I want to test the ability of students to read a passage and answer questions about it. Could you please come up with a good question for the passage "Hydrogen is commonly used in power stations as a coolant in generators due to a number of favorable properties that are a direct result of its light diatomic molecules. These include low density, low viscosity, and the highest specific heat and thermal conductivity of all gases."? 

A measure of how compact an object is, defined by mass per volume is called?