With reference to 'acid and bases', which of the following statements is/are correct ?
1. Acidic solutions have pH more than 7.
2. Basic solutions have pH less than 7.
Select the correct answer from t

Question

With reference to 'acid and bases', which of the following statements is/are correct ?
1. Acidic solutions have pH more than 7.
2. Basic solutions have pH less than 7.
Select the correct answer from the code given below :
Code :

UnlockIAS · Accepted Answer

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.

Why the other options are wrong — (a) 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) 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) 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.

Concept: 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.

State PCS 2025 Q78 — Science & Technology PYQ with Answer

State PCS 2025 Q78 — Science & Technology PYQ with Answer