Chemistry · Chapter 14

Hydronium and Hydroxide

Slide the pH and watch the two ions trade places. Their product never changes, because Kw holds it fixed at any one temperature.

Two ions on a log ladder each rung is a factor of ten

Hydronium and hydroxide bars on a logarithmic ladder

The pH strip acidic on the left, basic on the right

A pH strip with a moving marker

Live numbers

7.00pH neutral
pOH = pKw − pH: 7.00
[H3O+]: 1.00e-7 mol/L
[OH-]: 1.00e-7 mol/L
Product of the two: 1.0e-14
Ratio H3O+ to OH-: 1 to 1

Temperature changes Kw

Kw right now: 1.0e-14
pKw: 14.00
Neutral pH at this temperature: 7.00

Warm water splits a little more freely. So neutral moves below 7, even though the water is still perfectly balanced.

What you are seeing

Pure water at 25 °C holds equal amounts of both ions. Each one sits at 1.0e-7 mol/L.

Drop the pH by one unit and hydronium goes up ten times. Hydroxide drops ten times to keep the product fixed.