What is the purpose of a phase diagram?

What information are you able to learn from reading a phase diagram?

Sketch a phase diagram and label all of it parts including; solid, liquid, gas, critical point, supercritical fluid, and triple point. Upload your sketch here.

Why is it possible for there to be more than $1$ triple point on a phase diagram?

Can a phase diagram have more than one solid phase and explain your answer.

Look at water's phase diagram, explain why the freezing point curve goes backwards when all other phase diagrams it goes forward as the pressure and temperature increase. Estimate what the melting point and freezing point's would be at a pressure of $150$kPa.