Acidic and alkaline solutions

Acids

Acids form acidic solutions in water. Acids are sources of hydrogen ions, H+. For example, dilute hydrochloric acid forms hydrogen ions:

HCl(aq) → H+(aq) + Cl-(aq)

Alkalis

Alkalis form alkaline solutions in water. These contain hydroxide ions, OH-. For example, dilute sodium hydroxide solution contains hydroxide ions:

NaOH(aq) → Na+(aq) + OH-(aq)

The pH scale

The pH scale measures the acidity or alkalinity of a solution.

pHMeaning
< 7acidic
= 7neutral
> 7alkaline

Using an indicator

The pH of a solution can be estimated using universal indicator and a colour chart:

  1. dip a clean glass rod into the solution
  2. transfer one drop of solution to a piece of universal indicator paper on a white tile
  3. wait 30 seconds for the colour to develop, then match the colour to the colour chart
Universal indicator colours.Universal indicator colours

Using a pH meter

The pH of a solution can be measured using a pH meter:

  1. wash the pH probe with distilled water
  2. put the end of the probe into the solution
  3. record the reading on the pH meter
A pH meter with its pH probe in an acidic solution

The pH meter may need calibrating first. This is done by adjusting the reading to match the pH of a buffer solution, which is a solution with a known pH.