Reactions of metals with oxygen in air
Many metals react with oxygen to form metal oxides. Potassium and sodium are soft metal which are easily cut exposing a shiny surface which changes to dull rapidly. The change from shiny to dull is called The reaction of a metal with oxygen with air in which the surface of the metal changes from shiny to dull.
For example:
Potassium burns with a lilac flame when heated in air.
potassium + oxygen 鉃 potassium oxide
4K(s) + O2(g) 鉃 2K2O(s)
Magnesium reacts readily in air burning with a white light.
magnesium + oxygen 鉃 magnesium oxide
2Mg(s) + O2(g) 鈫 2MgO(s)
Magnesium reacts vigorously with oxygen to produce magnesium oxide