I believe that you think to alkali metals (ex. sodium).
The Rubidium valance shell is a radio active substance
potassium
Potassium is the only metal (alkali metal) where a flame is present. Lithium and sodium fizz but there is no flame. Caesium, francium and rubidium all explode on contact with water.
Throw water on it. The logical answer.
A soft, light weight metal called sodium, which we never see. It bursts into flames in air and can even burn under water.
The alkali metals sodium, potassium, rubidium, and cesium ignite on contact with water.
When a metal oxide reacts with an acid a slat and water are formed.
copper
Water
Hydrogen
rust
Metal hydroxide + hydrogen gas