Acids, bases & salts
pH scale — পিএইচ স্কেল
Quick answer
A 0–14 scale for acidity of water solutions: pH 7 neutral, below 7 acidic, above 7 basic; each unit is a tenfold change in H⁺.
Definition
The pH scale measures how acidic or basic a water solution is, running from 0 to 14. Pure water is neutral at pH 7; values below 7 mean acidic (more H⁺ ions), values above 7 mean basic. Each step of one pH unit means a ten-times change in H⁺ concentration, so pH 2 lemon juice is one hundred times more acidic than pH 4 tomato juice. Chemists read pH with indicators or a pH meter.
For example
Normal rain is slightly acidic, near pH 5.6, because it absorbs carbon dioxide from the air; acid rain can drop near pH 4.