Distinguish acid reactions by their products
Compare metal, alkali and carbonate reactions using salt formation, water and the gas produced.
Section 18 · Carboxylic acids and derivatives
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Track the carbon skeleton, name reagents and conditions, and show products and observations. Written practice is self-checked
Before you begin
- Use alcohol oxidation and carbonyl chemistry from Topics 16–17, functional-group names, acid–base equations and balanced reaction formulae.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which reagent gives a carboxylate, water and CO₂?
What you will learn
- Compare three acid reactions
- Infer reagent from products
- Balance a carboxylate equation
Each reagent class gives a distinct product pattern
Reactive metal: carboxylate salt + H₂. Alkali: carboxylate salt + H₂O. Carbonate: carboxylate salt + H₂O + CO₂.
These patterns distinguish the chemistry even when the same acid and metal cation appear in the salt.
Use balanced equations as evidence
A carbonate ion needs two protons, so two molecules of a monoprotic acid react per carbonate. A reactive metal's charge sets the carboxylate salt formula.
Effervescence alone does not identify a gas; use the known reagent or a suitable gas test to distinguish H₂ and CO₂.
Pause and explain
Which product pattern identifies neutralisation by an alkali?
Worked example
An acid forms sodium propanoate and a gas that turns limewater milky. Which reagent class was used?
Show the worked solution
- Milky limewater indicates CO₂.
- Carboxylic acids produce CO₂ with carbonates.
- The salt confirms the acid formed propanoate.
Answer A carbonate was used; carboxylate salt, water and CO₂ form.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
An acid yields potassium carboxylate and hydrogen. Which reagent class is consistent?
Give me a hint
Match the gas before considering the salt.
Compare my reasoning
- Hydrogen is the metal reaction gas.
- The metal supplies K⁺.
- The acid residue becomes RCOO⁻.
A reactive potassium-containing metal; the products include potassium carboxylate and H₂.
Look for these in your work
- Gas matched
- Reagent class identified
- Salt interpreted
A reaction gives sodium ethanoate and water, with no gas. Identify the likely reagent.
Give me a hint
Compare neutralisation with carbonate reaction.
Compare my reasoning
- Water is a neutralisation product.
- Carbonate would also give CO₂.
- NaOH supplies sodium ions.
Sodium hydroxide; sodium ethanoate and water form without a gas.
Look for these in your work
- Alkali identified
- Products stated
- No-gas evidence used
A learner identifies CO₂ after an acid reacts with magnesium because bubbles appear. What further evidence or correction is needed?
Give me a hint
Magnesium is a reactive metal.
Compare my reasoning
- Metal plus acid normally gives H₂.
- Use the reagent identity to predict hydrogen.
- A limewater test would distinguish CO₂ from H₂.
With magnesium, hydrogen is expected; bubbling alone does not prove CO₂. Test the gas or use the reagent-based product pattern.
Look for these in your work
- Gas prediction corrected
- Observation limitation noted
- Evidence suggested
Common mistakes
- Calling all acid bubbles hydrogen
- Forgetting water with carbonate
- Ignoring the reagent class
What can you explain now?
Which reagent gives a carboxylate, water and CO₂?
Compare with the explanation
A carbonate
Acid plus carbonate produces carbon dioxide and water.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
What products distinguish a reactive metal, alkali and carbonate reacting with a carboxylic acid?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.