Acid Reactions and Neutralisation
Other reactant → product pattern → evidence; check final pH separately.
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The core idea
Food Chemistry · Lesson 5 of 5 · about 25–35 min
What you need to understand
Other reactant → product pattern → evidence; check final pH separately.
Definitions
- neutralisation
- a reaction in which an acid reacts with a baseFor example: At this level, acid plus base can form salt and water only.
Key idea
- LookA solution at pH 3 is treated carefully with an alkali until universal indicator shows pH 7.
- ThinkOther reactant → product pattern → evidence; check final pH separately.
- DoUse reactant and gas evidence to distinguish metal, base and carbonate reactions and interpret the final pH.
Explanation
First identify what is reacting with the acid. A suitable metal gives salt and hydrogen; a carbonate gives salt, water and carbon dioxide; a suitable non-carbonate base gives salt and water only. Bubbles show gas production but do not identify the gas. In a teacher-led gas test, hydrogen gives a squeaky pop with a lighted splint, while carbon dioxide turns limewater milky.
Neutralisation is the reaction of an acid with a base. Adding alkali to acid reduces acidity, but too little leaves excess acid and too much leaves excess alkali. Check the final pH instead of assuming every acid–base mixture is neutral. At this level use the general product names salt and water; specific salt names and chemical equations are not needed.
Pause and say it: Other reactant → product pattern → evidence; check final pH separately.
Common mistake
Tempting wrong idea: Acid plus metal produces carbon dioxide.
Why it fails: Fizzing alone does not distinguish an acid–metal reaction from an acid–carbonate reaction. Nor does mixing an acid and a base prove pH 7.
Use this instead: The general products are a salt and hydrogen.
Worked Examples
Modelled example 1
Neutralise an acid
Problem
A solution at pH 3 is treated carefully with an alkali until universal indicator shows pH 7.
Study the worked solution
Reason from the evidence
Method
Use the pH change to identify neutralisation, then name the products from the reactants.
Reason
Identify the second reactant before naming the gas or products.
Working
- pH 3 is acidic.
- Adding alkali reduces acidity.
At the neutral point, acid and base have reacted to form salt and water only for this stated reaction.
State the conclusion
Working
The final solution is neutral; indicator colour evidence locates the endpoint approximately.
Common misconception 2
Which acid reaction makes which gas?
Learner response
“Acid with a metal produces carbon dioxide, while acid with a carbonate produces hydrogen.” Correct the swapped gases.
Use the second reactant
View solution step by step
Match both patterns
Method
Use hydrogen for a suitable metal and carbon dioxide for a carbonate.
Reason
The identity of the second reactant determines the characteristic gas.
Working
acid + metal → salt + hydrogen; acid + carbonate → salt + water + carbon dioxide
Challenge 3
Use indicator evidence in two situations
Problem
Sample A turns blue litmus red and universal indicator orange. Sample B turns red litmus blue. Classify both. Then predict the gas when A reacts with baking soda and name the reaction when A is carefully treated with B to neutral conditions.
Connect four pieces of evidence
Hints
Hint 1: classify from colour changes
Blue-to-red indicates acid; red-to-blue indicates alkali.
Hint 2: identify the other reactants
Baking soda is a carbonate; acid plus base can neutralise.
View solution step by step
Classify the samples
Method
Classify A as acidic and B as alkaline.Reason
Both litmus results point in the required directions; orange universal indicator also supports acidity.
Working
A: acid; B: alkaliPredict the reactions
Method
State carbon dioxide for A with baking soda and neutralisation for A with B.
Reason
A carbonate produces carbon dioxide, while a base can react with an acid to form salt and water at this level.
Working
acid + carbonate → CO₂; acid + base → neutralisation
Guided practice
Try it with support
A liquid turns blue litmus red and universal indicator suggests pH 3. Classify it, then predict the gas if it reacts with a suitable metal and with a carbonate.
- Use the second reactant to choose the gas.
Check the guided answer
Answer: It is acidic. With a suitable metal it forms hydrogen; with a carbonate it forms carbon dioxide as well as salt and water.
Check: Litmus classifies the liquid but does not provide the numerical pH.
Practise and continue
Practise this
An acid is treated carefully with a base until pH 7. Name the reaction and the G1 products.
Need a hint?
- Use the acid + base pattern.
Check your answer
Answer: The reaction is neutralisation, producing salt and water for the stated G1 reaction.
Check: Do not import chemical salt names or equations when they are not requested.
Think like a scientist
A little alkali is added to excess acid. Must the final mixture be pH 7? Explain.
Check the reasoning
No. Some acid remains, so the mixture can still be acidic even though neutralisation has occurred.
Remember: Excess alkali can instead leave an alkaline mixture; measure the final pH.
One-minute check
- Hide the page and explain acid reactions and neutralisation in your own words.
- Give a new example that is different from the worked example.
- Correct this common mistake: “Acid plus metal produces carbon dioxide.”