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.
	The products depend on the base: a hydroxide base forms salt and water; calcium carbonate forms a salt, water and carbon dioxide.
	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 how agricultural lime (calcium carbonate) reacts with acidity in the soil. Include the products formed and explain why the equation H⁺(aq) + OH⁻(aq) → H₂O(l) describes hydroxide bases rather than calcium carbonate.

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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. In a classroom model, calcium carbonate reacts with hydrochloric acid. Write the word equation and explain how carbonate ions consume H⁺ ions, including all products. Then distinguish this reaction from the net ionic equation H⁺(aq) + OH⁻(aq) → H₂O(l), which describes hydroxide bases.

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Wrap Up

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