an element that reacts vigorously with cold water

The element that reacts vigorously with cold water is Sodium (Na). Here are the steps of the reaction:

1. Sodium is cut to expose a fresh surface. The cut surface turns from shiny silver to dull grey as the sodium reacts with oxygen in the air.

2. The sodium is then dropped into cold water.

3. The reaction is very exothermic, producing a lot of heat. This heat causes the sodium to melt and it forms a small ball that moves around on the surface of the water.

4. As the sodium reacts, it produces sodium hydroxide and hydrogen gas. The equation for this reaction is: 2Na(s) + 2H2O(l) -> 2NaOH(aq) + H2(g)

5. The hydrogen gas produced can catch fire or explode if there is a flame or spark nearby. This is why this reaction must be done in a controlled environment under the supervision of a trained professional.