What is meant by pH value of a solution ? What is pH of neutral solution ?

Answer

### Step-by-Step Solution

1. **Definition of pH**:
- pH is a scale used to measure the acidity or alkalinity of a solution. It quantifies the concentration of hydrogen ions (H⁺) present in the solution.
- The formula for pH is given by:
\[ \text{pH} = -\log[H^+] \]
- The pH scale typically ranges from 0 to 14.

2. **Understanding Neutral Solutions**:
- A neutral solution is one where the concentration of hydrogen ions (H⁺) is equal to the concentration of hydroxide ions (OH⁻).
- In a neutral solution, the pH is equal to the pOH, which is a measure of hydroxide ion concentration.

3. **pH of Neutral Solution**:
- At 25°C (standard temperature), the pH of a neutral solution is defined to be 7.
- This is because at this temperature, the concentrations of H⁺ and OH⁻ are both \(1 \times 10^{-7} \, \text{mol/L}\).
- Therefore, using the pH formula:
\[ \text{pH} = -\log(1 \times 10^{-7}) = 7 \]

4. **pH Scale Context**:
- The pH scale is divided into three regions:
- **Acidic**: pH < 7
- **Neutral**: pH = 7
- **Basic**: pH > 7
- It is important to note that the value of 7 as neutral is valid under standard conditions (25°C and 1 atm pressure).

5. **Effect of Temperature**:
- The pH of a neutral solution can change with temperature. At temperatures other than 25°C, the pH of a neutral solution may not be exactly 7.
- For example, at higher temperatures, the pH of pure water decreases slightly, indicating a shift in the balance of H⁺ and OH⁻ concentrations.