Solutions are marvelous things that we use all the time. Water cleans well, but water with soap dissolved in it cleans even better. What exactly are solutions?
A solution is a uniform mixture of a solvent and a solute.
If you take some water and dissolve salt in it, you have salt water. (Solutions are named by the solute and then the solvent.) The solvent is the substance doing the dissolving. The solute is the stuff that gets dissolved. Usually, there is less of the solute than there is of the solution. There's more to the solvent, solute, solution story though, but first, let's have a review of atoms.
One of the first things a student of Chemistry must learn is atomic theory.
The atom is composed of a nucleus and electrons in orbitals outside the nucleus. The nucleus is composed of protons and neutrons. Protons have a positive charge and neutrons have no charge. Electrons have a negative charges, and neutral atoms have an equal number of protons and electrons. The type of matter, or the kind of atom it is (is it gold or is it aluminum?) is determined by the number of protons in the nucleus.
A molecule contains one or more atoms, bound together, acting as one substance.
How does atomic theory relate to solutions?
Solutions are made of two different substances, or two different kinds of molecules. Sugar water is made of sugar and water, for instance. Both sugar molecules and water molecules dance around in the solution. If, however, the sugar and the water molecule had bonded together, there'd be a different molecule, and you would not have a solution.
The substances in solutions can be separated.
Evaporation: To separate the sugar and the water in sugar water, just boil the sugar water. The water will turn to steam and evaporate and leave a solid, sugar residue in the container.
Decanting: With some solutions, you can just pour the solvent away.
Filtering: For some solutions, you can filter the solute out of the solution. If you take dye, for instance, and let it touch paper, the different colors of dye travel through the paper at different rates. (Try this by putting a dot on a damp coffee filter with a kid's marker.)
Fractional Distillation: Both substances in the solution have different boiling points. So, you can heat the solution to the lowest boiling point and keep it there for a while. One of the substances will turn to gas and leave the mixture.
What dissolves in what?
Not all solutes will dissolve in all solvents. Water is sometimes called the universal solvent because it will dissolve so many substances, but not even water can dissolve everything. (Try making a solution of oil and water.)
Likes dissolve likes. Water is very polar. That is, a water molecule, while electrically neutral, has one side that is more positive and one side that is more negative. Because of this, water dissolves substances like salts well. Table salt, for example, has both a Sodium and a Chloride ion. Sodium likes to lose an electron to get a stable electron configuration and Chlorine likes to gain an electron, so both readily dissolve into ions in water.
Oil dissolves non-polar substances well, like other oils.