CDS (I) 2023 General Knowledge · Question 51 of 120

Correct — D, Lemon juice < Coffee < Blood < Milk of magnesia. Sort the four by how acidic they are and the sequence writes itself. Lemon juice is the strongest acid on the list — citric acid at a pH of roughly 2. Coffee is acidic too but far more weakly so, around 5. Human blood is not acidic at all; it is held on the alkaline side of neutral, between 7.32 and 7.42, and the body defends that narrow band with buffers, breathing and the kidneys because proteins lose their shape outside it. Milk of magnesia is a suspension of magnesium hydroxide, a base, and comes in near 10 — which is why it is swallowed as an antacid when the stomach has made too much acid. Rising pH therefore runs lemon juice, coffee, blood, milk of magnesia.

• (a)Coffee < Lemon juice < Milk of magnesia < Blood — Two faults. It puts coffee below lemon juice, when lemon juice is much the stronger acid, and it ends with blood above milk of magnesia, which would make blood more alkaline than an antacid.
• (b)Milk of magnesia < Blood < Coffee < Lemon juice — The whole order reversed. This is what you get by ranking the four from most alkaline to most acidic and forgetting that pH rises as acidity falls.
• (c)Lemon juice < Blood < Coffee < Milk of magnesia — Right at both ends, wrong in the middle. It places blood below coffee, which would mean blood is the more acidic of the two; in fact coffee sits near 5 and blood above 7.

pH measures hydrogen ion activity on a scale that normally runs from 0 to 14, with 7 neutral. It is built so that the more hydronium ions a solution holds, the lower its number — NCERT puts it as 'Higher the hydronium ion concentration, lower is the pH value'. Each whole step is a factor of ten, so lemon juice at about 2 carries something like a thousand times the hydronium concentration of coffee at about 5.

An ordering question of this kind is answered by grouping first and refining afterwards. Two of the four substances are acids and two are not, so any option that mixes them up can go straight away. The one that needs care is blood, because people file it under 'body fluid' and think of the stomach, where the juice really is strongly acidic at about 1 to 3. Blood is the opposite case: slightly alkaline, and kept that way. The other snare is direction. Milk of magnesia is the most basic substance here and therefore has the highest pH, so it must come last in an ascending list, not first.

• Lemon juice is about pH 2, coffee about 5, human blood 7.32 to 7.42, milk of magnesia about 10.
• Milk of magnesia is magnesium hydroxide, described by NCERT as a mild base and used as an antacid.
• Blood pH outside roughly 7.32 to 7.42 is called acidemia below and alkalemia above; proteins are denatured outside the band.
• The scale is logarithmic — one pH unit is a tenfold change in hydrogen ion concentration.
• Gastric juice, at about pH 1 to 3, is the strongly acidic body fluid, not blood.

• Filing blood with gastric juice as an acidic body fluid. Blood sits above 7; gastric juice is near 1 to 3.
• Reading the ordering the wrong way and putting the most alkaline substance first.
• Treating coffee as strongly acidic because it tastes sharp. It is a weak acid, three units above lemon juice.

As an ordering of everyday substances by pH, as a single-substance pH value, or as an acidic-or-basic classification of an antacid or a body fluid.

• practice — not a real PYQ

  Which one of the following has the lowest pH?

  • (a)Milk of magnesia
  • (b)Pure water
  • (c)Gastric juice
  • (d)Human blood

  Answer(c) Gastric juice — it is about pH 1 to 3, well below pure water at 7, blood at about 7.4 and milk of magnesia at about 10.

• practice — not a real PYQ

  Milk of magnesia relieves indigestion because it is

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

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