Explanation:
Van der Waals interactions occur between any two or more molecules. They are caused by a fluctuation in electron density, as electrons are not actually fixed in a shell, but actually freely moving as a 'cloud of electron density'. This means that sometimes one end of a molecule can become more partially negatively charged as all electrons move to that side, and conversely it can attract the more partially positive end of a molecule (that has little electrons).
Hydrogen bonds only occur between molecules that contain oxygen, nitrogen and fluorine bonded to a hydrogen atom.
Hydrogen bonding is also the strongest intermolecular force there is, but not strong in comparison to ionic and covalent bonds. Therefore, hydrogen bonds are much stronger than Van der Waals forces. Hydrogen bonds only form if oxygen, nitrogen and fluorine are bonded to a hydrogen atom, as they have the greatest electronegativity differences (look at an electronegativity table), and when the overall molecule is polar (have unequal charges). This allows the molecule to be able to attract another molecule from one of the bonded atoms to a hydrogen atom.
"As the molecules in a sample move faster the temperature reading will be lower" is not true.
Yes they are a pure substance
