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 with a pH of 7, such as water, is described as being neutral


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.

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