An acid is a substance that produces H+ as the only positively charged ions when dissolved in water. In general, non-metalic oxides dissolve in water to produces acids.

Bases and Alkalis

A base is a substance that produces OH ions when dissolved in water. In general, metalic oxides and hydroxides are bases. A soluble base is called an alkali.

Neutral substances

A neutral substance is a substance that is neither acidic nor alkaline. Examples are pure water, solutions of many salts e.g. sodium chloride solution.


We used indicators to test for the acidity or alkalinity of a substance. Litmus, methyl orange and phenolphthalein are indicators used to test whether a substance is an acid or an alkali. They show us a colour change.

Colour changes of indicators
IndicatorColour in acidColour in alkali
methyl orangeredyellow

However all these indicators only tell us whether a substance is acid or alkaline. To measure how acidic or alkaline a substance is, we use another indicator known as a universal indicator. This indicator is a mixture of many other indicators. The colour shown by this indicator can be matched against a pH scale.

The pH scale runs from 0 to 14. A substance with a pH of less than 7 is an acid. One with a pH of greater than 7 is alkaline. One with a pH of 7 is said to be neither acid nor alkaline, that is neutral. Water is the most common example of a neutral substance.

pH scale vs colours on the universal indicator
Strong acidsWeak acidsNeutralWeak basesStrong bases
  • Hydrochloric acid
  • Sulphuric acid
  • Nitric acid
  • Citric acid
  • Ethanoic acid
  • Water
  • Sodium chloride solution
  • Limewater
  • Toothpaste
  • Ammonia solution
  • Sodium hydroxide solution

For more accurate work, a pH meter is used. A pH meter is a scientific instrument that measures the hydrogen-ion activity in water-based solutions , indicating its acidity or alkalinity expressed as pH.


781 pH/Ion Meter pH meter manufactured by Metrohm

Reactions of Acids and Bases


Neutralisation is a reaction of an acid and a base to produces water and a salt. The reaction of a carbonate with acids is called neutralisation but the reaction of an acid with a metal is not neutralisation.

Acid + base → salt + water

hydrochloric acid + sodium hydroxide → sodium chloride + water
HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)

Some common salts
AcidType of saltExample
Carbonic acidCarbonatesSodium carbonate (Na2CO3)
Ethanoic acidEthanoatesSodium ethanoate (CH3COONa)
Hydrochloric acidChloridesPotassium chloride (KCl)
Nitric acidNitratesPotassium nitrate (KNO3)
Sulphuric acidSulphatesSodium sulphate (Na2SO4)


  • A student makes magnesium sulphate by reacting magnesium with a dilute acid.
    1. Name the acid. ………[1]
    2. Describe the pH change of the mixture during the reaction. ……..[1]
    3. Name one other substance that reacts with this acid to make magnesium sulphate. [1]
    4. The reaction between magnesium and this acid is exothermic.
      State what is meant by the term exothermic. ….[1]
  • Another student reacts calcium with excess dilute acid in a beaker. Calcium sulphate forms as a solid in the beaker. Suggest the separation method that is used to separate the solid calcium sulphate from the excess acid. Explain how this separation method removes the solid from the liquid. ….. [2]
      1. Sulphuric acid
      2. The pH increases
      3. magnesium oxide  /  magnesium hydroxide  / magnesium carbonate
      4. releases heat energy
    1. filtration
      solid particles of calcium sulphate stays on the filter paper because they cannot pass through filter paper and the liquid passes through


    Titration is a method of volumetric analysis in which a volume of one reagent (for example an acid) is added to a known volume of another reagent (for example an alkali) slowly from a burette until an end-point is reached. If an acid and alkali are used, then an indicator is used to show that the end-point has been reached.

    Titration can be used to prepare salts from a soluble base and an acid. Because in this neutralisation reaction both reactants are in solution, titration is required. Acid is slowly and carefully added to a measured volume of alkali using a burette until the indicator, usually phenolphthalein, changes colour.

    An indicator is used to show when the alkali has been neutralised completely by the acid. This is called the end-point. Once you know where the end-point is, you can add the same volume of acid to the measured volume of alkali but this time without the indicator.


    The hazard symbol for corrosive substances according to directive 67/548/EWG by the European Chemicals Bureau, now known as the Consumer Products Safety
    and Quality (CPS&Q) Unit.

    Corrosive acids and alkalis, drops of sodium hydroxide
    solutions can readily decompose proteins and lipids in living tissues via amide hydrolysis and ester hydrolysis, which consequently cause chemical burns and may induce permanent blindness upon contact with eyes. Solid alkali can also express its corrosive nature if there is water, such as water vapor. Thus, protective equipment, like rubber gloves , safety clothing and eye protection , should always be used
    when handling these chemicals or their solutions. The standard first aid measures for alkali or acid spills on the skin is, as for other corrosives, irrigation with large quantities of water. Washing is continued for at least ten to fifteen minutes.

    Caustic burn

    Sodium hydroxide burn, image taken about 44 hours after exposure.

    Image source