I'd like a nice answer to my chemistry question....?

Question

How can molecule, such as CO2 or CH4 contain polar bonds yet still be a non polar substance?

Answer 1

All the polar bonds are pulling in opposite directions, so there is no net movement of charge as far as the whole molecule is concerned.

Answer 2

I'll answer this question to cover a few of your other questions as well.

The above answer is exactly correct. You need to understand the geometry of the molecule before you can determine if it has a net dipole moment (is polar). CO2 is a linear molecule. O is more electronegative than C, so it will be slightly more negative than C in a bond. But since the molecule is linear, the charge is distributed evenly at both ends. Because of this, there is no net dipole moment. Think of it as 2 guys pulling on a rope. If each one pulls with the same force, it doesn't go anywhere.

CH4 is esentially the same, except you are pushing on the central atom, and in 4 directions rather than 2. Still, all the individual dipoles cancel out.

Now look at something like NH3. N is more electronegative than H, so it pulls in the electrons. But, the atom isn't symmetrical, its trigonal pyrimidal. So there is a net dipole moment!

Polar molecules are more soluable in polar solvent, like water. They also tend to react (align) in a megnetic field die to the difference in electrical charge on the molecule. Polar molecules usually have higher melting and higher boiling points than nonpolar molecules due to stronger intermolecular forces between dipole moments.

Hope this helps!