Read the following and answer the questions:
During a science fair, students set up a display about the importance of pH in daily life. One student brings a chart showing: tooth enamel corroding below pH 5.5; the human body maintaining a pH of 7.0–7.8 for metabolic activity; acid rain having a pH below 5.6; and a stomach producing HCl to aid digestion. Another student points out that toothpastes are basic, and explains why we use them. A third student adds that farmers sometimes treat acidic soil with quicklime.
(i) Why does tooth decay begin only when pH of the mouth drops below 5.5 and not before? (1 mark)
(ii) How do toothpastes help in preventing tooth decay? (1 mark)
(iii) What is the source of the acid that lowers pH in the mouth after eating? (1 mark)
(iv) Write a balanced chemical equation showing how quicklime (CaO) could help neutralise excess acid (HCl) in soil. (1 mark)
Generated by claude-sonnet-4-6 · 2026-06-26 01:05 · grounding rag
Model Answer
(i) Tooth enamel is made of calcium hydroxyapatite, which does not dissolve in water. It is corroded only when the pH in the mouth falls below 5.5. Above this pH, the acid is not strong enough to corrode the enamel, so decay does not begin.
(ii) Toothpastes are generally basic in nature. They neutralise the excess acid produced in the mouth by bacteria, raising the pH above 5.5, thereby preventing the corrosion of tooth enamel and stopping tooth decay.
(iii) Bacteria present in the mouth degrade sugar and food particles left in the mouth after eating and produce acids. These acids lower the pH of the mouth.
(iv) Quicklime reacts with HCl as follows:
$$\text{CaO} + 2\text{HCl} \rightarrow \text{CaCl}_2 + \text{H}_2\text{O}$$
Source: Chapter 2, Section 2.3.1 (pH change as the cause of tooth decay); Chapter 5 (Dental caries box)
---
Explanation
- (i) The key fact is that enamel (calcium hydroxyapatite) is stable above pH 5.5 — state this explicitly.
- (ii) Always mention: toothpastes are basic, they neutralise the acid — both points needed for the mark.
- (iii) The source of acid is bacterial degradation of sugar/food particles — don't just say "bacteria" without mentioning what they break down.
- (iv) Write the fully balanced equation; note it is 2HCl (not 1). CaO + 2HCl → CaCl₂ + H₂O. Examiners check balancing.