When doing experiments, why do we fill the gas generator test tubes almost to the top with chemicals?

Question

According to my lab manual, which I am doing a lab involving manganese and HCl with hydrogen peroxide, it is asking to give two reasons why we fill the tubes up. I read nothing about it in my text or anywhere.

Answer 1

We don't, unless we specifically need to.

But you need to have the same measurements of liquid usually in each experiment.

Answer 2

So the gasses you are collecting are pure, they do not mix or react with air.