Lithium , sodium and potassium all react with cold water , forming a soluble alkaline hydroxide and hydrogen gas . Hydrogen is given off as a gas . All three metals float on water . Sodium melts and forms a ball which whizzes over the surface as it gives off hydrogen . Lithium is less reactive and does not melt but does give off hydrogen .
Potassium is the most reactive .
It melts , reacts vigorously and burns with a lilac flame as it gives off hydrogen . All the hydroxides are strongly alkaline and dissolve in the water . The alkali metals are so named because they form alkaline oxides and hydroxides .