AI-generated practice question — model-generated for extra practice, not a previous-year CBSE board question.
Tooth decay begins when the pH of the mouth falls below 5.5. At this pH, the tooth enamel (calcium hydroxyapatite) gets corroded by acids produced by bacteria.
Source: Chapter 2, Section 2.3.1
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The examiner expects the specific pH value (below 5.5) and ideally a brief reason (acid corrodes enamel). Since it's 1 mark, just the pH value with one supporting detail is sufficient. Do not write more than one or two lines.