January 13, 2010 by
Filed under nuclear medicine equipment
The world in which we live is a chemical world. Our own body is a complex chemical factory that uses chemical processes to change the food we eat and the air we breathe into bones, muscle, blood, and tissues and even into the energy that we use in our daily living. When illness prevents some part of these processes from functioning correctly, the doctor may prescribe as a medicine a chemical compound, either isolated from nature or prepared in a chemical laboratory by a chemist. The world around us is also a vast chemical laboratory. The daily news is filled with reports of acid rain, toxic wastes, the risks associated with nuclear power plants, and the derailment of trains, trucks etc. carrying substances such as vinyl chloride, sulfuric acid, and ammonia. However, not all chemical news is of disasters. The daily news also carries stories (often in smaller headlines) of new drugs that cure old diseases; of fertilizers, insecticides, and herbicides designed by chemists to allow the farmers to feed our growing populations and of other new products to make our lives more pleasant. The packages we buy at the grocery store list their contents, including what chemicals a package contains, such as preservatives, and the nutritional contents in terms of vitamins, minerals, fats, carbohydrates, and proteins. In fact our everyday life is besieged with chemicals.
A pure substance consists of a single kind of matter. It always has the same composition and the same set of properties. For example, baking soda is a single kind of matter, known chemically as sodium hydrogen carbonate. A sample of pure baking soda, regardless of its source or size, will be white solid containing 57.1% sodium, 1.2% hydrogen, 14.3% carbon, and 27.4% oxygen. The sample will dissolve in water. When heated to 270°C the sample will decompose, giving off carbon dioxide and water vapor and leaving a residue of sodium carbonate. Thus, by definition, baking soda is a pure substance because it has a constant composition and a unique set of properties that hold true for any sample of baking soda. These properties are the kinds in which we are interested. The word pure means a single substance, not a mixture of substances and the term pure also means “fit for human consumption.” Milk, whether whole, 2% fat, or skim, may be pure (fit for human consumption) by public health standards, but it is not pure in the chemical sense. Milk is a mixture of a great many substances, including water, butterfat, proteins, and sugars. Each of these substances is present in different amounts in each of the different kinds of milk.
A mixture consists of two or more pure substances. Most of the matter we see around us is composed of mixtures. Seawater contains dissolved salts; river water contains suspended mud; hard water contains salts of calcium, magnesium, and iron. Both seawater and river water also contain dissolved oxygen, without which fish and other aquatic life could not survive. Unlike the constant composition of a simple substance, the composition of a mixture can be changed. The properties of the mixture depend on the percentage of each pure substance in it.
Steel is an example of a mixture. All steel starts with the pure substance iron. Refiners then add varying percentages of carbon, nickel, chromium, vanadium, or other substances to obtain steels of a desired hardness, tensile strength, corrosion resistance, and so on. The properties of a particular type of steel depend not only on which substances are mixed with the iron but also on the relative percentage of each of the constituents. One type of chromium-nickel steel contains 0.6% chromium and 1.25% nickel. Its surface is easily hardened, a property that makes it valuable in the manufacture of automobile gears, pistons, and transmissions. The stainless steel used in the manufacture of surgical instruments, food-processing equipments, and kitchen wares is also a mixture of iron, chromium, and nickel; it contains 18% chromium and 8% nickel. Steel with this composition can be polished to a very smooth surface and is very resistant to rusting.
We can often tell from the appearance of a sample whether it is a pure substance or mixture. For example, if river water is clouded with mud or silt particles, we know it is a mixture. If a layer of brown haze lies over a city, we know the atmosphere is mixed with pollutants. However, the appearance of a sample is not always sufficient evidence by which to judge its composition. A sample of matter may look pure without being so. For instance, air looks like a pure substance but it is actually a mixture of oxygen, nitrogen, and other gases. Rubbing alcohol is a clear, colorless liquid that looks pure but is actually a mixture of isopropyl alcohol and water, both of which are clear, colorless liquids. As another example, we cannot look at a piece of metal and know whether it is pure iron or a mixture of iron with some other substance such as chromium or nickel.
Each kind of matter possesses a number of properties by which it can be identified. These properties fall into two large categories: (1) physical properties, those that can be observed without changing the composition of the sample, and (2) chemical properties, those whose observation involves a change in composition. Baking soda dissolves readily in water. If water is evaporated from a solution of baking soda, the baking soda is recovered unchanged; thus, solubility is a physical property. However, the decomposition of baking soda on heating is a chemical property. We can observe the decomposition of baking soda, but, after we make this observation, we no longer have baking soda. Instead we have carbon dioxide, water, and sodium carbonate. While, a physical change alters only physical properties, such as size and shape, a chemical change alters chemical properties, such as composition.
This discussion of properties points to another difference between pure substances and mixtures. A mixture can be separated into its components by differences in their physical properties. For example, a mixture of salt and sand can be separated because salt dissolves in water but sand does not. If we add water to a salt-sand mixture, the salt will dissolve, leaving the sand at the bottom of the container. If we pour off the water, the sand will remain. If we boil off the water from the salt solution, we will get the salt by itself. We have separated the two components of the mixture by a difference in their ability to dissolve in water. Thus solubility is a physical property.
Pure substances, on the other hand, can be separated into their components only by chemical changes. For example, when added to water, the pure substance sodium bicarbonate does not separate into sodium, hydrogen, carbon, and oxygen, although these components of sodium bicarbonate differ greatly in their solubilities in water. One of the important physical properties of a substance is its physical state at room temperature. The three physical states of matter are solid, liquid and gas. Most kinds of matter can exist in all three states. We are familiar with water as a solid (ice), a liquid, and a gas (steam). We have seen wax as a solid at room temperature and a liquid when heated. We also know that carbon dioxide exists as a solid (dry ice) too and we are aware of it as a colorless gas at higher temperatures.
The temperatures at which a given kind of matter changes from a solid to a liquid (its melting point) or from a liquid to a gas (its boiling point) are physical properties. For example, the melting point of ice (0°C) and the boiling point of water (100°C) are physical properties of the substance water. Like pure substances, mixtures can exist in the three physical states of solid, liquid, and gas. Air is a gaseous mixture of approximately 78% nitrogen, 21% oxygen, and varying percentages of several other gases. Rubbing alcohol is a liquid mixture of approximately 70% isopropyl alcohol and 30% water. Steel is a solid mixture of iron and other pure substances.
Dr Badruddin Khan teaches Chemistry in the University of Kashmir, Srinagar, india.
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