IFAP - Solutions and Mixtures (Lesson)
Solutions and Mixtures
Solids and liquids are frequently combined to make solutions. Recall that solutions are homogeneous mixtures which means that the particles are evenly mixed. In a solution, the macroscopic properties do not vary throughout the sample. This is in contrast to a heterogeneous mixture in which the macroscopic properties depend upon the location in the mixture. While solids dissolved in liquids are probably the most common type of solution, it is not the only type. Actually, every phase of matter can serve as either the solute or the solvent.
The relative amount of a given material dissolved in another is known as its concentration. Usually, a solution contains one component in a much greater amount than that of all other components. This component is called the solvent and may be viewed as the medium in which the other components are dispersed, or dissolved. A solution in which water is the solvent is called an aqueous solution.
For example, potassium dichromate, K2Cr2O7, is an ionic compound composed of colorless potassium ions, K+, and orange dichromate ions, Cr2O72−.
When a small amount of solid potassium dichromate is added to water, the compound dissolves and dissociates to yield potassium ions and dichromate ions uniformly distributed throughout the mixture. The ions are in small quantity compared to the water solvent and are called solutes.
When table sugar, sucrose, is added to water, it also dissolves forming an aqueous solution. However, as a molecular compound rather than a salt, the sucrose does not break into ions but individual sugar molecules are spread throughout the solution.
Electrical conductivity is often used to classify or describe solutions. In an aqueous solution of ionic salts, the ions are released from the crystalline lattice and thus are free to move about the solution resulting in a conductive solution. But in the case where molecular (covalent) substances are the solute particles in water, there are no charged particles because none of the substances dissociate into ions. While the molecules are also free to move about as are the ions in the first type of solution, there are no moving charged particles. This type of solution is not conductive.
Please watch the following video to learn more about electrical conductivity. Be sure your volume is turned on!
Distilled water is a very poor conductor of electricity since very little electricity flows through water. Highly ionized substances are strong electrolytes. Strong acids and salts are strong electrolytes because they completely ionize (dissociate or separate) in solution. The ions carry the electric charge through the solution thus creating an electric current.
Water is used so often as a solvent that the word solution has come to imply an aqueous solution to many people. However, almost any gas, liquid, or solid can act as a solvent.
Other Mixtures
Pure metals are useful but their applications are often limited to each individual metal's properties. However, metals can be mixed to form alloys. Alloys allow metal mixtures that have increased resistance to oxidation, increased strength, conductivity, and melting point. Essentially any property can be manipulated by adjusting alloy concentrations.
Brass is a common alloy. Brass is a mixture of pure zine and copper. Brass is stronger and resists corrosion better than pure zinc or copper. The combination also has a low melting point allowing it to be easily cast into many different shapes and sizes.
Air is a gaseous solution, a homogeneous mixture of nitrogen, oxygen, and several other gases. Oxygen (a gas), alcohol (a liquid), and sugar (a solid) all dissolve in water (a liquid) to form liquid solutions.
You Try It!
Return to the image at the top of this page. Write a series of captions about the dissolving process shown here.
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