Question

Which of the following observations is the best evidence that unknown liquid X is composed of only one pure substance?

Liquid X forms a solid when added to hydrochloric acid

Liquid X boils away completely at a single temperature

Liquid X appears clear and uniform throughout

Liquid X can be easily dissolved in water

Answer 1

Well, a pure substance is defined as being definite and uniform throughout. If it is a liquid you could say the same for it.
This link helps further explain it in a simple way :)
http://www.dummies.com/education/science/chemistry/how-to-distinguish-pure-substances-and-mixtures/
I hope this helps.