Question 1

Does pure water contain ions?

Accepted Answer

Yes. A small fraction of water molecules undergo reversible autoionisation, producing hydronium and hydroxide ions. Their equal concentrations in pure water at 25 °C make the solution neutral.

Question 2

What is the equation for the ionisation of water?

Accepted Answer

The reaction is written as 2H₂O ⇌ H₃O⁺ + OH⁻. The reversible arrow is important because hydronium and hydroxide can recombine to form water.

Question 3

How is pH related to hydronium concentration?

Accepted Answer

pH is the negative base-ten logarithm of hydronium-ion concentration: pH = −log[H₃O⁺]. It has no unit because the numerical concentration is used in the logarithmic expression.

Question 4

Why is a change of one pH unit important?

Accepted Answer

A one-unit change in pH corresponds to a tenfold change in hydronium-ion concentration. Lower pH means a higher hydronium concentration, while higher pH means a lower hydronium concentration.

Question 5

Are pH 0 and 14 absolute limits?

Accepted Answer

No. The commonly shown aqueous pH scale is labelled from 0 to 14, but these endpoints are not treated as absolute physical limits. The logarithmic definition remains the basis of pH.

pH & Water Lab — The H⁺/OH⁻ Balance That Never Stops

About this lab

Practice