Why is there a difference between sodium and potassium in the reactivity with water

Potassium reacts more vigorously because once it hits water it produces an iron called alkaline. Hydrogen is a release that causes big flames.

Sodium made a little flame and a little bit of a fizzing sound.

Potassium produced a purple flame as it is more reactive as well as sound and smoke came out.

Both of these group 1 metals let out hydrogen as they where in the water so if you put a universal indicator inside the water will turn blue/green/purple so that's y they are called Alkali metals.