What happens when sodium or potassium are left outside in the open?
Slow oxidation and hydrolysis of the surface of the metal.
When alkali metal is exposed in the lab, the surface of the metal is oxidized by air or water:
The hydrolysis reaction for both alkali metals is vigorous. The reaction of potassium is especially vigorous. You can destroy relatively large quantities of sodium with ethanol with little trouble; but this procedure would be extremely hazardous with potassium. Gram quantities of potassium (but not sodium) can be destroyed by