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A solution is a homogenous (uniform throughout) mixture, on a molecular level, of two or more substances. It is formed when one or more substances are dissolved in one or more other substances. The scientific nature of solutions is a relatively recent discovery, though solutions in one form or another have been used by people throughout history.

The substances (solids, liquids, or gasses) in a solution make up two phases, the solvent and the solute. The solvent is the substance which typically determines the physical state of the solution (solid, liquid or gas). The solute is the substance which is dissolved by the solvent. For example, in a solution of salt and water, water is the solvent and salt is the solute.

Solutions are formed because the molecules of the solute are attracted to the molecules of the solvent. When the attractive forces of the solvent are greater than the molecular forces holding the solute together, the solute dissolves. There are no rules which will determine whether substances will dissolve however, the cardinal rule of solubility is "like dissolves like." Oil and water don't mix, but oil in oil does.

The substances which make up a solution can be either solids, liquids, gasses, or a combination of any of these. Brass is a solution of solid copper and zinc. Gasoline is a complex solution of liquids. Air is a solution of gasses. Soda pop is a solution of solid sugar, liquid water and carbon dioxide gas. The properties of solutions are best understood by studying solutions with liquid solvents.

When water is the solvent, the solutions are called aqueous solutions. In aqueous solutions, dissolved material often separates into charged components called ions. For example, salt (NaCl) ionizes into Na+ ions and Cl-ions in water. The ionic nature of liquid solutions was first identified by Svante Arrhenius (1859-1927) who, in the early 1880s, studied the way electricity passed through a solution. His ionic theory states that charged particles in a solution will conduct electricity. At the time, his theory was controversial and scorned by the majority of scientists. In the late 1890s, however, when scientists discovered that atoms contained charges, the ionic theory was accepted. He was awarded the Nobel prize in 1903 for his work in understanding the nature of solutions.

Because of molecular interaction, the physical properties of a solution are often different from the properties of the pure substances of which they are composed. For example, water freezes at 32°F (0°C), but a solution of water and salt freezes below 32°F. This is why salt melts ice.

Unlike pure substances, solutions do not have a definite composition. Their composition is dependent on the amount of solute dissolved in the solvent. Concentrated solutions have relatively high amounts of solute dissolved in the solvent while dilute solutions have relatively low amounts. The concentration of a solution is typically expressed in terms of grams of solute per liter of solvent. The concentration of a solution of 0.2 oz (5 g) of sugar dissolved in 3.5 oz (100 g) of water is 0.05 or 5%.

Every solute has a certain degree of solubility in a solvent. Solubility is a number which indicates the normal concentration, at a certain temperature, in which no more dissolving will take place. For example, if a teaspoon of sugar is added to a glass of water, it dissolves, and an unsaturated solution is created. However, if more and more sugar is added, it eventually forms a pile of undissolved sugar on the bottom of the glass. At this point, the normal maximum concentration is exceeded and a saturated solution is created.

The solubility of a solute in a solvent is affected by various factors. Molecular structure, pressure, and temperature all affect the solubility of a system. Heating a solution can increase or decrease solubility. Increasing pressure has a similar effect.

A solution is an important form of matter and is the basis of many of the products we use everyday. From glues to shampoos, soda pops to medicines, solutions will undoubtedly be used by people forever.

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