Tooth enamel of humans is made up of:

Correct — A, Calcium phosphate. Enamel is the outer shell of the crown of a tooth and it is more than 95 per cent mineral by weight, which is what makes it the hardest substance the human body produces — harder than bone. That mineral is calcium hydroxyapatite, formula Ca10(PO4)6(OH)2, a crystalline form of calcium phosphate laid down as tightly packed rods by cells called ameloblasts before the tooth erupts. Two consequences follow from the chemistry and both are testable. First, because the crystal is a salt of a weak acid, it does not dissolve in water but is corroded by acid: NCERT fixes the threshold at a mouth pH below 5.5, and the acid comes from bacteria fermenting sugar left on the teeth. Second, because enamel is made before eruption and the ameloblasts are then lost, the body cannot grow it back — a cavity has to be filled. Fluoride toothpaste works on the same mineral, replacing some hydroxide in the crystal with fluoride to give fluorapatite, which resists acid better.

• (b)Sodium hydrogen carbonate — Baking soda, a mild base. It turns up in the same chapter of the syllabus and even inside some toothpastes, where its job is to neutralise the acid attacking the enamel — so it is the enamel's ally, not its material.
• (c)Sodium hydroxide — Caustic soda, a strong alkali used in soap and paper manufacture. It is corrosive to living tissue; no part of the human body is built from it.
• (d)Lactic acid — Reverses the relationship. Lactic acid is one of the acids that mouth bacteria produce from sugar, and it is what dissolves enamel once the pH drops below about 5.5. It destroys enamel rather than composing it.

A tooth has three hard layers and one soft core. Enamel is the outermost, almost entirely calcium hydroxyapatite, non-living and unable to repair itself. Dentine lies beneath it, softer and containing more organic material. Cementum covers the root. At the centre is the pulp, with blood vessels and nerves. Hydroxyapatite is also the mineral of bone, but bone is only about two-thirds mineral and is remodelled all life long by living cells, which is why a broken bone heals and a chipped tooth does not.

The item is a one-line recall question, and the sensible way to attack it is by class of compound rather than by memory. A hard, insoluble, load-bearing biological solid has to be a mineral salt; that removes an acid at once and removes a caustic alkali on grounds of chemistry and of survival. Of the two salts offered, sodium hydrogen carbonate is a soft, water-soluble powder, which cannot be the hardest tissue in the body. Calcium phosphate is left, and it is also the mineral of bone, so the answer sits inside a fact most candidates already carry. It is worth holding the acid fact alongside it: mouth pH below 5.5 corrodes enamel, which is why the same NCERT chapter treats tooth decay under acids and bases rather than under biology.

• Enamel is calcium hydroxyapatite, Ca10(PO4)6(OH)2 — a crystalline form of calcium phosphate — and is the hardest substance in the human body.
• It does not dissolve in water but is corroded when the pH in the mouth falls below about 5.5.
• Bacteria in the mouth make that acid by breaking down sugar and food residue left after eating.
• Toothpastes are generally basic, so brushing neutralises the excess acid; fluoride converts some hydroxyapatite into the more acid-resistant fluorapatite.
• Hydroxyapatite is also the mineral of bone, but bone contains far more organic matrix and is continuously remodelled, while enamel once formed cannot be regenerated.

• Picking lactic acid because it is genuinely part of the tooth-decay story — it is the attacker, not the material.
• Assuming bone and enamel are the same tissue because they share a mineral; enamel has almost no living cells and cannot heal.
• Confusing sodium hydrogen carbonate with calcium carbonate, another common toothpaste ingredient used as a mild abrasive.

As a one-line composition question, or turned around into a tooth-decay item asking at what pH enamel begins to corrode or why toothpaste is basic.

• practice — not a real PYQ

  Tooth decay begins when the pH of the mouth falls below approximately which value?

  • (a)3.5
  • (b)5.5
  • (c)7.0
  • (d)8.5

  Answer(b) 5.5 — enamel does not dissolve in water but is corroded once the mouth becomes more acidic than about pH 5.5, which is why cleaning the mouth after eating prevents decay.

• practice — not a real PYQ

  Toothpastes are generally basic in nature. The main reason is that they

  • (a)dissolve the enamel to expose a fresh surface
  • (b)neutralise the excess acid produced in the mouth
  • (c)supply calcium directly to the pulp of the tooth
  • (d)kill every micro-organism present in the mouth

  Answer(b) neutralise the excess acid produced in the mouth — the acid comes from bacteria acting on sugar, and neutralising it keeps the pH above the level at which enamel corrodes.