Introduction to Solution
Solution, a mixture of two or more substances in which the particles of each substance are of molecular or atomic size. A solution is always a homogeneous mixture - that is, every part of the mixture is exactly like every other part. Solutions are formed by dissolving one substance, called the solute, in another substance, known as the solvent. One of the world's most abundant and common solutions is seawater, which consists primarily of salt (the solute) dissolved in water (the solvent).
Solutions can occur in any of the three states of matter: solid, liquid, and gaseous. The most common solutions are liquid, composed of a solid, liquid, or gas dissolved in a liquid solvent. Other solutions result from the dissolving of one gas in another, a liquid in a gas, or even one solid in another. (Solid solutions are formed while the materials—copper and zinc for brass—are heated to the liquid state.) Water is the most abundant and widely used solvent. Its solutions are usually called aqueous solutions.
The fluids found in plants and animals are solutions of great complexity. Many drugs, cleaning compounds, beverages, and cosmetics are supplied in solution form. In industry, hundreds of products are made with the help of chemical reactions that occur in solutions.
In a solution, the solute breaks down into tiny particles that become uniformly distributed among particles of the solvent. In some solutions, such as sugar in water, the solute particles are molecules. In solutions of acids, bases, or salts in water, the solute breaks down into electrically charged particles known as ions. Ionic solutions, called electrolytes, conduct electricity and are very active chemically. Because of their extremely small size, solute particles (such as the sugar in a solution of sugar in water) will not settle out, nor can they be filtered out, as is possible with other mixtures such as colloids and suspensions.
Depending on their nature, the solute and solvent may or may not undergo a chemical change when a solution is formed. When sugar is dissolved in water, the change is only physical, since when the water is evaporated away, the sugar remains. When hydrochloric acid dissolves a metal such as zinc, there is a chemical change. If the solution is evaporated, it is zinc chloride, not the original zinc, that remains.
Solubility
Solubility is usually defined as the amount of a solute that will dissolve in a particular weight of a solvent. The solubility of most solutes varies with different solvents. Sugar, for example, is very soluble in water but only slightly soluble in methyl alcohol. The solubility of most solids and liquids increases with an increase in temperature. Gases, however, generally decrease in solubility as the temperature increases, provided the pressure remains the same. If the pressure is increased, most gases become more soluble. This is why carbonated drinks are bottled under pressure. When the pressure is released by opening the container, the carbon dioxide dissolved in the drink loses most of its solubility and begins to escape.
Types of Solutions
A solution containing only a small proportion of the solute that could be dissolved is called a dilute solution. If a relatively large amount of solute is present, the solution is considered to be concentrated. Both dilute and concentrated solutions are said to be unsaturated; that is, more solute could be added to a given amount of the solution. A saturated solution, however, holds all the dissolved solute it can accommodate at a particular temperature and is said to be in a state of equilibrium.
If the temperature of a saturated solution is lowered, the solubility of the solute is usually reduced and some of the solute returns to its undissolved state in crystalline form. Under certain conditions, however, all of the solute may remain in solution despite a considerable lowering of the solution's temperature. Such a solution is said to be supersaturated, since it contains more dissolved solute than would ordinarily be possible at the lower temperature. A supersaturated solution is highly unstable; a slight jarring of the container, or the addition of a crystal of the solute, will usually cause rapid crystallization of the excess solute.
Concentration of Solutions
The amounts of solute and solvent in a solution are expressed as concentration. The concentration value of a solution is the mass (commonly referred to as the weight) or volume of solute in a specified amount of solvent or total solution. There are three common methods of expressing concentration:
Percentage Concentration,the per cent of solute by weight or by volume in 100 parts of solution. For example, an aqueous solution of 5 per cent sodium chloride by weight contains 5 grams of sodium chloride in 95 grams of water. Percentage is used to express the concentration of liquid, solid, or gaseous solutions.
Molarity, or Molar Concentration,the number of moles of solute per liter of total solution. (One mole of a substance is the amount whose weight in grams is numerically equal to the atomic weight or molecular weight.) For example, a 1 molar (1 M) aqueous solution of sodium chloride (molecular weight, about 58.44) contains one mole (in this instance, about 58.44 grams) of sodium chloride in enough water to make one liter of solution. Molarity is used to express the concentration of liquid solutions.
Molality, or Molal Concentration,the number of moles of solute in 1,000 grams of solvent. For example, 1 molal (1 m) aqueous solution of sodium chloride contains one mole of sodium chloride in 1,000 grams of water. Molality, like molarity, is used to express the concentration of liquid solutions.