Why isn't this test question on pure substances correct?

Hi Guys, the picture below is from a test and the answer was C but I chose B as pure substances really do contain one type of atom! It's still correct right?
enter image source here

1 Answer
Jan 12, 2017

Option #"B"# is incorrect.


Your answer was indeed incorrect.

That is the case because a pure substance is simply a material that contains only one type of compound or element.

For example, a sample of pure iron contains only atoms of iron and nothing else.

On the other hand, a sample of pure water contains only molecules of water and nothing else. Now, water is a compound, which means that it is made up of different elements.

In this case, every molecule of water, #"H"_2"O"#, is made up

  • one atom of oxygen, #1 xx "O"#
  • two atoms of hydrogen, #2 xx "H"#

Therefore, a sample of pure water contains two types of atoms, oxygen atoms and hydrogen atoms.

Another example would be sucrose, #"C"_12"H"_22"O"_11#. A sample of pure sucrose would contain three types of atoms, carbon, hydrogen, and oxygen atoms, all bonded together in sucrose molecules.

So keep in mind that we use the term pure substance for pure elements and for pure compounds.

Option #"C"# is the correct because pure substances tend to have exact melting points, meaning that the pure substance goes from solid to liquid at an exact temperature.

On the other hand, mixtures tend to not to have exact melting points, meaning that mixtures go from solid to liquid at a range of temperatures, i.e. an interval of temperatures, depending on the actual composition of the mixture.