Science (086) — AI-generated practice question

(i) A low pH (around 4) means the soil is highly acidic. Most crops grow best in a neutral to slightly acidic pH (near 7). Highly acidic soil reduces the availability of essential nutrients and is harmful to beneficial soil organisms, causing poor crop growth.

(ii) Slaked lime is calcium hydroxide, Ca(OH)₂, which is a base. When added to acidic soil, it undergoes a neutralisation reaction with the acids present:

$$\text{Ca(OH)}_2 + 2\text{H}^+ \rightarrow \text{Ca}^{2+} + 2\text{H}_2\text{O}$$

The base (slaked lime) neutralises the excess acid in the soil, raising the pH.

(iii) Yes, a salt has been formed. Neutralisation is the reaction between an acid and a base to produce a salt and water. When Ca(OH)₂ reacts with the acids in the soil, the calcium ions (Ca²⁺) combine with the negative ions of the acid to form a salt (e.g., calcium sulphate or calcium chloride, depending on the acid present). A pH of 7 confirms that the reaction has reached neutrality, which is characteristic of a neutral salt formed from a strong acid and a strong base.

Source: Chapter 2, Acids, Bases and Salts — Introduction; Section 2.4.2 pH of Salts

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• (i) Living beings carry out metabolic activities within an optimal pH range — this is the key idea. Low pH disrupts nutrient availability and soil biology.
• (ii) Examiners expect you to name the reaction as neutralisation, write the ionic equation (or word equation), and identify Ca(OH)₂ as the base. Even a word equation with a balanced formula earns marks.
• (iii) This is a justify question — stating "yes" alone is insufficient. You must link it to the definition: acid + base → salt + water. The textbook states: "Salts of a strong acid and strong base are neutral with pH = 7," which supports the justification. Always connect the pH observation back to the chemistry.