One of the frequent activities on expedition is melting snow or making “Snowball Soup” as we often called it. For this it is always best to have some liquid water to start with.
If you just pack snow into a billy can and then start heating it the first thing that happens is that the melt water at the bottom of the pan is absorbed into the dry snow leaving a void in the bottom of the pot.
An aluminium pot will often melt a hole at this point and even a steel or titanium one can warp. Oddly enough it will also taint the water with a taste that is sometimes described as “burnt snow”.
By starting with a good quantity of liquid water, this can then be increased by packing snowballs two or three at a time and dropping them into the water to melt until the pot is full of hot water.
I always fill my 24hr. flask and a couple of spare water bottles with this first to ensure that I always have some sacrificial water to start the process again next time before using the remains for the immediate applications.