Which compound is more polar: water or dicloromethane? I know that water is a polar molecule but I don't know the polarity of dicloromethane or how to figure out which one is more polar than the other?

Answer 1

You can get a guess by comparing the electronegativities of the elements.
For example, in H2O, H has EN of about 2.1 and O of about 3.5 for a difference of about 1.4. In dichloromethane, C is about 2.5 and Cl is about 3 for a difference of about 0.5. We might guess from this that water is more polar. This doesn't take into account the 3-D arrangement of the atoms nor the number of different atoms attached to the central atom (2 H to O in H2O and 2 Cl to C in CH2Cl2) but since both dichloromethane and water are essentially tetrahedral and both have the same number of atoms attached to the central atom (the H atoms to C in CH2Cl2 have little difference in EN) then we would stick with water. The only REAL test, I suppose, is to look up the dipole moment of the molecule. Those tables are not widespread and most don't have extensive listings. However, I found water and dichloromethane on the web; water is 1.85 Debye and CH2Cl2 is 1.4 so our best guess was right.