2.3Unit 1 · Inorganic Chemistry
Acids, alkalis & titrations — Edexcel IGCSE Chemistry Modular - UNIT 1 Revision Notes & Topical Questions
10 min read 7 parts Lesson 3 of 4 1 quiz
Lesson summary
Describe the colours of common indicators in acids and alkalis.Acids in solution contain H+ ions; alkalis contain OH− ions.Ionic equation for neutralisationH+(aq) + OH−(aq) → H2O(l).acid + alkali → salt + water, e.g.
- Phenolphthalein
- Two-colour indicators
- Titrations
- Not every acid reaction is neutralisation
- Neutralising acidic soil
- Accurate burette technique
Section 1 of 7Acids, alkalis & titrations
1Indicators
Indicators show whether a solution is acidic, neutral or alkaline. Litmus turns red in acid and blue in alkali. Universal indicator gives a range of colours from red (pH 0) through green (pH 7) to purple (pH 14), so it estimates pH.
2Two-colour indicators
Two-colour indicators have only two colours and change sharply over a small pH range: phenolphthalein is colourless in acid and pink in alkali; methyl orange is red in acid and yellow in alkali. This sudden change makes them ideal for titrations.
3Acids
Acids release hydrogen ions (H+) when dissolved in water, so acids are proton donors. Bases are proton acceptors, such as metal oxides and hydroxides. Alkalis are soluble bases that release OH− ions.
4Neutralisation
H+ ions from the acid react with OH− ions from the alkali to make water. The remaining ions form a salt, e.g. H2SO4 + 2KOH → K2SO4 + 2H2O.
5
The lower the pH, the higher the concentration of H+ ions; the higher the pH, the higher the concentration of OH− ions.
6Titrations
Titrations measure exactly how much acid neutralises a known volume of alkali. A pipette measures a fixed volume accurately; a burette measures variable volumes to 0.05 cm3. A white tile under the flask makes the colour change easier to see.
7
A first rough titration finds the approximate end point; accurate runs then add acid dropwise near it. Results are concordant when within 0.10 cm3; anomalous titres are left out of the mean.
8
Titrations can be used to prepare a pure soluble salt: find the volumes needed with an indicator, then repeat with the same volumes but no indicator, and crystallise the salt.
9Litmus is not used in titrations
Litmus changes colour gradually and looks purple near neutral, so the end point is hard to see. It is useful instead as red or blue indicator paper for testing solutions and damp gases.
10Not every acid reaction is neutralisation
Neutralisation must produce water from H+ and OH− ions. A metal reacting with an acid makes a salt and hydrogen but no water, so it is not a neutralisation.
11Neutralising acidic soil
Farmers add bases such as powdered limestone (calcium carbonate) or quicklime (calcium oxide) to soil that is too acidic, because many crops grow poorly at low pH.
12Accurate burette technique
Remove the filling funnel before starting, run a little solution through the tap to clear air bubbles, and read the bottom of the meniscus at eye level. Record readings to the nearest 0.05 cm3.
Section 2 of 7Process & sequence
1
Rinse a pipette with the alkali, then use it to transfer exactly 25.0 cm3 into a conical flask.
2
Add a few drops of a suitable two-colour indicator (e.g. phenolphthalein or methyl orange).
3
Rinse and fill a burette with acid; record the initial reading to 0.05 cm3.
4
Add acid while swirling the flask; add dropwise near the end point.
5
Stop when the indicator just changes colour permanently; record the final reading.
6
Repeat until you have concordant titres (within 0.10 cm3).
7
Calculate the mean titre from concordant results only.
Acid–alkali titrationwrite these steps in this order; each step is a marking point.
1
Pipette 25.0 cm3 alkali
2
Add two-colour indicator
3
Fill burette with acid
4
Add acid, swirl
5
Stop at colour change
6
Repeat for concordant titres
Section 3 of 7Compare
| Indicator | Colour in acid | Colour in alkali |
|---|---|---|
| Litmus | Red | Blue |
| Methyl orange | Red | Yellow |
| Phenolphthalein | Colourless | Pink |
Universal indicator is not used in titrations because its gradual colour change gives no sharp end point.
| pH | Colour | Type of solution |
|---|---|---|
| 0–2 | Red | Strong acid |
| 3–6 | Orange / yellow | Weak acid |
| 7 | Green | Neutral |
| 8–11 | Blue | Weak alkali |
| 12–14 | Purple | Strong alkali |
Section 4 of 7Worked examples
Titres
24.60 (rough), 23.85, 23.95, 23.90 cm3. Calculate the mean titre.
- Ignore the rough titre.
- Concordant results: 23.85, 23.95, 23.90 (all within 0.10).
- Mean = (23.85 + 23.95 + 23.90) ÷ 3 = 23.90 cm3.
Sodium nitrate
Write the ionic equation for the reaction between nitric acid and sodium hydroxide, and name the salt.
- H+(aq) + OH−(aq) → H2O(l). The salt is sodium nitrate, NaNO3.
Section 5 of 7Key terms
Section 6 of 7Command terms
Command terms
Give / State / Name
Recall a fact; one idea per mark.
Define
Give the exact meaning using the specification keywords.
Describe
Give an account of what happens / the pattern in data, quoting values; no reasons needed.
Explain
Give reasons with linked statements (cause → effect). Each linked step is usually a separate marking point.
Suggest
Use knowledge in a new context; more than one answer may be accepted.
Compare
Similarities and differences; each point must refer to both items.
Calculate
Show working; include units; check significant figures. Method marks are given even if the final answer is wrong.
Discuss
Explore the issue from different angles and link the points.
Evaluate / Assess
Weigh strengths and weaknesses and reach a supported judgement.
Deduce
Draw a conclusion from the information given.
Justify
Support a statement/choice with evidence.
Section 7 of 7Exam technique
Keywords the mark scheme looks for in this lesson
Keywords the mark scheme looks for in this lesson
Acid · Base · Alkali · Neutralisation · pH scale · Indicator · Universal indicator · Titration · End point · Concordant results
Learn each definition in the Key words tab exactly; a missing keyword in a definition usually loses the mark.
How to get full marks (examiner guidance)
How to get full marks (examiner guidance)
Use specification keywords exactly (e.g. electronegativity, nucleophile, dynamic equilibrium, activation energy).
Link ideas
Link ideas for “explain” each marking point is usually a cause → consequence step; unlinked facts often score nothing.
Use the information in the question
“discuss/evaluate” answers must use the data provided and match the number of marks.
Convert units carefully (mg → g, cm³ → dm³) and show working for calculations.
Avoid vague words
“it”, “stuff”, “things”, “goes up”, “energy is made”. Name the substance and the process.
Do not extrapolate a graph line beyond the data or to zero unless asked.
Six-mark extended responses are marked on content AND a logical, linked line of reasoning.
Marked practiceQuizzes for this lesson
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