Question

pH value of pure water at 25°C temperature is:

• A. Less than 7 and more than 0
• B. More than 7 and less than 14
• C. 0
• D. 7

Hint:

pH less than 7 indicates acidic solution, pH equal to 7 indicates neutral, and pH greater than 7 indicates basic solution.

Answer 1

The Correct Option is D

At \( 25^\circ \mathrm{C} \), pure water is considered
neutral, which means it is neither acidic nor basic. This neutrality is quantitatively represented by its
pH value of exactly 7, which is the midpoint on the pH scale. \medskip 1.
Understanding pH:

The pH of a solution is a logarithmic measure of the concentration of hydrogen ions (\( \mathrm{H^+} \)) or more accurately hydronium ions (\( \mathrm{H_3O^+} \)) in that solution.
It is mathematically defined as: \[ \mathrm{pH} = -\log_{10} [\mathrm{H^+}] \]
For pure water at \( 25^\circ \mathrm{C} \), the concentration of hydrogen ions is: \[ [\mathrm{H^+}] = 1 \times 10^{-7} \, \mathrm{mol/L} \]
Substituting this value gives: \[ \mathrm{pH} = -\log_{10}(1 \times 10^{-7}) = 7 \]
2.
Ionization of Water:

Water undergoes a process called
self-ionization or
auto-ionization, where a very small fraction of water molecules dissociate into hydrogen ions and hydroxide ions: \[ \mathrm{H_2O (l)} \rightleftharpoons \mathrm{H^+ (aq)} + \mathrm{OH^- (aq)} \]
At \( 25^\circ \mathrm{C} \), the concentration of both \( \mathrm{H^+} \) and \( \mathrm{OH^-} \) ions is equal, each being \( 1 \times 10^{-7} \, \mathrm{mol/L} \).
Because the concentrations of \( \mathrm{H^+} \) and \( \mathrm{OH^-} \) ions are equal, the solution is
neutral.
3.
Neutrality and the pH Scale:

The pH scale generally ranges from 0 to 14,
where:


pH less than 7 indicates an
acidic solution (more \( \mathrm{H^+} \) ions).
pH greater than 7 indicates a
basic or
alkaline solution (more \( \mathrm{OH^-} \) ions).
pH exactly 7 corresponds to a
neutral solution.

Pure water is the standard for neutrality, making pH 7 the reference point.
4.
Effect of Temperature on pH:

The ionization of water is an endothermic process, meaning it depends on temperature.
At temperatures higher or lower than \( 25^\circ \mathrm{C} \), the ionization constant of water changes, leading to a shift in the concentrations of \( \mathrm{H^+} \) and \( \mathrm{OH^-} \).
As a result, the pH of pure water is not always exactly 7; it may be slightly lower or higher depending on the temperature.
However, at any temperature, pure water remains neutral because the concentrations of \( \mathrm{H^+} \) and \( \mathrm{OH^-} \) remain equal.
5.
Importance of Neutral pH in Nature and Industry:

Neutral pH is critical for many biological systems and chemical reactions, as extreme acidity or alkalinity can denature enzymes or disrupt chemical equilibria.
Water with a pH close to 7 is safe for most plants, animals, and human use.
Monitoring and adjusting pH is essential in water treatment, agriculture, and various industrial processes to maintain proper conditions.
\medskip In conclusion, at \( 25^\circ \mathrm{C} \), pure water has equal concentrations of hydrogen and hydroxide ions due to self-ionization, resulting in a neutral pH value of exactly 7. This neutrality is fundamental to many chemical, biological, and environmental processes.