Question

Which of the following statement(s) is/are correct regarding pH?

• A. pH of pure water at ${25}^{o}C$ is 7.
• B. pH of water decreases with rise in temperature
• C. pH does not change with the dilution of an acidic solution.
• D. pH of a basic solution is greater than 7 at ${25}^{o}C$.

Answer 1

Answer: (A), (B), (D)

The correct options are
A pH of pure water at ${25}^{o}C$ is 7.
B pH of water decreases with rise in temperature
D pH of a basic solution is greater than 7 at ${25}^{o}C$.

• In a neutral medium , $\left[H^{+}\right]=\left[O{H}^{-}\right]=1\times {10}^{-7}mol{L}^{-1}$ at ${25}^{o}C$. So , $pH=-\log \left[H^{+}\right]=7$ , at ${25}^{o}C$ is correct.
• As the temperature increases, $K_w$ increases. For example, pH range is from $0to14$ at ${25}^{o}C$ and $K_w={10}^{-14}$ and neutral pH point (or pH of pure water) is 7.
  But pH range is from $0to13$ at ${80}^{o}C$ and $K_w={10}^{-13}$ and neutral pH is 6.5 or pH of water is 6.5 at this temperature.
  So, pH of water decreases with rise in temperature but water will still remain neutral.
• pH depends upon the concentration of $H^{+}$. Again concentration is inversely proportional to the volume of the solution. So , as dilution occurs volume increases and concentration decreases , and therefore pH will decrease. Therefore the given statement is incorrect.
• For a basic solution the pH should be greater than the neutral pH (7). So , $pH>7$ is correct for a basic solution.