[biochem]

The metal you are referring to could be any of the alkali metals (group 1 metals-see the periodic table for the whole list).

Sodium being itself a group 1 metal is a good example. Since elements in the same group have similar chemical properties and chemical reactions, any other group 1 metal would do as well. These metals are usually soft meaning that they are malleable. They also have a lustrous appearance. The oxides of these metals are also soluble in water to give soluble bases called alkalis hence their name as alkali metals. These features fit the description of the metal that you want.

So, substance Y would be the oxide of a group 1 metal and substance Z would be the hydroxide of a group 1 metal.