NDA (I) 2018 General Ability Test · Question 65 of 150

Correct — D, Gastric juice. Hydrogen ion concentration is what the pH scale measures, and the lower the pH the more hydrogen ions are present. Two of the four choices are bases and can be set aside at once. Of the two acidic ones, gastric juice is the more acidic — the hydrochloric acid secreted by the stomach lining puts it at a pH of roughly 1 to 2, while lemon juice, whose acid is the far weaker citric acid, sits at about 2·2 or above. The lower pH belongs to gastric juice, so it supplies the highest concentration of hydrogen ions.

• (a)Sodium hydroxide solution — Sodium hydroxide is a strong base with a pH near 14. It supplies hydroxide ions in abundance and hydrogen ions in the smallest quantity of all four, so it is the exact opposite of what the question wants.
• (b)Milk of magnesia — Milk of magnesia is a suspension of magnesium hydroxide with a pH of about 10, which makes it basic. It is used as an antacid precisely because it removes hydrogen ions from the stomach rather than supplying them.
• (c)Lemon juice — Lemon juice is genuinely acidic and is the runner-up here, but citric acid is a weak acid that ionises only partly. Gastric juice contains hydrochloric acid, which is strong and ionises almost completely, so it holds the lower pH.

The pH of a solution is a measure of its hydrogen ion concentration, and the scale is logarithmic — a fall of one pH unit means ten times as many hydrogen ions. Neutral water sits at 7; acids lie below and bases above. A strong acid such as hydrochloric ionises almost completely in water and so gives a very low pH, while a weak acid such as citric or ethanoic ionises only in part and gives a higher one at the same concentration.

The item is really a two-step sort. First separate acids from bases, which removes sodium hydroxide and milk of magnesia and halves the problem. Then rank the two acids, and here the strong-versus-weak distinction decides it. The logarithmic nature of the scale is worth taking seriously, because it means the gap between gastric juice at about pH 1 and lemon juice at about pH 2·2 is not a small one — it is more than a tenfold difference in hydrogen ion concentration. It is also useful to remember why the stomach can hold an acid that strong without harm: a layer of mucus protects the lining, and the acid is there to activate pepsin and to kill swallowed microbes. When that protection fails, an antacid such as milk of magnesia is given, which is why two of the options in this very item are related as problem and remedy.

• pH measures hydrogen ion concentration; the lower the pH, the more hydrogen ions.
• The pH scale is logarithmic, so one unit lower means ten times as many hydrogen ions.
• Gastric juice contains hydrochloric acid and has a pH of roughly 1 to 2.
• Milk of magnesia is a suspension of magnesium hydroxide with a pH near 10 and is used as an antacid.
• The desirable pH range for drinking water is 6·5 to 8·5.

• Reading a higher pH number as a stronger acid; the relationship runs the other way.
• Ranking two acids by how sour they taste rather than by how completely they ionise.
• Forgetting that the scale is logarithmic, so small pH differences are large differences in concentration.

NDA asks for the most acidic or most basic item from a mixed list, or gives a named substance and asks for its approximate pH.

• practice — not a real PYQ

  A solution has a pH of 3. Compared with a solution of pH 5, its hydrogen ion concentration is

  • (a)two times greater
  • (b)ten times greater
  • (c)hundred times greater
  • (d)the same

  Answer(c) hundred times greater — the scale is logarithmic, so two units correspond to a factor of ten squared.

• practice — not a real PYQ

  The acid present in gastric juice is

  • (a)citric acid
  • (b)hydrochloric acid
  • (c)lactic acid
  • (d)oxalic acid

  Answer(b) hydrochloric acid — secreted by the stomach lining, it activates pepsin and kills swallowed microbes.