Topic outline

  • Recall that that there are three basic states of matter: solid, liquid, and gas. 

    In general, compounds with similar bonds have different physical properties depending on their mass.  The heavier a compound is, the more London dispersion forces it has and thus it will be more like a solid.  We compare how a solid will be a solid, liquid or gas by comparing their melting points (mp) or boiling points (bp).  Melting point is the temperature that a substance is converted from a solid to a liquid, and the boiling point is the temperature that substance is converted from a liquid to a gas. 

    Have a look at this short animation to help you understand the difference.   

    The lighter a substance is, the lower the boiling point.  For example, the boiling point of CH4 is much lower   than SiH4 as the elements in the family get progressively heavier, they keep increasing.

    Carbon family boiling point trends

    BUT, look at the graph below.  

     boiling points of elementsSome substances clearly are not following this trend.  In particular, look at H2O compared to H2S.  H2O clearly has a smaller mass than H2S but H2O has the higher boiling point!  Water is clearly an exception to the trends we just discussed for London Dispersion.  And the reason is… there is much more going on than just London Dispersion!  What is the more predominate intermolecular bond in H2O?  Well, we have hydrogen directly attached to oxygen so that can only be… hydrogen bonds!  Water is known as the universal substance and is an important part of all biological beings.  Water has a higher boiling point because of its intermolecular forces! 

    Similarly, NH3 and HF, which also hydrogen bond, are anomalies in the trends of analogous compounds in their groups.