At `25^(@)C` the pH of water is 7. When temperature of water is increased to `70^(@)C` than pH of water and nature of water is

Options

A. pH will decrease and the sample becomes acidic
B. pH will increase but the sample will remain neutral
C. pH will remain constant as 7.
D. pH will decrease but the sample will remain neutral.

Answer

To solve the problem, we need to analyze the effect of temperature on the pH of water and its nature. Here’s a step-by-step breakdown:

### Step 1: Understand the pH of Water at 25°C
At 25°C, the pH of pure water is 7. This indicates that the concentrations of hydrogen ions \([H^+]\) and hydroxide ions \([OH^-]\) are equal, each being \(10^{-7} \, \text{mol/L}\).

### Step 2: Effect of Temperature on Water Dissociation
As the temperature increases, the dissociation of water increases. This means that more water molecules dissociate into hydrogen ions and hydroxide ions. The equilibrium constant for the dissociation of water, \(K_w\), increases with temperature.

### Step 3: Calculate the pH at 70°C
At 70°C (which is 343 K), the concentration of \(H^+\) and \(OH^-\) ions will be higher than at 25°C. However, the pH will not remain at 7. Instead, the pH will decrease.

For example, at higher temperatures, the concentration of \(H^+\) ions can be approximately \(10^{-6} \, \text{mol/L}\). Therefore, the pH can be calculated as:
\[ \text{pH} = -\log[H^+] = -\log(10^{-6}) = 6 \]

### Step 4: Determine the Nature of Water
Even though the pH is lower than 7, the solution remains neutral because the concentrations of \(H^+\) and \(OH^-\) ions are still equal. This means that water is still neutral at this temperature, despite the decrease in pH.

### Conclusion
At 70°C, the pH of water decreases to 6, but the nature of water remains neutral.

### Final Answer
- **pH of water at 70°C:** 6
- **Nature of water:** Neutral

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