CDS (I) 2020 General Knowledge · Question 4 of 120

Correct — B, highly acidic solution. The pH scale runs from 0 at one end to 14 at the other, and NCERT labels the ends plainly: 0 is very acidic and 14 very alkaline, with 7 in the middle for a neutral solution. pH is a measure of hydronium ion concentration, and the higher that concentration, the lower the pH — so zero is not the absence of acidity but the extreme of it. A solution at pH 0 holds hydronium ions at about 1 mole per litre, which is roughly what you get from concentrated hydrochloric acid.

• (a)highly alkaline solution — That is the other end of the scale. Alkalinity climbs as pH rises above 7, and a highly alkaline solution reads close to 14, not 0.
• (c)weakly acidic solution — Weakly acidic means a pH somewhere between about 4 and 7 — coffee or milk territory. pH 0 is as far from weak as the scale allows.
• (d)neutral solution — Neutral is pH 7, where hydronium and hydroxide ions are present in equal measure. Pure water sits there.

The p in pH stands for the German potenz, meaning power, and the scale measures hydrogen ion concentration in a solution. It is inverse and logarithmic: every step down the scale is a tenfold rise in hydronium ion concentration. Values below 7 are acidic, 7 is neutral, and values above 7 mark rising hydroxide ion concentration and so rising alkalinity.

The trap is the arithmetic instinct that treats zero as 'none' or as a midpoint. On the pH scale zero is an endpoint, and it is the acidic endpoint. Anchoring a few real values keeps the direction straight — gastric juice around 1 to 2, lemon juice about 2, pure water 7, blood a shade above 7, milk of magnesia around 10. If the trend from lemon juice to milk of magnesia is clear in your head, pH 0 has only one place to sit.

• NCERT states the working range of the scale as 0, very acidic, to 14, very alkaline, with 7 as neutral.
• The higher the hydronium ion concentration, the lower the pH value.
• A drop of one pH unit means a tenfold increase in hydronium ion concentration.
• Tooth decay begins when the pH in the mouth falls below 5.5, which is why basic toothpastes help.
• Diluting an acid lowers the number of hydronium ions per unit volume, so the pH moves up towards 7.

• Treating pH 0 as neutral or as 'no pH', when it is the acidic extreme of the scale.
• Reading the scale the wrong way round — pH falls as acidity rises.
• Assuming the scale cannot go outside 0 to 14; very concentrated acids and alkalis do go past both ends, but school work stays inside them.

As a direct 'a solution of pH x is ___' item, or as an ordering question that asks you to rank four everyday liquids by pH.

• practice — not a real PYQ

  A solution turns red litmus blue. Its pH is most likely to be

  • (a)1
  • (b)4
  • (c)5
  • (d)10

  Answer(d) 10 — red litmus turning blue marks a base, so the pH must be above 7.

• practice — not a real PYQ

  If the pH of a solution falls from 5 to 3, the concentration of hydronium ions in it becomes

  • (a)2 times
  • (b)10 times
  • (c)100 times
  • (d)unchanged

  Answer(c) 100 times — each unit drop in pH is a tenfold rise in hydronium ion concentration, so two units is ten times ten.