CDS (II) 2022 General Knowledge · Question 12 of 120

Correct — B, Mild base. Milk of magnesia is a suspension of magnesium hydroxide, Mg(OH)₂, in water, and NCERT names it exactly this way — 'Magnesium hydroxide (Milk of magnesia), a mild base, is often used' as an antacid. It works by neutralising the excess hydrochloric acid the gastric glands have secreted, which relieves the pain of indigestion. It has to be mild for that job. Magnesium hydroxide is only slightly soluble, so the suspension holds a small concentration of hydroxide ions and its pH sits at about 10; a strong base would neutralise the acid and then attack the stomach lining as well.

• (a)Strong base — A strong base such as sodium hydroxide ionises completely and would be corrosive. Nothing swallowed for indigestion can be strongly basic — the point is to remove excess acid, not to swing the stomach to the other extreme.
• (c)Strong acid — Reverses the treatment. The stomach already has too much hydrochloric acid, which is the complaint; adding a strong acid would make it worse.
• (d)Mild acid — Still the wrong side of neutral. A weak acid would not neutralise the gastric acid, and the whole logic of an antacid is that it is basic.

Neutralisation is the reaction between an acid and a base to give a salt and water, and antacids are its everyday application. The stomach secretes hydrochloric acid to activate pepsin and digest protein; when it secretes too much, the pain is treated with a mild base such as magnesium hydroxide, aluminium hydroxide or sodium hydrogencarbonate. Strength here means degree of ionisation, not concentration — a base is strong if it dissociates fully in water, and magnesium hydroxide does not.

Two of the four options can be discarded the moment you read 'indigestion', because both name acids and the problem being treated is excess acid. That leaves a choice between strong and mild, and the deciding argument is that whatever is swallowed has to be safe for the stomach lining. Carrying the approximate pH is useful for the harder version of this question — milk of magnesia sits near 10, which puts it above blood at about 7.4 and far above lemon juice at about 2. Sodium hydrogencarbonate, baking soda, is the other mild base that shows up in the same chapter, both as an antacid ingredient and as the remedy for a bee sting.

• Milk of magnesia is a suspension of magnesium hydroxide, Mg(OH)₂, in water.
• NCERT describes it as a mild base used to neutralise excess acid in the stomach.
• Its pH is approximately 10.
• Sodium hydrogencarbonate is also an ingredient in antacids.
• Strength of a base refers to how completely it ionises in water, which is different from how concentrated the solution is.

• Reading 'mild' as 'dilute'; strength describes ionisation, concentration describes how much is dissolved.
• Choosing an acid because the ailment is called acidity — the cure has to be on the other side of neutral.
• Confusing milk of magnesia with milk of lime, which is calcium hydroxide and is not swallowed.

As a straight classification of a named substance as acid or base, strong or mild; as a pH-ordering question; or as a match of compound to use.

• practice — not a real PYQ

  Milk of magnesia is chemically

  • (a)calcium hydroxide
  • (b)magnesium hydroxide
  • (c)magnesium carbonate
  • (d)sodium hydrogencarbonate

  Answer(b) magnesium hydroxide — Mg(OH)₂, suspended in water and taken as an antacid.

• practice — not a real PYQ

  Which one of the following is used to treat a bee sting, which is acidic?

  • (a)Vinegar
  • (b)Lemon juice
  • (c)Baking soda
  • (d)Common salt

  Answer(c) Baking soda — sodium hydrogencarbonate is a mild base and neutralises the acid injected by the sting.