Solutes And Precipitates

Type of physical science: Chemistry

Field of study: Chemical reactions

Solutions abound in nature and are involved in daily living. They participate in processes as varied as the carrying of nutrients to the cells, household cleaning, and pollution.

Overview

A solution is a uniform (homogeneous) mixture of two or more substances (components). Solutions are very common in nature and represent an abundant form of matter. In discussing solutions, it is common to differentiate their components. The component present in the largest amount is called the solvent; all the others are called solutes. In the majority of cases, the solutes present in a solution are of more interest than the solvent, because solutes participate in chemical reactions or other processes when two or more solutions are mixed.

The properties of a solution are uniform throughout; that is, any sample taken from a particular solution will have the same physical and chemical properties (color, density, taste, smell, boiling point) as the solution as a whole. The properties of a solution can be changed by changing its composition--that is, by changing the relative amount of solute(s) and solvent present in it.

"Miscible" substances are those that completely mix with each other in any and all proportions, giving rise to many possible combinations or solutions. "Concentration," which is used to describe solutions, refers to the amount of solute present in a particular amount of solvent. Qualitative terms such as "dilute" (small proportion of solute) and concentrated (large proportion of solute) are commonly used. Another way to express the concentration of a solution is by using the terms "saturated" and "unsaturated." A saturated solution is one in which the solvent has dissolved the maximum amount of the solute it can dissolve at that temperature. An unsaturated solution contains less solute than a saturated one. It is also possible to have a supersaturated solution, by, for example, preparing a solid-liquid solution at a high temperature, removing any undissolved solute, and letting the solution cool down. The cool solution may contain more solute than it normally would. Such solutions are unstable, and by agitating or adding a crystal of the solute, the excess solute can be made to come out of the solution, leaving a saturated solution in its place. For quantitative purposes, the concentration of a solute in a solution is expressed in units of amount of solute per amount of solvent, such as grams per 100 milliliters of solvent, grams per liter of solvent, and so forth. "Dilution" refers to the process of adding more solvent to a solution in order to lower its concentration.

The concentration of a gas in a liquid solution depends on the nature of the gas and the solvent, as well as on the pressure of the gas and the temperature, with solubility decreasing as the temperature increases. Unless gases are involved, the effect of pressure on solubility is of little importance, and the only major factor affecting solubility is temperature; normally, solubility will increase as temperature increases.

The most common solutions found in nature are those in which the components are in liquid form, those in which water is the solvent (aqueous solutions). There are many different types of solutions, however, and they can be classified according to the physical state of the solute and solvent before the solution was formed (the physical state of a solute becomes that of the solvent when a solution is formed).

Examples of liquid solutions include gas in a liquid (such as carbonated beverages--carbon dioxide in water), liquid in a liquid (such as vinegar--acetic acid in water), and solid in liquid (such as sugar in water). Solid solutions include gas in solid (such as hydrogen in platinum), liquid in solid (such as dental filling--mercury dissolved in silver), and solid in solid (such as steel--carbon in iron). Gaseous solutions include gas in gas (such as air), liquid in gas (such as humid air--water vapor in air), and solid in gas (such as moth balls sublimed into air).

When the term "solution" is used, it is normally understood that a liquid solution is meant and that water is the solvent. (The remainder of this presentation will deal with water solutions. Keep in mind that most of the principles presented here will apply to all solutions.)

For a solute to dissolve in a solvent, the attractions between the solute particles (solute-solute interactions) and the attractions between the solvent particles (solvent-solvent interactions) must be overcome and replaced by the new solute-solvent attractions (solute-solvent interactions). These new solute-solvent interactions are the primary driving force behind the formation of a solution. How much of a substance will dissolve in a given solvent will depend on how much the new solute-solvent interactions can compensate for the energy needed to overcome the solute-solute and solvent-solvent interactions.

Substances are held together by three major kinds of intermolecular forces: dispersion or London forces (weak), dipole forces (for polar molecules), and hydrogen bonding. Depending on the nature of the solute and solvent, one of these forces will have to be overcome.

In dissolving a nonpolar substance such as octane (an organic compound present in gasoline) in water (a polar substance), it is necessary to consider the London forces holding the octane molecules together and the hydrogen bonding present in the water. When the two liquids are brought together, the forces of attraction between the octane molecules and the water molecules are not strong enough to overcome the hydrogen bonding between the water molecules, and the solution does not form. Even after vigorous stirring, the water and the octane will separate into two separate layers.

As an example, consider the process of dissolving an ionic solid in water--in this case, sodium chloride (NaCl). Water is a polar molecule, with a partially negative and a partially positive end. As the sodium chloride crystal is put in water, the negative portion of the water molecule (oxygen atom) orients itself so that it is closer to the positive ion in the NaCl, the sodium. The positive part of the water molecule, the hydrogen atoms, orient toward the negative chloride ions. As the polar water molecules surround the ions in the surface of the NaCl crystal, they exert sufficient attraction to cause these ions to break away from the crystal surface and become dissolved, or form part of the solution. As each ion leaves, others are exposed to the water molecules, and the process continues. The new solvent-solute interactions (water-ion) have overcome the old solute-solute and solvent-solvent interactions, and a solution has formed.

The observations performed on the formation of different solutions and the type of intermolecular forces present in them gave rise to the rule of thumb "like dissolves like." That is, polar solvents will dissolve polar substances and nonpolar solvents will dissolve nonpolar substances.

The amount of solute entering a solution per unit of time is called the rate of solution of a solute, and it depends on several factors: More soluble substances dissolve more rapidly than less soluble ones; a solid dissolves only at its surface--the more finely dissolved the solute, the greater the exposed surface area per unit of mass and the more rapidly it will dissolve; dissolved molecules diffuse away from the solid relatively slowly--therefore, the solution next to the solid becomes saturated rather quickly, and stirring, shaking, or heating the mixture will bring the unsaturated solution in contact with the solute and increase the rate of solution; and heating increases the rate of solution--the vapor pressure is lower because the surface of the solution now has solute molecules that do not escape into the vapor phase as easily as solvent molecules. Since a liquid boils when its vapor pressure equals the atmospheric pressure, a solution that has a lower vapor pressure than the pure substance will need a higher temperature for this to happen and will then have a higher boiling point. Each solvent has its characteristic boiling-point elevation constant, since this phenomenon is independent of the nature of the solute. The freezing point of a substance is the point at which the vapor pressure of the liquid equals that of the solid. If the vapor pressure of the solution is lower than that of the pure solvent, the crossing point will be at a lower temperature--hence, the freezing point depression. Osmosis refers to the flow of solvent from a region of low solute concentration to one of high solute concentration through a semipermeable membrane. This flow depends only upon the amount or concentration of solute in the solution. Osmotic pressure is the pressure needed to stop this flow of solvent. It is of critical importance in the fluid transport in all living systems throughout the plant and animal kingdom.

A precipitate is any solid that comes out of a liquid solution. The reasons for the precipitation of a solid can be various, such as an increase in the concentration of the solute until the solubility is exceeded--the excess solute will come out or precipitate out of solution; changes in the nature of the solute that will alter its solubility, such as denaturing a protein through heating; the combination of ions in a solution to form an insoluble or slightly soluble compound; and changing the conditions of a solution (such as pH) so that the solubility of the solute decreases.

Applications

Pure substances and solutions are the two major categories of matter. The importance of solutions, especially water solutions, cannot be overemphasized. Most chemical reactions in the laboratory are carried out in solution, and most of the body's chemistry also occurs in solution.

Because of its ability to dissolve a large number of substances, water is sometimes called the universal solvent. This ability to dissolve many substances has great importance in human life, since human bodies are mainly composed of water, and most body chemistry takes place in water solution. It also has some drawbacks, however, especially regarding water pollution. All sources of contamination to the water supply are called pollutants. A pollutant is a substance that makes water unfit for drinking, unfit to support aquatic life, or unfit for any other specific purpose.

Oxygen from the air dissolves in the water of rivers, lakes, and streams. Aquatic animals and plants depend on this oxygen supply for life. If the oxygen content is too low, the life-forms may not be able to survive. Modern industrial processes in the paper, food, chemical, and other industries have added a large amount of organic waste materials to lakes and streams.

The bacteria present in water use some of the oxygen dissolved in the water to convert this organic material into simple molecules and ions, such as carbon dioxide and sulfide ions. The oxygen supply in the water is therefore depleted, and the aquatic life is affected.

Fertilizers contain large amounts of phosphate, which is dissolved and carried away from fields into rivers and waterways. This poses a special pollution problem, because algae can grow rapidly in solutions containing phosphate and "choke off" a waterway. Nitrates, also present in fertilizers, can reach drinking water and present an additional hazard, since they are changed in the human body to nitrite ions, which destroy the ability of hemoglobin to transport oxygen to the cells.

Many industries produce large amounts of highly poisonous waste products that include organic solvents, dyes, and heavy-metal ions. These substances can dissolve in water and pollute it. In order to get rid of these solutes in the water solution, the water has to be treated. The contaminated water is put into a sedimentation tank, where any solids that were not dissolved will settle down. The water is then filtered to remove any remaining particles, aerated, and chlorine is added to kill the bacteria. These examples show the drawbacks of the great ability of water to dissolve different substances, since many of them are harmful and difficult to remove from the solution.

The shape of the earth was caused by the action of solutions, since rainwater dissolves substances in stones and rocks and deposits them somewhere else. Mineral deposits, the result of reactions that have taken place in solution, are left after the solvent has evaporated.

Metal alloys used in processes ranging from dental fillings to jewelry are other examples of solutions--in this case, solid solutions. Since the properties of a solution depend on its composition, changing the amount of one of the metals changes the properties of an alloy. For example, the gold used in jewelry is not pure gold, but an alloy whose composition is indicated by the karat number. Twenty-four-karat gold is pure gold, while 18- or 10-karat gold has other metals that increase the strength of the alloy and affect its shine. White gold is really an alloy of gold and silver, and rose-colored or pink gold is an alloy of gold and copper. A ruby is a solid solution that contains about 1 percent of chromium oxide in an aluminum-oxide matrix.

Sodas are effervescent because they contain dissolved carbon dioxide. The carbon dioxide remains in solution as a result of the high pressure inside the can of soda. When the can is opened, the pressure decreases and the gas comes out of the solution, creating bubbles and eventually leaving the soda flat.

There are many practical applications involving the colligative properties of solutions.

The freezing-point depression is the basis of the antifreeze solutions used in cars. The antifreeze is usually a solution of ethylene glycol in water, which has a lower freezing point than pure water and will allow the car to perform at lower than normal temperatures. (It also increases the boiling point, allowing the engine to perform at higher temperatures.) The same principle is used to make ice cream at home: The salt-water mixture that is used provides a lower temperature than plain ice would, facilitating the process. Sodium chloride or calcium chloride is added to the ice formed in the streets in winter. These salts form a solution with the water that has a lower freezing point, permitting the ice to melt at a lower temperature, clearing the streets.

When food is converted into a form that the body can use, useful nutrients go into solution and pass through the walls of the digestive tract into the blood by means of osmosis.

Once in the blood, the nutrients are carried through the body in a solution until they reach their final destination. Water and nutrient intake from the soil through the roots of plants also occurs by means of osmosis. When intravenous solutions are used, extreme care must be taken that they have the same osmotic pressure as blood (are isotonic), so that blood cells will not burst or shrink when the solution is added, as a result of water flow from one region to another. Since the osmotic pressure is directly proportional to the molecular weight of the solute, it is used to determine the molecular weights of macromolecules such as proteins and plastics. One of the methods used in the desalination of water is reverse osmosis. The exertion of a hydrostatic pressure greater than the osmotic pressure causes water to flow through a semipermeable membrane from the more concentrated salt solution to the pure-water side.

The formation of a precipitate can be used as an antidote. If a person ingests crystals of barium nitrate, it will dissolve in water and the person will end up with barium ions in the stomach. These ions are highly poisonous, but they can be removed easily by giving the person a drink of a sodium-sulfate solution. The barium ions will react with the sulfate ions and produce a precipitate of barium sulfate, removing the barium ions from solution. The use of precipitation reactions in analytical chemistry is an extremely common and useful method for the determination of the concentration of species in a solution. A known amount of precipitating ion is added, and the concentration of the ions in solution can be determined from the amount of precipitate formed.

Context

The study of solutions by chemists dates back to the 1700's. In 1748, Abbe Nollet described experiments in which pure water and aqueous solutions were separated by animal membranes. He observed that the pure solvent moved through the membrane and entered the solution, never the other way around. This first description of the process of osmosis generated great interest in the study of solutions. In 1885, Jacobus Henricus van't Hoff noted that the osmotic pressure is directly related to the concentration of the solution, and in 1886, he showed that the molecules of dissolved substances moved randomly throughout the liquid in which they were dissolved, much as gases do. For this work, he was awarded the first Nobel Prize in Chemistry in 1901.

In the mid-1800's, Francois-Marie Raoult found the relationship between the molecular weight of a dissolved substance and the vapor pressure of the solution. This gave rise to the idea that some properties of a solution depend only upon the nature of the solvent, not that of the solute--that is, the concept of colligative properties. Svante August Arrhenius first postulated that electrolyte solutions result from the separation of the neutral species into positively and negatively charged ions. He received the Nobel Prize in Chemistry in 1903 for his efforts.

The detailed study of the interactions present in solutions has been an important part of chemical research. The theories used to explain the behavior of solutions can be used to develop better methods to purify polluted water; stronger, lighter metal alloys for use in aircraft; more soluble drugs and synthetic nutrients; more selective and precise methods for the determination of the concentration of solutes; and to determine the optimal conditions under which chemical reactions can take place in solution. The study of solutions and their properties has fascinated scientists, chemists in particular. This field of study has many practical applications.

Principal terms

COLLIGATIVE PROPERTIES: physical properties of a solution that depend only on the number of solute particles in a given quantity of solvent, not on the chemical identities of those particles

CONCENTRATION: the amount of substance present in a given amount of total solution

MISCIBLE SUBSTANCES: two substances that completely mix with each other in any and all proportions

PRECIPITATE: a solid that forms in a solution

SOLUBILITY: the maximum amount of a substance (solute) that will dissolve in a medium (solvent)

SOLUTE: the dissolved substance present in a solution, always of lesser volume or concentration than the solvent

SOLUTION: a homogeneous mixture of two or more substances

SOLVENT: the substance present in the largest amount in a solution, the medium in which the solute is dissolved

Bibliography

Bauer, R. D., and R. L. Loeschen. CHEMISTRY FOR THE ALLIED HEALTH SCIENCES. Englewood Cliffs, N.J.: Prentice-Hall, 1980. Chapter 6, "Solutions," gives a good presentation of the basic theory, plus related health and biochemistry examples.

Fine, Leonard W., and Herbert Beall. "Properties of Solutions and the Colloidal States." CHEMISTRY FOR ENGINEERS AND SCIENTISTS. Philadelphia: Saunders College Publishing, 1990. This excellent presentation of the properties of solutions at the molecular level includes a discussion of colligative properties, with examples. Excellent graphics and pictures.

Kroschwitz, Jacqueline I., and Melvin Winokur. "Solutions." CHEMISTRY: GENERAL, ORGANIC, BIOLOGICAL. 2d ed. New York: McGraw-Hill, 1989.

Gives a complete presentation of the theory of solutions with a good balance between qualitative and quantitative discussions.

Mohrig, J. R., and W. C. Child. CHEMISTRY IN PERSPECTIVE. Boston: Allyn & Bacon, 1987. Chapter 10, "Salts and Solutions," gives a good nonscience-oriented presentation of the subject with good examples.

Stoker, H. S. PREPARATORY CHEMISTRY. New York: Macmillan, 1990. Chapter 13, "Solutions: Terminology and Concentrations," presents basic information on the process of solution formation, plus detailed sections on the calculation of the concentration of solutions.

Concentrations in Solutions

Diffusion in Gases and Liquids

The Chemistry of Water Pollution