6m nh3 a basic solution cannot be found on the reagent shelf, but 6m naoh also a base is available, what would be observed if the 6m naoh is substituted for the 6m nh3, in testing the reference solution

The difference will be that whatever chemical reaction is taking place, it will take place faster. For example, if a titration was being carried out to neutralize an acid, the acid will neutralize faster with NaOH. 
The reason for this is because NaOH is a stronger base. It dissociates more than does ammonia; therefore, it reacts faster.

Yes, endothermic means it turns cold. and some chemical changes include change of tempature.

The density would be 1.622