Question

The pH value of pure water is ______.
1. 4
2. 7
3. 12
4. 18

Answer 1

Correct Answer - Option 2 : 7

The correct answer is 7.

• The scientific definition of pH is that it is a hydrogen ion activity indicator (H+) and is stated as the inverse of the hydrogen ion activity logarithm.
• Therefore there are 10-7 moles per litre of hydrogen ions in water with a pH of 7; while a pH of 6 is 10-6 moles per litre. The scale of the pH varies between 0 and 14.
• Water with a pH of < 7 is considered acidic, and water with a pH of > 7 is considered basic.
• The standard range of pH is 6.5 to 8.5 for surface water systems and 6 to 8.5 for groundwater systems.
• Alkalinity is a measure of the water's capacity to tolerate a pH change that appears to make the water more acidic.
• To calculate the corrosivity of the water, the calculation of alkalinity and pH is required. The pH of pure water (H₂0) at 25^oC is 7, but this combination results in a pH of about 5.2 when exposed to carbon dioxide in the atmosphere.