# Acids, alkalis and titrations

> Chemistry · Edexcel IGCSE
> Source: https://www.owlsprep.com/study/edexcel-igcse-chemistry-s2-acids-alkalis-and-titrations/

This guide covers all Edexcel IGCSE 4CH1 required content for acids, alkalis and titrations, including indicator colours, pH classification, neutralisation and Chemistry-only titration procedures.

**Prerequisites:** [Ionic equation writing basics](https://www.owlsprep.com/study/edexcel-igcse-chemistry-s1-ionic-equations/); [Aqueous solution and state symbol definitions](https://www.owlsprep.com/study/edexcel-igcse-chemistry-s1-chemical-formulae-equations/)

## Learning objectives

- Recall exact colour changes of litmus, phenolphthalein and methyl orange in acidic and alkaline solutions
- Classify solutions using the official 0-14 pH scale bands for Edexcel IGCSE
- Explain how universal indicator measures approximate pH of aqueous solutions
- Link acids to H+ ions and alkalis to OH- ions in aqueous solution
- Recall and apply the ionic equation for neutralisation
- Describe the full acid-alkali titration procedure (Chemistry-only Paper 2C)

## Indicators for Acid/Alkali Identification

**Indicator** — A substance that undergoes a distinct, observable colour change when placed in acidic or alkaline solutions, used to distinguish between the two.

You are required to memorise the exact colour changes of three indicators for your exam, as specified by Edexcel:

| Indicator | Colour in acidic solution | Colour in alkaline solution |
| --- | --- | --- |
| Litmus | Red | Blue |
| Phenolphthalein | Colourless | Pink |
| Methyl orange | Red | Yellow |

> **tip**
>
> Exam questions will not accept alternative colour descriptions, so memorise the pairs in the table above exactly to gain full marks.

**Worked example:** A student adds phenolphthalein to a beaker of dilute sulfuric acid, then adds excess potassium hydroxide solution. State the colour changes observed.

1. First, identify the initial solution: dilute sulfuric acid is acidic. Phenolphthalein is colourless in acid, so the initial colour is colourless.
2. Excess potassium hydroxide is alkaline. Phenolphthalein turns pink in alkali, so the final colour is bright pink.
3. Answer: Colourless → Pink

## The pH Scale and Universal Indicator

**pH scale** — A 0-14 descriptive scale used to classify aqueous solutions based on their acidity or alkalinity, with 7 as the neutral midpoint.

- 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 different dyes that changes colour across the full 0-14 pH range. When added to an aqueous solution, its colour is matched to a reference chart to find the approximate pH of the solution.

> **warning**
>
> You must use the exact pH bands listed above for all exam answers. Alternative band classifications (e.g. classifying pH 3 as weakly acidic) will be marked incorrect.

**Worked example:** A solution of lemon juice has a pH of 2, while a solution of baking soda has a pH of 9. Classify each solution using the official Edexcel pH bands.

1. Lemon juice pH 2 falls in the 0–3 band, so it is strongly acidic.
2. Baking soda pH 9 falls in the 8–10 band, so it is weakly alkaline.
3. Answer: Lemon juice = strongly acidic; Baking soda = weakly alkaline

## Acids, Alkalis and Neutralisation Reactions

**Neutralisation** — The exothermic reaction between an acid and an alkali that produces only water and a salt, driven by the combination of hydrogen and hydroxide ions.

*Notation:* H^+_{(aq)} + OH^-_{(aq)} \rightarrow H_2O_{(l)}

- Acids dissociate in aqueous solution to release hydrogen ions ($H^+_{(aq)}$)
- Alkalis are soluble bases that dissociate in aqueous solution to release hydroxide ions ($OH^-_{(aq)}$)

The ionic equation for neutralisation is the same for all acid-alkali reactions, as spectator ions (ions that do not take part in the reaction) are omitted. Full balanced equations for specific acid reactions and salt naming are covered in the next sub-topic.

**Worked example:** Write the ionic equation for the neutralisation of dilute hydrochloric acid with sodium hydroxide solution.

1. Identify the reactive ions: hydrochloric acid provides $H^+_{(aq)}$ ions, sodium hydroxide provides $OH^-_{(aq)}$ ions.
2. Spectator ions ($Na^+$ and $Cl^-$) are omitted from the ionic equation, as they do not react.
3. $$H^+_{(aq)} + OH^-_{(aq)} \rightarrow H_2O_{(l)}$$

## Acid-Alkali Titration Procedure (Chemistry-only)

Titration is a quantitative practical technique used to find the exact volume of acid and alkali that neutralise each other, used to calculate unknown concentrations (calculations are covered in a separate sub-topic on moles and concentration). You must recall the full step-by-step procedure for structured exam questions.

1. Use a pipette and pipette filler to measure a fixed, known volume of one solution (e.g. alkali) into a clean conical flask.
2. Add 2–3 drops of a suitable indicator to the conical flask, swirl to mix evenly.
3. Fill a clean burette with the second solution (e.g. acid), record the initial burette reading to 0.05 cm³ precision (2 decimal places, ending in 0 or 5).
4. Slowly add the solution from the burette to the conical flask, swirling constantly, until the indicator changes colour permanently (the end-point).
5. Record the final burette reading, calculate the titre (volume added) by subtracting the initial reading from the final reading.
6. Repeat the titration until you get **2 concordant titres** (results within 0.10 cm³ of each other).
7. Calculate the mean titre using only concordant results, discard any anomalous results.

> **tip**
>
> Exam questions often ask for practical details, so make sure you mention 0.05 cm³ precision and concordant titre rules to gain full marks.

**Worked example:** A student collects the following titration results: 22.05 cm³, 22.10 cm³, 23.15 cm³. Identify concordant results and calculate the mean titre.

1. Concordant results are within 0.10 cm³ of each other: 22.05 cm³ and 22.10 cm³ are 0.05 cm³ apart, so they are concordant. The 23.15 cm³ result is anomalous and discarded.
2. Calculate the mean of the concordant results: $(22.05 + 22.10) / 2 = 22.075$ cm³, rounded to 22.08 cm³ or 22.10 cm³ as appropriate.
3. Answer: Concordant titres = 22.05 cm³ and 22.10 cm³; Mean titre = 22.08 cm³

*Calculator:* allowed

## Common pitfalls

- **Wrong:** Stating phenolphthalein is pink in acid, or methyl orange is orange in alkali.
  - Why it fails: The Edexcel specification only accepts exact, predefined colour changes for the three required indicators; alternative answers are marked incorrect.
  - Correct: Memorise the three required colour pairs exactly: litmus red/blue, phenolphthalein colourless/pink, methyl orange red/yellow.
- **Wrong:** Classifying a pH 3 solution as weakly acidic, or pH 10 as strongly alkaline.
  - Why it fails: Edexcel uses fixed, non-negotiable pH bands for classification that must be followed exactly to gain marks.
  - Correct: Recall the official bands: 0-3 strong acid, 4-6 weak acid, 7 neutral, 8-10 weak alkali, 11-14 strong alkali.
- **Wrong:** Writing a full balanced equation including spectator ions when asked for the neutralisation ionic equation.
  - Why it fails: The neutralisation ionic equation only includes the ions that react to form water, spectator ions are always omitted.
  - Correct: Always write the neutralisation ionic equation as $H^+_{(aq)} + OH^-_{(aq)} \rightarrow H_2O_{(l)}$ with full state symbols.
- **Wrong:** Recording burette readings to 1 decimal place, e.g. 24.1 cm³ instead of 24.10 cm³.
  - Why it fails: Burettes have a precision of 0.05 cm³, so readings must be recorded to 2 decimal places to be considered accurate.
  - Correct: Always record burette readings to 2 decimal places, ending in 0 or 5, e.g. 24.05 cm³, 24.10 cm³.
- **Wrong:** Including anomalous titration results when calculating the mean titre.
  - Why it fails: Anomalous results (not within 0.10 cm³ of other titres) are caused by practical error and will skew the mean calculation.
  - Correct: Only use concordant titres (within 0.10 cm³ of each other) to calculate the mean titre, discard all anomalous results.

## Cheatsheet

| Content Area | Key Exam Recall |
| --- | --- |
| Indicator Colours | Litmus: Red (acid)/Blue (alkali); Phenolphthalein: Colourless (acid)/Pink (alkali); Methyl orange: Red (acid)/Yellow (alkali) |
| pH Scale Bands | 0-3: Strong acid; 4-6: Weak acid; 7: Neutral; 8-10: Weak alkali; 11-14: Strong alkali |
| Neutralisation | Ionic equation: $H^+_{(aq)} + OH^-_{(aq)} \rightarrow H_2O_{(l)}$ |
| Titration Rules (Chemistry-only) | Read burettes to 0.05 cm³; Concordant titres = within 0.10 cm³; Discard anomalies for mean calculation |

## What's next

Now you have mastered the core content for acids, alkalis and titrations, you can move on to the next sub-topic covering reactions of acids with metals, bases and carbonates, including salt naming and preparation methods. You can also practise titration calculation questions, which use the procedure you learned here to find unknown concentrations of acids or alkalis. Make sure you memorise all required recall content (indicator colours, pH bands, neutralisation equation, titration steps) as these are frequently tested in both multiple choice and structured answer questions. For Chemistry-only candidates, practise writing full titration procedure answers to ensure you include all required steps for full marks.

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