01Resisting change
Added H⁺ reacts with A⁻ to make HA; added OH⁻ reacts with HA to make A⁻ and water. The ratio changes only a little, so pH barely moves.
Diluting a buffer with water barely changes pH because the ratio stays the same.
02Notebook box
Buffer pH depends on ratio; capacity depends on amounts.
Worked example: Add 0.01 mol HCl to a buffer with 0.10 mol A⁻ and 0.10 mol HA → 0.09 A⁻ / 0.11 HA → pH drops only 0.09.