Question

An unknown blue liquid was boiled. A blue solid was left after the liquid boiled away. Is the unknown material a mixture or a pure substance?
I think the unknown liquid is a ___________ because...

Answer 1

It is a mixture. 
take salt water for example if you boil the water the salt will be left behind. salt water on its own is not a pure substance a pure substance would be just the water by its self. 
hope that helps.