← All topics
Environmental Chemistry
Acid–Base Equilibria & pH
pH, pOH, the ion product of water and buffering — the acid–base backbone of water-quality chemistry.
PART 1
Topic Breakdown & Traps
The Engineering Principle
pH measures the hydrogen-ion activity of a solution on a logarithmic scale. At 25 °C the ion product of water is , so . Neutral water has pH 7; acids lower it, bases raise it. Because the scale is logarithmic, each pH unit is a ten-fold change in .
The Core Formula Matrix
pH:
pOH:
Water relation (25 °C):
Ten-fold rule: changes by a factor of 10.
pOH:
Water relation (25 °C):
Ten-fold rule: changes by a factor of 10.
The ‘IIT Traps’
- ⚠pH 7 is neutral only at 25 °C. rises with temperature, shifting neutral pH below 7.
- ⚠A change of 2 pH units is 100×, not 2×, in hydrogen-ion concentration.
- ⚠pH + pOH = 14 holds at 25 °C only.
📚 Standard references
- Environmental Chemistry — A.K. De
- Chemistry for Environmental Engineering and Science — Sawyer, McCarty & Parkin
PART 2
Progressive 3-Tier Question Suite
Q1BASIC1 Mark · NAT
The hydrogen-ion concentration of a sample is . Its pH is _____.
Q2MEDIUM1 Mark · NAT
At 25 °C a solution has pOH = 4.2. Its pH is _____.
Q3HARD2 Marks · MCQ
If the pH of a water sample drops from 7 to 5, its hydrogen-ion concentration: