Like dissolves like

Michael is trying to find out which substances can be dissolved in water. Salt can be dissolved, and alcohol. And vinegar. And soap. But not oil.

What is going on here? The answer lies in the molecular structure of the substances. A water molecule consists of two hydrogen atoms bonded to an oxygen atom. The hydrogen atoms are on the same side of the oxygen atom. And this is important.

Why? Around and between the atoms, there are electrons. And the oxygen atom has a stronger pull on electrons than hydrogen has. The electrons are pulled away from the "hydrogen side" of the molecule, and toward the "oxygen side". We end up with one side more positively charged and the other more negatively charged.

The water molecule is a polar molecule. These areas of plus and minus charge are very good at attracting other molecules with plus and minus charges. - other polar substances. Here's sodium chloride, or table salt. It consists of a crystal lattice of positive and negative ions. Since it has positive and negative areas, it's a polar substance, just like water.

See what happens when we mix it with water. The positive sides of the water molecules are attracted to the negative ions. While the negative sides of the water molecules are attracted to the positive ions. The crystal lattice is broken down as the ions float freely in the water. A polar substance, like sodium chloride, often mixes well with other polar substances, such as water.

And it doesn't even need to be the whole molecule that has areas of plus and minus, it's enough that one part of it is polar. Here is an alcohol molecule and a molecule of acetic acid, found in vinegar. Just like in water, there are oxygen atoms that pull electrons from the nearby atoms. And just like in water, there are hydrogen atoms that become positive. These are also polar molecules, that can be dissolved in water.

Now look at this substance: iodine. An iodine molecule only consists of one type of atom. Neither side pulls electrons harder than the other. No side gets more negative than the other. The structure is symmetrical.

So we have an even distribution of electrons. Which means no polarity. We call this a nonpolar molecule. When we mix the powder with waterō ... it does not dissolve that much.

That's because polar and nonpolar substances do not mix well together. What would be a good substance to dissolve iodine? Let's look at cooking oil. The molecules in oil and fat also have some oxygen atoms, that pull electrons toward them. But look, the oxygen atoms are tucked in between carbon atoms, and not attached to any hydrogen atoms that they can pull electrons from.

And the long "arms" of carbon and hydrogen are totally symmetrical, so no side pulls the electrons more than the other sides - it has an even distribution of electrons. This means that oil, just like iodine, is a non-polar substance. When we add the iodine powder, it dissolves in the oil. Sodium chloride can be dissolved in water but not in oil. Iodine can be dissolved in oil, but not in water.

Why? Because sodium chloride and water are both polar substances, ... while iodine and oil are both non-polar substances. Polar solvents dissolve polar substances. Nonpolar solvents dissolve nonpolar substances.

This is summarized in a rule that states: Like Dissolves Like