NDA (II) 2016 General Ability Test · Question 102 of 150

Correct — B, it hardens the enamel of the tooth. Enamel is built of hydroxyapatite, a calcium phosphate mineral that carries a hydroxide group. When fluoride ions from the paste reach the enamel surface, they take the place of that hydroxide and turn the mineral into fluorapatite. Fluorapatite is harder and, more importantly, far less soluble in acid than the mineral it replaced, so the acid that oral bacteria make from sugar has to work much harder to dissolve the tooth. Fluoride also helps redeposit mineral on surfaces that have already started to dissolve, so the enamel is both toughened and repaired.

• (a)prevents plaque formation — Plaque is a sticky film of bacteria and their secretions that builds up on the tooth surface, and it is removed mechanically — by the brushing action and the mild abrasives in the paste — not by the fluoride ion. Fluoride works on the tooth mineral, not on the film sitting over it.
• (c)kills pathogenic bacteria — Fluoride at toothpaste strength is not sold or recommended as a germicide. It can slow some bacterial enzymes, but the mouth is not sterilised by brushing and the reason dentists give for recommending fluoride is protection of the mineral, not killing of the flora.
• (d)prevents tooth ache — This confuses a cause with a consequence. Toothache is a symptom, usually of decay reaching the dentine or pulp; a paste that protects enamel may mean less decay and so less pain later, but relieving or preventing pain is not the mechanism by which fluoride acts, and no fluoride paste is an analgesic.

Tooth enamel is the hardest tissue in the human body and is about 96 per cent mineral, almost all of it hydroxyapatite. Decay begins when bacteria in the mouth ferment sugars into acids that dissolve this mineral, a process called demineralisation. Fluoride shifts the chemistry in the tooth's favour twice over — it converts surface hydroxyapatite into the more acid-resistant fluorapatite, and it speeds up the redeposit of calcium and phosphate on partly dissolved enamel.

Every option here is something people loosely associate with brushing, which is what makes the item work. The discipline it rewards is asking what fluoride specifically does, as distinct from what the toothbrush does, what the abrasive does and what the pain-relief tablet does. Note also the dose relationship, because the exam has asked about the other side of it: fluoride in small amounts strengthens enamel, but chronic intake of too much — usually from groundwater rather than from paste — mottles and damages the teeth and eventually the bones, a condition called fluorosis.

• Enamel is roughly 96 per cent mineral by weight and is the hardest tissue in the human body.
• Fluoride converts hydroxyapatite in the enamel surface into fluorapatite, which resists acid attack much better.
• Tooth decay starts with acid produced when oral bacteria ferment sugars, so the sugar in the diet, not the fluoride, is the driver of the problem.
• Excess fluoride intake over long periods causes dental and skeletal fluorosis, which is a public-health problem in several Indian states with high-fluoride groundwater.

• Reading the question as 'what does brushing do' rather than 'what does the fluoride ion do'.
• Assuming that more fluoride is always better — the same ion causes fluorosis in excess.

The GAT keeps returning to everyday chemistry with a named active ingredient, so learn the specific job of the named substance rather than the general benefit of the product.

• practice — not a real PYQ

  Long-term consumption of drinking water containing an excess of fluoride causes

  • (a)goitre
  • (b)fluorosis
  • (c)scurvy
  • (d)anaemia

  Answer(b) fluorosis — chronic excess fluoride mottles and damages the teeth and, in severe cases, the bones and joints.

• practice — not a real PYQ

  The principal mineral of which tooth enamel is made is

  • (a)calcium carbonate
  • (b)hydroxyapatite
  • (c)sodium chloride
  • (d)silica

  Answer(b) hydroxyapatite — a calcium phosphate mineral; fluoride converts its surface into the tougher fluorapatite.