Properties of Pure Substances

It would be nice if you could see now some Iodine. Maybe at school? Note that you may use Iodine for desinfection, but that Iodine is not pure; it is Idone(s) dissolved in a liquid!
Pure Iodine is a solid, built up of dark purple, almost black cristals!
You might observe what happens when pure cristals of Iodine do evaporate. Iodine does not melt, but changes, when heated, directly form solid to gas.
If you dissolve Iodine in alcohol, you get the Iodine that the chemist is selling. Because of the alcohol, it hurts when put on a wound.
There is another Iodine solution available that does not hurt. Then Iodine was dissolved in water using a trick (because normally Iodine does not dissolve in water. Water is polar en Iodine is non polar. We come tot that 'trick' later.

Now look at the scheme below: you see models of compounds with bonds between Hydrogen and elements of the second period of the Periodic Table (C, N and O)

methane ammonia(g) water
CH4 NH3 H2O
tetraedric
no dipole		 dipole dipole
b.p. 112 K b.p. 240 K b.p. 373 K

The bonding between C and H is hardly polar. The bonds between O and H and the bonds between N and H are polar enough.
If a molecule contains polar bondings, then there are two options:
  1. In the three dimensional shape of the molecule, the two centre points (the positive and the negative one) have not the same position, do not coincide. This is the case in molecules of water and ammonia.
  2. The centre points of the negative and the positive charges do coincide, have the same position. The little bit of polar character between C and H in methane do not deliver a dipole molecule; the molecule is too symmetrical for that.
Ammonia will dissolve in water, methane will not.

symmetrical non symmetrical

electronic structure of carbon dioxyde.
There are real polar bonds,
but in the total molecule
the charges do annule each other.
There is no dipole molecule.
electronic structure of sulphur dioxyde.
This molecule is non symmetric
and the two polar bonds between S and O
result in a dipole molecule.

SO2 will certainly better dissolve in water then CO2

Just to remembrance:
The very important data of a pure substance is that it is built up of only one kind of particles.
For example, pure sugar is built up of just only molecules of saccharose (C12H22O11).