Tooth enamel begins to corrode when the pH of the mouth falls below 5.5 after a meal. (i) Identify the substance responsible for this drop in pH and explain its origin. (ii) How does the chemical nature of toothpaste counteract this effect? (iii) Why is it advisable to rinse your mouth with water immediately after consuming acidic foods or drinks, even before brushing?
Generated by claude-sonnet-4-6 · 2026-06-26 01:06 · grounding rag
Model Answer
(i) Bacteria in the mouth act on sugars (from food) and produce acids. These acids lower the pH of the mouth below 5.5, causing corrosion of tooth enamel.
(ii) Toothpaste is basic (alkaline) in nature. It neutralises the acid produced in the mouth, raising the pH above 5.5 and preventing damage to the enamel.
(iii) Rinsing with water immediately dilutes and washes away the acids before they can act on the enamel. This provides quick, temporary relief even before brushing can remove the bacterial plaque.
Source: Chapter 2 (Acids, Bases and Salts); Chapter 5, Section 5.2.4 (Dental caries — More to Know!)
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Explanation
- (i) The key point is bacteria + sugars → acid. The source passage (Dental caries box) states bacteria acting on sugars produce acids.
- (ii) The examiner wants the word basic/alkaline and the concept of neutralisation.
- (iii) This part tests application: water dilutes acid quickly. Plaque prevents saliva from neutralising acid (per the passage), so rinsing physically removes it.
- Do not write more than what is shown above — three focused points is exactly what a 3-mark answer needs (1 mark each).