UPPSC 2025 Prelims GS Paper-I · Question 78 of 150

Correct — C, Neither 1 nor 2. Both statements are the scale read backwards. On the pH scale, which runs from about 0 to 14 for ordinary aqueous solutions at 25 °C, pure water is neutral at pH 7; an acidic solution has pH LESS than 7, and a basic (alkaline) solution has pH MORE than 7. Statement 1 claims acids sit above 7 and statement 2 claims bases sit below 7 — each is the exact inverse of the truth, so neither is correct and the answer is (c). The reason the scale runs that way is that pH is defined as the negative logarithm of the hydrogen-ion concentration, pH = -log10[H+]. A larger concentration of H+ therefore gives a SMALLER pH number, and it is a large H+ concentration that makes a solution acidic. Because the scale is logarithmic, each whole unit is a tenfold change: a solution at pH 3 has ten times the H+ of one at pH 4 and a hundred times that of one at pH 5.

• (a)Only 2 — Statement 2 is false. Basic or alkaline solutions lie ABOVE pH 7 — a dilute solution of sodium hydroxide is near pH 13, household ammonia around 11, a soap solution around 9-10. Nothing basic sits below 7.
• (b)Both 1 and 2 — This accepts both halves of an inverted scale. If acids were above 7 and bases below 7, then lemon juice (about pH 2) would be alkaline and caustic soda would be an acid — the everyday values contradict the statements at once.
• (d)Only 1 — Statement 1 is false. Acidic solutions lie BELOW pH 7: gastric juice in the stomach is roughly pH 1.5-3.5, lemon juice about 2, vinegar about 3. A pH above 7 is the mark of a base, not of an acid.

pH is a measure of how much free hydrogen ion a solution carries, defined as pH = -log10[H+]. The minus sign is what puts acids at the bottom of the scale: the more H+ present, the more acidic the solution and the lower its pH number. In pure water at 25 °C the hydrogen-ion and hydroxide-ion concentrations are equal at 10^-7 mol per litre, which fixes the neutral point at pH 7. Below 7 the solution is acidic, above 7 it is basic or alkaline. The notation was introduced by the Danish chemist S. P. L. Sorensen in 1909 while he was working on brewing chemistry.

This is a pure inversion question, and it is answered fastest by testing the statements against a value you already know rather than by recalling a rule. Everyone knows the stomach is strongly acidic and that its pH is about 1.5-3.5 — a small number. That single fact kills statement 1. Everyone also knows soap and antacids are basic, and both sit well above 7 — that kills statement 2. Note one honest refinement worth carrying: pH 7 is the neutral point only at 25 °C. Because water's self-ionisation increases with temperature, neutral water at 100 °C sits near pH 6.1 and is still neutral — the neutral point moves, even though 'acidic below neutral, basic above neutral' always holds.

• pH = -log10[H+]; the negative logarithm is why a higher hydrogen-ion concentration gives a LOWER pH number
• Acidic: pH below 7. Neutral: pH 7 (pure water at 25 °C). Basic or alkaline: pH above 7
• The scale is logarithmic — one pH unit is a tenfold change in hydrogen-ion concentration
• Typical values: gastric juice about 1.5-3.5, lemon juice about 2, vinegar about 3, blood 7.35-7.45 (slightly alkaline), household ammonia about 11
• Acids turn blue litmus red and bases turn red litmus blue; an acid and a base react in a neutralisation reaction to give a salt and water
• pH 7 marks neutrality only at 25 °C — the neutral point shifts with temperature because water's self-ionisation is temperature-dependent

• Reversing the scale because the minus sign in pH = -log10[H+] is forgotten — more acid means a lower, not a higher, number
• Treating pH 7 as neutral under all conditions; it is the neutral point at 25 °C, and the neutral value shifts with temperature
• Confusing strength with concentration — a strong acid is one that ionises completely, which is not the same as a concentrated one; a dilute strong acid can have a higher pH than a concentrated weak acid

UPPSC asks the scale directly, usually as a two-statement item with one or both halves inverted, or as a 'which is NOT correctly matched' list of everyday acids. UPSC prefers applied settings — the pH of blood, soil acidity and fertiliser use, acid rain, ocean acidification — where you must know the direction of the scale before you can reason about the effect.

• practice — not a real PYQ

  A solution has a pH of 4. Compared with a solution of pH 6, its hydrogen-ion concentration is:

  • (a)2 times greater
  • (b)10 times greater
  • (c)100 times greater
  • (d)20 times greater

  Answer(c) 100 times greater — the pH scale is logarithmic, so a difference of two units means a hundredfold difference in hydrogen-ion concentration.

• practice — not a real PYQ

  Which one of the following is arranged in order of increasing pH?

  • (a)Gastric juice, pure water, household ammonia
  • (b)Household ammonia, pure water, gastric juice
  • (c)Pure water, gastric juice, household ammonia
  • (d)Gastric juice, household ammonia, pure water

  Answer(a) Gastric juice, pure water, household ammonia — roughly pH 2, then 7, then 11, moving from acidic through neutral to alkaline.