Fun with chemistry!
The Vari-etch Powder, when mixed with water will create an endothermic reaction within the chemical bonds of the mixture. This means that the chemical reaction occurs by drawing heat out of the surrounding area. If you don't use boiling water to mix the powder into solution, it won't actually mix.
The chemical reaction will run out of heat before the mixture goes into suspension.
Once you mix the Vari-etch one time, there is no need to heat the mixture again. All you need to do is use boiling water for the initial mixing (and the water will basically immediately cool down anyway once mixed).
Then you're done. Ready to etch, and etch, and etch, and etch.....