To determine the pH values of water before and after the addition of sodium hydroxide (NaOH), we can look at how NaOH behaves in water. Sodium hydroxide is a strong base, and when it is dissolved in water, it dissociates completely into sodium ions (Na⁺) and hydroxide ions (OH⁻). This increases the concentration of hydroxide ions in the solution, leading to an increase in pH.
Pure water at neutral pH is around 7. If you add NaOH to it, the pH will rise above 7, making the solution basic. Therefore, we anticipate:
- The pH of pure water before adding NaOH is 7.
- The pH after adding NaOH will be greater than 7.
Given the options provided:
- before = 7, after = 9 (This is possible)
- before = 7, after = 4 (This is not possible; adding a base cannot lower pH below 7.)
- before = 4, after = 7 (This is not relevant because pure water is not at a pH of 4 before adding NaOH.)
- before = 9, after = 7 (This is not possible; if you start with a basic solution, adding more base would not lower the pH.)
The most reasonable answer is:
before = 7, after = 9.
2 answers