How can water be a pure substance even though it contains two different elements ?

Endeleev saw trends in the physical and chemical properties of elements when he organized them by

How much heat is released upon converting one mole of steam (18.0 g) from 100.0 ∘c to water at 25.0 ∘c? show work and constants, trying to figure out how it works. only given the heat capacity for steam and water so try to only use that

You have a sample of a gas that occupies a volume of 17ml at -111 degrees celsius. what volume does the sample occupy at 88 degrees celsius? show all work asap