Let's analyze the situation step by step to understand the changes in pH when sodium hydroxide (NaOH) is added to pure water.
1. Understanding pH Scale:
- The pH scale ranges from 0 to 14.
- A pH value of 7 is considered neutral (like pure water).
- A pH value less than 7 indicates an acidic solution.
- A pH value greater than 7 indicates a basic (alkaline) solution.
2. Properties of Sodium Hydroxide (NaOH):
- Sodium hydroxide (NaOH) is a strong base.
- When NaOH is added to water, it dissociates into Na⁺ and OH⁻ ions, increasing the concentration of hydroxide ions (OH⁻) in the solution.
- The increase in hydroxide ions (OH⁻) makes the solution more basic.
3. Effect of Adding NaOH to Pure Water:
- Pure water has a neutral pH of 7.
- Adding a base like NaOH to pure water will increase the pH, making the solution more basic.
Given these points, we can determine the pH values before and after adding NaOH to pure water:
- Before the Addition:
- Since pure water is neutral, the pH before adding NaOH would be 7.
- After the Addition:
- Adding NaOH to the water increases the pH, making the water more basic.
- A reasonable change in pH due to the addition of a strong base like NaOH results in a pH value greater than 7. A pH of 9 indicates a basic solution that aligns with adding NaOH.
Thus, the most likely pH values are:
- Before adding NaOH: 7 (since pure water is neutral).
- After adding NaOH: 9 (increased due to the basic nature of NaOH).
Therefore, the answer is:
Before = 7
After = 9
