The pH value of a substance indicates its acidity or alkalinity. It is measured on a scale from 0 to 14. A pH value below 7 indicates acidity, a value of 7 indicates neutrality, and a value above 7 indicates alkalinity.

Option 1 suggests a pH value of 1 for pure water. However, this is incorrect, as a pH value of 1 indicates a strong acid, which is not true for pure water.

Option 2 suggests a pH value of 6 for pure water. Although this is closer to neutrality, pure water has a pH value of 7, which makes this option incorrect.

Option 3 provides the correct answer that the pH value of pure water is 7. This means that pure water is neutral and neither acidic nor alkaline.

Option 4 suggests a pH value of 10 for pure water. However, this is incorrect as a pH value of 10 indicates alkalinity, which is not applicable to pure water.

Therefore, option 3 is the correct answer.