pH scale
- CBSE · NCERT Class 10, Ch 2 Acids, Bases and Salts
The pH scale is a number scale from 0 to 14 that shows how acidic or basic a solution is, with 7 as neutral.
pH tells you the strength of an acid or a base, not just which one it is. The p comes from a German word for power, and H stands for hydrogen ions.
- Below 7: acidic. The lower the number, the stronger the acid.
- Exactly 7: neutral, like pure water.
- Above 7: basic. The higher the number, the stronger the base.
More hydrogen ions in a solution means a lower pH. A universal indicator shows the pH by its colour.
A small example
Three samples read pH 2, pH 7 and pH 11. The pH 2 sample is a strong acid. The pH 7 one is neutral. The pH 11 one is a fairly strong base. NCERT gives a real case: tooth decay starts when the pH in the mouth falls below 5.5. Brushing with a paste that is a base helps bring the pH back up.
Where marks go
CBSE's 2026-27 Science sample paper has no pH question, so we cannot point to a pH split. In its Question 27, the 2026-27 Science marking scheme gives 1 mark for each "give reason" part. Each marked answer names a cause.
So a pH answer should say what the number means, acid or base, and why. A bare number gives the examiner less to mark. See how step marking works.
Sources
Facts last checked against these sources on 30 September 2026.
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