Carboxylic Acids
Carboxylic acids for IGCSE 0620: CnH2n+1COOH, ethanoic acid as a weak acid, reactions with metals, bases and carbonates, and the route to esters.
The IGCSE Chemistry Specialist Team · founded by Rig
Written to the Cambridge IGCSE Chemistry (0620) syllabus and mark-scheme conventions. Last updated 2026-06-11.
Carboxylic acids are where organic chemistry and acid chemistry meet, and examiners exploit the join: a Paper 4 question can demand the formula of sodium ethanoate, the gas from a carbonate reaction, and an explanation of “weak”: three different chapters of revision in one part-question. The content is compact; the trick is recognising that ethanoic acid plays by every acid rule already learned.
The series
Carboxylic acids have the general formula CnH2n+1COOH and the −COOH (carboxyl) functional group. Named to four carbons: methanoic acid HCOOH, ethanoic acid CH3COOH, propanoic acid C2H5COOH, butanoic acid C3H7COOH. Counting trap: the COOH carbon is part of the name’s carbon count but not part of the n in the general formula. Full naming rules are on the homologous series page.
In a displayed formula of ethanoic acid, the COOH group must show the C=O double bond and the O−H bond. Examiner reports single out displayed COOH groups as a frequent source of dropped marks.
Ethanoic acid is made by oxidising ethanol: either with acidified aqueous potassium manganate(VII), or by bacterial oxidation (vinegar from wine). Both routes are detailed under alcohols.
A weak acid
Ethanoic acid is a weak acid: in aqueous solution it is only partially dissociated into ions:
CH3COOH ⇌ CH3COO− + H+
A strong acid, such as hydrochloric acid, is completely dissociated. The observable consequences: at the same concentration, ethanoic acid has a higher pH than hydrochloric acid and reacts more slowly with magnesium or a carbonate (same volume of gas eventually, slower fizz). The dissociation explanation in those terms is Supplement; “ethanoic acid is a weak acid” as a fact is Core.
Reactions: the standard acid set
Ethanoic acid does everything in the acid playbook from acids, bases and salts. Its salts are ethanoates.
| Reacts with | Products | Example |
|---|---|---|
| Reactive metals | salt + hydrogen | 2CH3COOH + Mg → (CH3COO)2Mg + H2 |
| Bases / alkalis | salt + water | CH3COOH + NaOH → CH3COONa + H2O |
| Carbonates | salt + water + carbon dioxide | 2CH3COOH + Na2CO3 → 2CH3COONa + H2O + CO2 |
Word equations are accepted at Core; the Extended paper expects symbol equations, and the 2:1 ratios with magnesium and carbonates are the balancing detail that separates grades. Test the gases as standard: hydrogen pops a lighted splint, carbon dioxide turns limewater milky.
On to esters (Supplement)
Heated with an alcohol and a few drops of concentrated sulfuric acid as catalyst, a carboxylic acid forms an ester plus water:
CH3COOH + C2H5OH ⇌ CH3COOC2H5 + H2O
Ethanoic acid + ethanol → ethyl ethanoate + water. The naming, structures and uses have their own page: esters.
Worked exam question
Dilute ethanoic acid and dilute hydrochloric acid of the same concentration are each added to excess sodium carbonate. (a) Name the gas produced and describe its test. [2] (b) State two differences you would observe or measure between the two reactions. [2] (c) Explain, in terms of ions, why ethanoic acid is described as a weak acid. [2]
Model answer, mark by mark:
- (a) M1: carbon dioxide. M2: bubble through limewater; it turns milky (cloudy/white precipitate).
- (b) M3: the ethanoic acid reaction is slower / fizzes less vigorously. M4: ethanoic acid has the higher pH (lower H+ concentration). (Same final volume of gas, since the moles of acid are equal: a third acceptable point.)
- (c) M5: ethanoic acid is only partially dissociated (ionised) in solution. M6: so the concentration of H+ ions is low / most molecules remain undissociated. “It’s not very strong” or “it’s diluted” scores zero. Weak and dilute are different words in chemistry.
The mistakes that cost marks
- Weak confused with dilute. Weak = partially dissociated; dilute = few moles per dm³. A concentrated weak acid exists. Mixing the terms voids the explanation mark.
- Hydrogen named from the carbonate reaction. Acid + carbonate gives CO2, not H2. Match the gas to the reactant: metals give hydrogen, carbonates give carbon dioxide.
- Salt names mangled. The salt of ethanoic acid and sodium hydroxide is sodium ethanoate, not “sodium ethanate” or “ethanoic sodium”.
- COOH drawn with single-bonded oxygens only. The displayed carboxyl group has one C=O and one O−H. Two singly bonded OH groups is a different (wrong) molecule.
How examiners want it phrased
| Student wording | Mark-scheme wording |
|---|---|
| ”Ethanoic acid isn’t very strong" | "Ethanoic acid is a weak acid: it is only partially dissociated into ions in aqueous solution" |
| "It fizzes with carbonate" | "Effervescence; the gas turns limewater milky, so it is carbon dioxide" |
| "It makes a salt with alkali" | "Ethanoic acid is neutralised by sodium hydroxide, forming sodium ethanoate and water" |
| "Vinegar comes from wine going off" | "Bacterial oxidation of ethanol produces ethanoic acid” |
The whole subtopic is the acid rules wearing an organic name. Revise the two chapters together, the way the organic chemistry pillar maps it. If acid-reaction products still need a guess rather than a rule, a free trial lesson sorts the metal/base/carbonate product grid in under an hour.
Test yourself
Cover the page above and answer these three. Click each answer to check.
Q1 (2 marks). Write the symbol equation for the reaction between ethanoic acid and sodium hydroxide, and name the salt formed.
Show answer
• CH3COOH + NaOH → CH3COONa + H2O [1] • sodium ethanoate [1]
Q2 (3 marks). A piece of magnesium ribbon is added to dilute ethanoic acid. State what you would observe, name the gas produced, and describe the test for it.
Show answer
• effervescence (bubbles) and the magnesium dissolves [1] • hydrogen [1] • a lighted splint gives a squeaky pop [1]
Q3 (2 marks). Name the oxidising agent used to convert ethanol to ethanoic acid in the laboratory, and state the colour change observed.
Show answer
• acidified aqueous potassium manganate(VII), with heating [1] • purple to colourless [1]
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Frequently asked questions
Why is ethanoic acid called a weak acid?
It is only partially dissociated into ions in aqueous solution; most molecules stay as CH3COOH. A strong acid like hydrochloric acid is completely dissociated. The strong/weak distinction in those terms is Supplement content.
What does ethanoic acid react with?
Everything a dilute acid reacts with, just more slowly: metals give a salt and hydrogen, bases and alkalis give a salt and water, carbonates give a salt, water and carbon dioxide. The salts are called ethanoates.
How is ethanoic acid made?
By oxidation of ethanol: either with acidified aqueous potassium manganate(VII) on heating, or by bacterial oxidation, which is how vinegar forms from wine.