Acids produce hydrogen ions (H+) in aqueous solution. Aqueous solutions of alkalis contain hydroxide ions (OH-). The pH scale runs from 0 to 14 and measures how acidic or alkaline a solution is. A solution with pH 7 is neutral. Aqueous solutions of acids have a pH less than 7 and aqueous solutions of alkalis have a pH greater than 7.
You can measure pH in two ways. Universal indicator, or a wide range indicator, is added to the solution and changes colour: red, orange or yellow for acids, green for neutral, and blue or purple for alkalis. You compare the colour with a chart and so get only an approximate pH. A pH probe is placed in the solution and gives a numerical reading on a meter.
In a neutralisation reaction between an acid and an alkali, hydrogen ions react with hydroxide ions to produce water:
H+(aq) + OH-(aq) → H2O(l)
As alkali is added to an acid, the pH rises towards 7. At pH 7 the hydrogen ions and hydroxide ions have exactly reacted.
