Acids, alkalis and titrations

Chemistry· 2.28–2.33 (sub-topic 2(f), 2017 spec)· 25 min read

1. Indicators for Acid/Alkali Identification★★☆☆☆⏱ 5 min

📘 Definition

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

📐 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

2. The pH Scale and Universal Indicator★★☆☆☆⏱ 5 min

📘 Definition

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.

📐 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

3. Acids, Alkalis and Neutralisation Reactions★★★☆☆⏱ 7 min

📘 Definition

Neutralisation

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

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.

Acids dissociate in aqueous solution to release hydrogen ions ( $H_{(a q)}^{+}$ )
Alkalis are soluble bases that dissociate in aqueous solution to release hydroxide ions ( $O H_{(a q)}^{-}$ )

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 $H_{(a q)}^{+}$ ions, sodium hydroxide provides $O H_{(a q)}^{-}$ ions.
2
Spectator ions ( $N a^{+}$ and $C l^{-}$ ) are omitted from the ionic equation, as they do not react.
3
$H^+_{(aq)} + OH^-_{(aq)} \rightarrow H_2O_{(l)}$

4. Acid-Alkali Titration Procedure (Chemistry-only)★★★★☆HL only⏱ 8 min

✓ Calculator OK

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.

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

📐 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

5. Common Pitfalls

Wrong move:

Stating phenolphthalein is pink in acid, or methyl orange is orange in alkali.

Why:

The Edexcel specification only accepts exact, predefined colour changes for the three required indicators; alternative answers are marked incorrect.

Correct move:

Memorise the three required colour pairs exactly: litmus red/blue, phenolphthalein colourless/pink, methyl orange red/yellow.

Wrong move:

Classifying a pH 3 solution as weakly acidic, or pH 10 as strongly alkaline.

Why:

Edexcel uses fixed, non-negotiable pH bands for classification that must be followed exactly to gain marks.

Correct move:

Recall the official bands: 0-3 strong acid, 4-6 weak acid, 7 neutral, 8-10 weak alkali, 11-14 strong alkali.

Wrong move:

Writing a full balanced equation including spectator ions when asked for the neutralisation ionic equation.

Why:

The neutralisation ionic equation only includes the ions that react to form water, spectator ions are always omitted.

Correct move:

Always write the neutralisation ionic equation as $H_{(a q)}^{+} + O H_{(a q)}^{-} \to H_{2} O_{(l)}$ with full state symbols.

Wrong move:

Recording burette readings to 1 decimal place, e.g. 24.1 cm³ instead of 24.10 cm³.

Why:

Burettes have a precision of 0.05 cm³, so readings must be recorded to 2 decimal places to be considered accurate.

Correct move:

Always record burette readings to 2 decimal places, ending in 0 or 5, e.g. 24.05 cm³, 24.10 cm³.

Wrong move:

Including anomalous titration results when calculating the mean titre.

Why:

Anomalous results (not within 0.10 cm³ of other titres) are caused by practical error and will skew the mean calculation.

Correct move:

Only use concordant titres (within 0.10 cm³ of each other) to calculate the mean titre, discard all anomalous results.

6. Quick Reference 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: $H_{(a q)}^{+} + O H_{(a q)}^{-} \to H_{2} O_{(l)}$
Titration Rules (Chemistry-only)	Read burettes to 0.05 cm³; Concordant titres = within 0.10 cm³; Discard anomalies for mean calculation

7. Frequently Asked

Which indicator colours do I need to memorise for the exam?

You only need to recall three exact indicator colour pairs:

Litmus: red in acid, blue in alkali
Phenolphthalein: colourless in acid, pink in alkali
Methyl orange: red in acid, yellow in alkali No other indicators are assessed for this sub-topic.

Do I need to calculate pH using -log[H+] for this specification?

No, pH is only used as a descriptive 0-14 scale for this Edexcel IGCSE sub-topic. All pH calculation content is out of scope and will not be tested.

Is the titration procedure tested for Double Science candidates?

No, the full titration method (syllabus point 2.33C) is only assessed in the Chemistry-only 4CH1 Paper 2C. Double Science 4SD0 candidates only need to learn core content 2.28–2.32.

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.