When the alkali metals are cut, they initially appear shiny grey but quickly become dull and white as they react with oxygen in the air. This is known as tarnishing.
Lithium tarnishes slowly due to its relatively slow reaction with oxygen.
lithium + oxygen → lithium oxide
4Li(s) + O2(g) → 2Li2O(s)
Sodium tarnishes more quickly than lithium, which is further evidence for the greater reactivity of sodium when compared to lithium.
sodium + oxygen → sodium oxide
4Na(s) + O2(g) → 2Na2O(s)
Potassium tarnishes so quickly that it is difficult to see that potassium is actually a shiny metal. This is further evidence that potassium is a more reactive metal than both lithium and sodium.
potassium + oxygen → potassium oxide
4K(s) + O2(g) → 2K2O(s)
The alkali metals can also be set alight and burn. When any substance burns in oxygen it is called a combustion reaction. Potassium (lilac) burns most vigorously followed by sodium (orange-yellow) and then lithium (red), as you might expect.