Year 10 Science · Unit 2 · Lesson 4

Neutralisation Reactions

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Learning Goals

Trace the sequential steps of how an antacid neutralises stomach acid
Apply neutralisation chemistry to an agricultural soil pH problem
Calculate and explain lime application using neutralisation principles

Warm Up

Order the steps

Number the steps from 1 to 6 to show the correct sequence of how an antacid tablet works in the stomach. Step 1 = what happens first.

Order	Step
	The pH of the stomach contents rises toward a less acidic level, reducing discomfort.
	Stomach is acidic (HCl present at pH ~1.5) causing discomfort.
	Salt and water are formed as products of the neutralisation reaction.
	The antacid tablet (containing Mg(OH)₂ or CaCO₃) dissolves in the stomach liquid.
	The base in the antacid reacts with the H⁺ ions from HCl, a neutralisation reaction.
	Person swallows an antacid tablet.

Your Turn

Real-world context

In the Western Australian wheat belt, decades of cropping and nitrogen fertiliser use have left many soils with a pH of 4.5–5.0, far too acidic for wheat, which grows best at pH 6.0–6.5. Farmers apply agricultural lime (calcium carbonate, CaCO₃) to raise soil pH through a neutralisation reaction. According to the WA Department of Primary Industries and Regional Development (2023), approximately 2 million hectares of WA cropland require liming treatment each year.

(a) Write the word equation for the reaction between calcium carbonate and sulfuric acid (one of the acids causing soil acidity). Name the salt produced in this neutralisation reaction.

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(b) A farmer needs to raise the soil pH from 4.8 to 6.5 to grow wheat. Explain using neutralisation chemistry why applying lime (a base) achieves this. Your answer should reference H⁺ ions and the ion equation: H⁺(aq) + OH⁻(aq) → H₂O(l).

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(c) Why would adding too much lime (raising pH well above 7) also harm wheat crops? Use your knowledge of pH and plant nutrition to explain.

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Show What You Know

1. A bee sting is acidic (formic acid). A wasp sting is alkaline. Explain why a baking soda paste (a base) helps relieve a bee sting, but vinegar (an acid) helps relieve a wasp sting. Use the concept of neutralisation in your answer.

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Wrap Up

In one sentence, what was the main idea of this lesson?