undergo decomposition when warmed with an alkali
  • Even though ammonia is itself a weak base, it is very volatile and can easily be displaced from the salt by another alkali
  • A salt, water and ammonia are produced


NH4Cl + NaOH →NaCl + H2O + NH3
  • This reaction is used as a chemical test to confirm the presence of the ammonium ion (NH4+)
  • Alkali is added to the substance with gentle warming followed by the test for ammonia gas using damp red litmus paper
  • The litmus paper will turn from red to blue if ammonia is present

Neutrality, Relative Acidity & Alkalinity

The pH scale

  • The pH scale is a numerical scale which is used to show how acidic or alkaline a solution is
  • It goes from 1 – 14 (extremely acidic substances can have values of below 1)
  • All acids have pH values of below 7, all alkalis have pH values of above 7
  • The lower the pH then the more acidic the solution is
  • The higher the pH then the more alkaline the solution is
  • A solution of pH 7 is describe as being neutralg. water


The pH scale, IGCSE & GCSE Chemistry revision notesThe pH scale showing acidity, neutrality and alkalinity


Universal indicator

  • Universal indicator is a mixture of different indicators which is used to measure the pH
  • A drop is added to the solution and the colour is matched with a colour chart which indicates the pH which matches specific colours


Universal-indicator-and-the-pH-scale2, IGCSE & GCSE Chemistry revision notesThe pH scale with the Universal Indicator colours which can be used to determine the pH of a solution


The importance of pH and soil acidity

  • Soil pH is analysed to indicate the acidity or alkalinity of soil
  • Most plants favour a pH value of between 5 and 8
  • Changes in soil which cause a pH to be outside this range adversely affect plant processes resulting in reduced growth and crop yield
  • Soils may become acid from acid rain, overuse of fertilisers which contain ammonium salts or by the excessive breakdown of organic matter by bacteria
  • Crushed or powdered limestone (calcium carbonate) or lime (calcium oxide) or slaked lime (calcium hydroxide) is added to neutralise the excess acidity in the soil
  • The addition process must be carefully monitored though, as if added in excess, further damage could be done if the pH goes too high
Extended Only

Proton Transfer, Weak & Strong Acids & Bases

Proton transfer

  • The earlier definition of an acid and a base can be extended
  • In terms of proton transfer, we can further define each substance in how they interact with protons


  • Acids are proton donors as they ionize in solution producing protons, H+ ions
  • These H+ ions make the aqueous solution acidic

Bases (Alkalis)

  • Bases (alkalis) are proton acceptors as they ionize in solution producing OH ions which can accept protons
  • These OH ions make the aqueous solution alkaline


Transfer of protons in reaction between acid & base, IGCSE & GCSE Chemistry revision notesDiagram showing the role of acids and bases in the transfer of protons


Strong acids and bases

  • Acids and alkalis can be either strong or weak, depending on how many ions they produce when dissolved in water
  • Strong acids and bases ionize completely in water, producing solutions of very low pH for an acid or very high pH for a base
  • Strong acids include HCl and H2SO4 and strong bases include the Group I hydroxides

Weak acids and bases

  • Weak acids and bases partially ionize in water and produce pH values which are closer to the middle of the pH scale
  • Weak acids include organic acids such as ethanoic acid, CH3COOH and weak bases include aqueous ammonia
  • For both weak acids and bases, there is usually an equilibrium set-up between the molecules and their ions once they have been added to water
  • Example of a weak acid: propanoic acid
  • Example for a weak base: aqueous ammonia
NH3 + H2O ⇌ NH4+ + OH-
  • In both cases the equilibrium lies to the left, indicating a high concentration of intact acid / base molecules, with a low concentration of ions in solution

Effect of concentration on strong and weak acids and alkalis

  • A concentrated solution of either an acid or a base is one that contains a high number of acid or base molecules per dm3 of solution
  • It does not necessarily mean that the acid or base is strong though, as it may be made from a weak acid or base which does not dissociate completely
  • For example a dilute solution of HCl will be more acidic than a concentrated solution of ethanoic acid, since most of the HCl molecules dissociate but very few of the CH3COOH do

Author: Morgan

Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.

Join Save My Exams

Download all our Revision Notes as PDFs

Try a Free Sample of our revision notes as a printable PDF.

Join Now