why is sodium metal used for lassaigne's test? why not any other element?

rn

 

Sodium is basically used in Lassaigne's test in order to convert the compounds from the covalent form to the ionic form. It is used in preference to other metals because :

1) It is quite reactive. Moreover, its reactivity is appropriate for the reaction, that is it is neither too reactive like potassium nor less reactive like lithium. 

2) It is cheaper in comparison to other metals of the same group like potassium, cesium etc. 

Also note that we cannot use transition metals as they are not sufficiently electropositive to convert the compounds from covalent form to ionic form.