posted by janice .
In the qualitative test for nitrate anion, the test goes like this: -
Add dilute sodium hydroxide, then add a little aluminium powder. Warm.
Why do we need to warm the mixture?
Adding the aluminum powder to the nitrate reduces the nitrate ion to ammonia (gas). At cool temps, two things happen:1) the reaction is slower, 2) the low amount of gas produced can be held in solution in the water. As one increases temp, the reaction is much faster, and 2, the solubility of the ammonia in water is greatly reduced, thus the test (releasing ammonia gas) is easier (testing by smell, or the change in color of wet red litmus)