NDA (I) 2018 General Ability Test · Question 64 of 150

Correct — C, It is basic. Toothpastes are formulated to be mildly basic, and the reason is the chemistry of tooth decay. Bacteria in the mouth act on sugar and food left between the teeth and produce acids. Once the pH in the mouth falls below about 5·5, that acid begins to dissolve the calcium phosphate of the enamel, and decay sets in. A mildly basic paste neutralises the excess acid and brings the mouth back above the danger line, which is exactly what the NCERT chapter on acids, bases and salts gives as the reason for brushing.

• (a)It is acidic — An acidic paste would add to the very acid that attacks enamel. It would work against the purpose of brushing rather than for it.
• (b)It is neutral — A neutral paste would clean mechanically but would do nothing to the acid already sitting on the teeth. Neutralising that acid needs a base, which is why the formulation is deliberately pushed above pH 7.
• (d)It is made up of Calcium phosphate, the material of tooth enamel — The second half is true — enamel is largely calcium phosphate, in the form of hydroxyapatite. The first half is not. A toothpaste is a mixture of an abrasive, a detergent, a humectant, a binder, a fluoride compound and flavouring, and it is not made up of the enamel material. The option is built by hanging a false claim on a true one, which is a standard way of making a distractor.

The pH scale runs from 0 to 14, with 7 neutral, values below 7 acidic and values above 7 basic. Tooth enamel is the hardest substance in the body and is made mainly of calcium phosphate as hydroxyapatite. It is attacked when the pH of the mouth falls below about 5·5, which happens when oral bacteria ferment sugar into acid. Brushing with a mildly basic paste both removes the food and neutralises the acid, and the fluoride in most pastes converts some of the hydroxyapatite into fluorapatite, which resists acid better.

Two different mechanisms are at work in a tube of toothpaste and it pays to keep them apart, because NDA has asked about each. The basic nature of the paste handles the acid that is already there — straightforward neutralisation. The fluoride handles the enamel itself, hardening it against future attack. A later NDA paper set precisely this question and its official key accepted both mechanisms as correct answers, which tells you the examiner regards them as two genuine and separate lines of defence. Note also how option (d) is constructed here, since the pattern recurs across the paper: take a fact the candidate knows to be true, attach it to a claim that is not, and offer the pair as one option.

• Toothpastes are generally basic, so that they neutralise the acid produced in the mouth.
• Tooth decay begins when the pH of the mouth falls below about 5·5.
• Tooth enamel is made mainly of calcium phosphate, and is the hardest material in the human body.
• Fluoride in toothpaste hardens enamel by converting hydroxyapatite to the more acid-resistant fluorapatite.
• On the pH scale, 7 is neutral, below 7 is acidic and above 7 is basic.

• Accepting an option because the second half of it is a fact you recognise.
• Assuming a cleaning product must be neutral so as not to harm the mouth.
• Confusing the roles of the base and the fluoride in a toothpaste.

NDA asks either for the chemical nature of the paste or for the mechanism by which it prevents decay, and has set both forms.

• practice — not a real PYQ

  Tooth decay begins when the pH of the mouth falls below about

  • (a)7·0
  • (b)6·5
  • (c)5·5
  • (d)3·5

  Answer(c) 5·5 — below that value the acid begins to dissolve the calcium phosphate of the enamel.

• practice — not a real PYQ

  A person suffering from acidity is given milk of magnesia because it is

  • (a)a strong acid
  • (b)a mild base
  • (c)neutral
  • (d)a salt of a strong acid

  Answer(b) a mild base — magnesium hydroxide neutralises the excess hydrochloric acid in the stomach.