Acid reactions and neutralisation
Predict products from the other reactant, identify gases with evidence, and distinguish neutralisation from the final pH.
On this page
Food Chemistry · 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 base
For example: An acid reacting with a metal hydroxide base forms salt and water.
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
Start with the substance reacting with the acid. The three familiar patterns below have different products, so the word “acid” alone does not tell you which gas, if any, will form.
- Acid with a suitable metal
The products are a salt and hydrogen. Some metals do not react readily, so use the suitable metal specified in the question.
- Acid with a metal oxide or metal hydroxide base
The products are salt and water. Gas production is not required for this neutralisation.
- Acid with a carbonate
The products are salt, water and carbon dioxide. Baking soda is a hydrogencarbonate and follows this gas pattern when it reacts with acid.
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. Carbon dioxide turns limewater milky. Use the gas-test evidence with the reactants, rather than deciding from fizzing alone.
A base can neutralise an acid. Metal oxides and hydroxides are examples of bases; carbonates can also act as bases, although their reactions additionally release carbon dioxide. Some antacids contain hydroxides and others contain carbonates, so the ingredients determine whether gas forms.
Adding alkali to acid reduces acidity as the two react. Too little leaves excess acid; too much can leave excess alkali. Neutralisation can therefore occur without the final mixture being neutral. Measure the final pH instead of assuming that every acid–base mixture reaches pH 7. Use the general product names here; you do not need specific salt names or chemical equations to explain these patterns.
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: A suitable metal reacting with acid produces hydrogen. Carbon dioxide belongs to the carbonate pattern. Fizzing shows that a gas formed, but the reactant or a gas test is needed to identify it.
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 suggests pH 7. The alkali is a dissolved metal hydroxide. Name the reaction and its products, and interpret the final indicator evidence.
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.
The acid and hydroxide base react to form salt and water. The indicator suggests that the final mixture is approximately neutral.
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 follows the carbonate gas pattern; acid plus base is neutralisation.
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
Baking soda reacting with acid releases carbon dioxide. The reaction between an acid and a base is neutralisation; final pH must be checked.
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 metal hydroxide base. The measured final pH is 7. Name the reaction and its products.
Need a hint?
Use the acid + base pattern.
Check your answer
Answer: This is neutralisation. Acid reacting with a metal hydroxide forms salt and water, and the measured final pH indicates a neutral mixture under these conditions.
Check: The reactants give the product pattern; the measured pH establishes the final classification.
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
Name the gas formed with a suitable metal and with a carbonate, and describe how each gas is identified.
Why can neutralisation occur while the final mixture remains acidic?
Correct this common mistake: “Acid plus metal produces carbon dioxide.”
Syllabus and review details
- SEC G1 Science 2027 · 2027
Content structure and syllabus content, PDF pages 8–17
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