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Doc Brown's Chemistry  GCSE/IGCSE Science–Chemistry Revision Notes

pH scale of acidity and alkalinity, acids, bases–alkalis, salts and neutralisation

2. pH Scale, indicators, acids, alkalis (bases), neutralisation its ionic theory introduction

This page introduces and explains the pH scale measuring the relative acidity and alkalinity of aqueous solutions, that is solutions of substances dissolved in water. The use of indicators is described and several well known indicators are tabulated showing their different colours in solutions of different pH. The ionic theory of acids, bases and neutralisation is simply described and why explains why solutions are either acid, neutral or alkaline.

GCSE/IGCSE Sub–index: Index of all pH, Acids, Alkalis, Salts Notes 1. Examples of acid–alkali chemistry : 2. pH scale, indicators, ionic theory of acids–alkali neutralisation : 3. pH examples of acid, neutral or alkaline solutions : 4. Acid reactions with metals/oxides/hydroxides/carbonates and neutralisation reactions : 5. Reactions of bases–alkalis like sodium hydroxide : 6. Four methods of making salts : 7. Changes in pH in a neutralisation : 8. Important formulae, salt solubility and water of crystallisation : 9. Further examples of word/symbol equations for salt preparations : 10. More on Acid–Base Theory and Weak and Strong Acids

See also Advanced Level Chemistry Students Acid–Base Revision Notes – use index

2. The pH scale, indicators acids, alkalis (bases), neutralisation & ionic theory

2a. Introduction to the pH scale What is the pH scale?

(c) doc b

The colours observed in solutions when universal indicator is added

  • The pH scale is a measure of the relative acidity or alkalinity of a solution (see diagram).

    • So, knowing the pH of a solution, you know how acid or alkaline it is by reference to the pH scale (diagram above) or whether the solution is neutral.

    • The smaller the pH number, the more acid it is, the greater the pH number, the more alkaline it is, and if the pH is close to 7, you have a more or less neutral solution that has neither acidic or alkaline chemical properties.

    • Lots of examples of solution pH values are tabulated in section 3. and everyday examples of acid/alkaline chemistry are described in section 1.

  • pH can be approximately measured using indicator solution by putting a few drops of universal indicator into a solution and comparing the colour formed with a standard chart (picture above).

  • You can also used paper impregnated with an indicator solution (pH paper), the paper is dipped in the solution and again the colour matched with a pH chart.

    • This is quite handy for testing soil mixed and shaken with water.

    • You can get special soil testing kits which use indicator solution and the colour of the indicator in the water is matched with a chart after the soil has settled out.

  • pH can be very accurately measured with a special instrument called a pH meter using a glass electrode which is calibrated with buffer solutions of accurately known pH.


  • An indicator is a dye substance or mixture of coloured substances that when added to the solution gives a different colour depending on the pH of the solution.

    • Universal indicator solution or paper, is prepared from mixing several indicators to give a variety of colours to match the pH.

    • The mixture of dyes responds to changes in pH, so depending on what the pH is, i.e. how acid, how alkaline or neutral the solution is, the indicator tells which it is.

      • Not only that, an indicator like universal indicator' can tell you how strongly acid or strongly alkaline the solution is by giving you the pH to about the nearest indicator.

      • A calibrated instrument called a pH meter can give the pH to two decimal places.

        • pH meters are calibrated using buffer solutions which have an accurately known pH.

    • It is a very handy indicator for showing whether the solution is very weakly/strongly acidic (pH <7) or alkaline (pH > 7) or neutral (pH = 7) and gives the pH to the nearest pH unit.

    • (c) doc b

    • The diagram above gives the sort of range of colours you get from using universal indicator, which is a complex mixture of different dye molecules that respond to changes in pH.

  • Theoretically there is no limit to the pH scale, but most solutions are between pH 0 and pH 14.

    • For example, looking at the 'extremes', 1M hydrochloric acid (HCl) has a pH of 0 and 10M HCl has a pH of –1 and these would be described as strongly acidic solutions.

    • 1M sodium hydroxide (NaOH) has a pH of 14, but 10M potassium hydroxide (KOH) has a pH of 15 and these would be described as strongly alkaline solutions.

    • The closer the pH is to 7, the less strong is the acid or alkali.

    • However the solubility limits of substances in water ensures that its almost impossible to get below –1 or above 15 and most laboratory measurements will be in the range pH 1 to pH 14

    • .
  • Note 1: M is the old shorthand for solubility in mol/litre or mol dm–3.

  • Note 2: The pH scale is known as a logarithmic scale of base 10.

    • At GCSE/IGCSE level, to put it more simply, a change of one pH unit means a 10x change in the acidity or alkalinity of the solution

      • e.g. from pH 5 to pH 2 means an increase in acidity of 1000x

      • or to change from pH 13 to pH 11 means to become 100x less alkaline.).

Other common indicators used in the laboratory (c) doc b often used in titrations – e.g. salt preparation (a)


colour in acid pH<7 colour in neutral pH=7 colour in alkali pH >7


red 'purple' blue
phenolphthalein* colourless colourless >9 pink
methyl orange* <3.5 red, orange about pH 5, > 6 yellow yellow yellow
methyl red* <5 red, orange, >6 yellow yellow yellow
bromothymol blue* <6 yellow green >8 blue

2b. Introduction to Acid–Base (including Alkalis) Theory

  • Water is a neutral liquid with a pH of 7 (green with universal indicator).

  • When a substance dissolves in water it forms an aqueous (aq) solution that may be acidic, neutral or alkaline.

  • Acidic solutions have a pH of less than 7, and the lower the number, the stronger the acid it, or the more acidic the solution.

    • The colour can range from orange–yellow (pH 3–6) for partially ionised weak acids like ethanoic acid (vinegar) and carbonated water.

    • Strong acids like hydrochloric, sulphuric and nitric are fully ionised and give a pH 1 or less and a red colour with universal indicator or litmus paper.

  • Neutral solutions have a pH of 7. These are quite often solutions of salts, which are themselves formed from neutralising acids and bases.

  • The 'opposite' of an acid is called a base. Some bases are soluble in water to give alkaline solutions – these are known as alkalis.

  • Alkaline solutions have a pH of over 7 and the higher the pH the stronger is the alkali, the more alkaline is the solution.

    • Weak alkalis (soluble bases) like ammonia give a pH of 10–11 but strong alkalis (soluble bases) like sodium hydroxide give a pH of 13–14.

    • Alkalis give blue–purple–violet colour with universal indicator or litmus paper.

  • NEUTRALISATION usually involves mixing an acid (pH <7) with a base or alkali (pH > 7) which react to form a neutral SALT solution of pH 7

    • in general the word equation for a neutralisation reaction is


  • More details below


2c. More advanced Acid–Base Theory

  • THE IONIC THEORY of ACIDS and ALKALIS – a brief introduction and a few technical terms

    • The proton (H+) donation–acceptance theory of acids and bases (Bronsted–Lowry) is covered in Section 10.

    • Acids are substances that form hydrogen ions, H+(aq), when dissolved in water e.g.

      • hydrochloric acid HCl gives H+(aq) and Cl(aq) ions in water.

        • (aqueous solution of hydrogen ions and chloride ions, pH reduced to <7)

      • sulfuric/sulphuric acid H2SO4 gives 2H+(aq) and SO42–(aq) ions in water.

        • (aqueous solution of hydrogen ions and sulfate ions, pH reduced to <7)

      • nitric acid HNO3 gives H+(aq) and NO3(aq) ions in water.

        • (aqueous solution of hydrogen ions and nitrate ions)

    • Alkalis are substances that form hydroxide ions (OH(aq)) in water e.g.

      • sodium hydroxide NaOH gives Na+(aq) and OH(aq) ions in water.

        • (aqueous solution of sodium ions and hydroxide ions, pH increased to >7)

      • calcium hydroxide Ca(OH)2 gives Ca2+(aq) and 2OH(aq)  ions in water.

        • (aqueous solution of calcium ions and hydroxide ions, pH increased to >7))

      • Note that an alkali is a base soluble in water.

        • An insoluble base like copper(II) oxide, CuO, will NOT affect the pH of water (pH 7 neutral), i.e. it will not cause the formation of either hydrogen ions or hydroxide ions on mixing with water.

    • The majority of liquid water consists of covalent H2O molecules, but there are trace quantities of H+ and OH ions from the self–ionisation of water,

      • H2O(l) H+(aq) + OH(aq)

        • (about 1 in 200 million does this!, the reaction is reversible, so the longer half-arrow to the left tells you that most water remains as water molecules!)

      • BUT they are of equal concentration and so water is neutral at pH 7.

    • In acid solutions there are more H+ ions than OH ions.

    • In alkaline solution there are more OH ions than H+ ions.

    • When alkalis and acids react, the 'general word' or 'molecular formula' equation might be for NEUTRALISATION ...

      • ACID + ALKALI ==> SALT + WATER... e.g.

      • hydrochloric acid + sodium hydroxide ==> sodium chloride + water

      • HCl(aq) + NaOH(aq) ==> NaCl(aq) + H2O(l)

      • BUT the ionic equation for ANY neutralisation is

      • hydrogen ion + hydroxide ion ==> water

      • H+(aq)  + OH(aq)  ==> H2O(l)

      • because all acids form hydrogen ions in water and all alkalis (soluble bases) form hydroxide ions in water.

      • and, in this case, the remaining ions e.g. sodium Na+(aq) and chloride Cl(aq) become the salt crystals of sodium chloride NaCl(s) on evaporating the water.

        • So the salt is formed from the residual ions when all the hydrogen ions and hydroxide ions have reacted.

    • BASES e.g. oxides, hydroxides and carbonates, are substances that react and neutralise acids to form salts and water.

      • Bases which are soluble in water are called alkalis e.g. NaOH sodium hydroxide, KOH potassium hydroxide or Ca(OH)2 calcium hydroxide.

      • Bases which are water insoluble include CuO copper(II) oxide, MgO magnesium oxide.

    • After a neutralisation, the salt solutions consist of a mixture of positive and negative ions (and their names are in the salt name!) e.g.

      • sodium chloride (NaCl) is a mixture of  Na+ and Cl ions in the ratio 1:1

      • calcium chloride (CaCl2) is a mix of Ca2+ and Cl ions of ration 1:2

      • magnesium nitrate (Mg(NO3)2) is a mix of Mg2+ and NO3 ions in the ratio 1:2

      • aluminium sulphate (Al2(SO4)3) consists of Al3+ and SO42– ions in the ratio 2:3

    • More in the notes (c) doc b GCSE/IGCSE chemistry higher level notes for the advanced proton/hydrogen ion theory of acids and bases (Bronsted–Lowry theory


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How to measure the pH of solutions – indicator paper, pH meter, measuring against the standard pH scale, explaining the ionic theory of neutralisation

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