Polarity

The molecule of H2CO is polar because the oxygen end of the bond is more negative than the hydrogen ends of the bond. 
         <-|-  <-|-
          O = C - H
              |
              H
The arrows above are pointing from the more positively charged components to the more negatively charged components of the molecule; there is a arrow pointing from the carbon to the oxygen and arrows pointing from each of the hydrogen to the carbon. 


*There should be an arrow pointing from the  bottom hydrogen to the carbon but I was unable to put it onto this diagram.