Section 1 - What Is a Solution?Mixtures

You have learned about the difference between pure substances and mixtures.Mixtures can either be heterogeneous or homogeneous. The particlesof a heterogeneous mixture are large enough to see under a microscope.In a homogeneous mixture, however, the particles are molecule-sized, sothe mixture appears uniform, even under a microscope. A homogenousmixture is also known as a solution.

Solutions Are Stable Homogeneous Mixtures

Two substances when mixed together to create a solution will not separateover time. If separation occurs between two substances it is not a true solution.

Solution Is a Broad Term

Any mixture that is homogeneous on a microscopic level is a solution.According to that definition, air is a gaseous solution. However, whenmost people use the word solution, they are usually referring to a homogeneousliquid mixture. A homogeneous liquid mixture has one maincomponent—a liquid—as well as one or more additional ingredients thatare usually present in smaller amounts. The primary ingredient in a solutionis called solvent, the and the other ingredients are the solutes andare said to be dissolved in the solvent. Water is the most common solvent.Although it is a very common substance, water is a unique solventbecause so many substances can dissolve in it. Solutions in which water isthe solvent are called aqueous solutions.

Separating Mixtures

There are many ways to separate mixtures into their components. Thebest method to use in a particular case depends on the kind of mixtureand on the properties of the ingredients.

Method 1 –DecantingDecanting separates a liquid from solids that have settled. To decant a mixture, carefully pour off the liquid to leave the solids behind.

Method 2 – CentrifugeA centrifuge is used to separate substances of different densities. The centrifuge spins rapidly, and the denser substances collect at the bottom of the tube.

Method 3 – FiltrationGround coffee is separated fromliquid coffee by filtration. Thefiltrate—the liquid and whateverpasses through the filter—collectsin the coffeepot. The solid groundsstay on the filter.

Method 4 – EvaporationIn saltwater ponds such as this one, sea water evaporates, and salts, mainly sodium chloride, are left behind.

Method 5 – DistillationIf the boiling points of the components are different, distillation can separate them based on their boiling points. As one component reaches its boiling point, it evaporates from the mixture and is allowed to cool and condense.