All revision notes topics

2.6 Acids, Alkalis & TitrationsEdexcel IGCSE Chemistry: Revision notes

Section 1

How can indicators distinguish acids and alkalis?

Indicators change colour depending on whether a solution is acidic, alkaline or neutral:

IndicatorAcidAlkaliNeutral
LitmusRedBluePurple
PhenolphthaleinColourlessPinkColourless
Methyl orangeRedYellowOrange

These simple indicators show whether a solution is acidic or alkaline, but do not give a precise pH value.

Key termsindicator
Common mistake

Phenolphthalein is colourless in BOTH acidic and neutral solutions — it only turns pink in alkali, so it cannot distinguish acid from neutral.

Section 2

What is the pH scale?

The pH scale runs from 0 to 14 and measures how acidic or alkaline a solution is:

  • Strongly acidic: pH 0–3
  • Weakly acidic: pH 4–6
  • Neutral: pH 7
  • Weakly alkaline: pH 8–10
  • Strongly alkaline: pH 11–14

Universal indicator is a mixture of dyes that gives a range of colours across the whole pH scale (red through orange/yellow/green to blue/purple), allowing the approximate pH of a solution to be estimated by comparing the colour produced to a chart.

Key termspH scaleuniversal indicator

Section 3

What makes a solution acidic or alkaline?

  • Acids in aqueous solution are a source of hydrogen ions (H+).
  • Alkalis in aqueous solution are a source of hydroxide ions (OH-).

Alkalis can neutralise acids: the H+ ions from the acid react with the OH- ions from the alkali to form water, in the ionic equation:

H+(aq) + OH-(aq) → H2O(l)

Key termshydrogen ionhydroxide ionneutralisation

Section 4

How do you carry out an acid-alkali titration?

A titration finds the exact volume of one solution needed to neutralise a known volume of another, often to find an unknown concentration.

  1. Use a pipette to measure an accurate, known volume of the alkali into a conical flask.
  2. Add a few drops of a suitable indicator (e.g. phenolphthalein or methyl orange).
  3. Fill a burette with the acid and record the initial reading.
  4. Add the acid from the burette to the flask slowly, swirling constantly, until the indicator just changes colour (the end point).
  5. Record the final burette reading; calculate the volume of acid added (titre).
  6. Repeat the titration until concordant results (within 0.10 cm3 of each other) are obtained, and calculate a mean titre from the concordant results.
Key termstitrationend pointtitre
Exam tip

Examiners give marks for practical precision: swirling the flask, adding acid dropwise near the end point, and repeating for concordant results.

Must Know

  • Litmus: red in acid, blue in alkali. Phenolphthalein: colourless in acid, pink in alkali. Methyl orange: red in acid, yellow in alkali.
  • pH scale (0–14): strongly acidic 0–3, weakly acidic 4–6, neutral 7, weakly alkaline 8–10, strongly alkaline 11–14.
  • Universal indicator gives a range of colours to estimate approximate pH.
  • Acids are a source of H+ ions in solution; alkalis are a source of OH- ions in solution.
  • Neutralisation: H+(aq) + OH-(aq) → H2O(l).
  • Titration procedure: pipette a known volume of alkali, add indicator, titrate with acid from a burette to the end point, repeat for concordant results, calculate mean titre.

That's the notes covered.

Carry on to the next subtopic.