pHs of oxide solutions. Aqueous solutions of the oxides of some elements with universal indicator (UI) added to show their pH (acidity). The element forming each oxide is labelled. The oxide of sulphur produces sulphuric acid, which has the lowest pH (strongest acidity). UI changes colour according to the pH, from red (most acidic) through yellow (mildly acidic) to blue (slightly alkaline) to purple (strongly alkaline). Sulphur oxide forms sulphuric acid in water, which is highly acidic. Sodium oxide forms sodium hydroxide (a strong alakai). Oxide pH increases from left to right across the periodic table.