What is true of a molecule?

a. Each molecule of a substance has different properties

b. All molecules of a substance have the same properties

c. All molecules of a substance have one type of atom

d. Each atom of a molecule is the same size

The correct answer is that all molecules of the same substance have the same properties - because they are composed of the same kinds of atoms.

Atoms of different pure substances (such as the ones from the periodic table) have different sizes - so not all atoms are equal. But all molecules of water, which is a substance is always composed of two hydrogen atoms and one oxygen atom - that gives all the molecules the same properties.