Mixtures and Solutions

A mixture is what you get when two or more substances are combined but do not chemically join together. Each substance keeps its own properties. A solution is a special kind of mixture where one substance dissolves completely into another, so evenly that you cannot see the parts anymore. Both are everywhere in daily life, from the air you breathe to the lemonade in your glass.

Think about a salad. Lettuce, tomatoes, and carrots are all tossed together, but each piece is still itself. You can pick out a single carrot slice with a fork. Scientists call this kind of mixture a heterogeneous mixture, which just means the parts are not evenly spread. Sand mixed with iron filings, cereal with milk, and a rocky beach are all heterogeneous mixtures.

A solution is different. When you stir sugar into warm water, the sugar seems to vanish. It has not disappeared. Its tiny particles have spread out evenly among the water particles. You cannot see them, but you can still taste them. A solution is called a homogeneous mixture, which means the same all the way through. Salt water, apple juice, and even the air are all solutions.

In a solution, the thing that dissolves is called the solute. The thing doing the dissolving is called the solvent. In salt water, salt is the solute and water is the solvent. Water is such a good solvent that scientists sometimes call it the universal solvent. It dissolves more kinds of substances than almost any other liquid on Earth.

Not everything dissolves in water, though. Oil does not. If you pour oil into a glass of water, the oil floats in blobs on top. No matter how hard you shake it, the two will separate again. Oil and water form a mixture, but never a solution.

Because mixtures are not chemically bonded, you can usually pull them apart again. A magnet can lift iron filings out of sand. A filter can catch coffee grounds and let the liquid through. To separate salt from water, you can boil the water away and leave the salt crystals behind. This is how people harvest sea salt in shallow pools along warm coasts.

Chemical reactions are different from mixing. In a reaction, substances change into something brand new. In a mixture, the ingredients stay themselves, just hanging out together.