CHEM 1200 - 004
CHEM 1200 - 004 PHYS 1101 - B02
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This 6 page Class Notes was uploaded by Mindavi Rey Barnig on Wednesday January 13, 2016. The Class Notes belongs to PHYS 1101 - B02 at University of North Carolina - Charlotte taught by Awad Samir Gerges in Fall 2015. Since its upload, it has received 27 views. For similar materials see Introductory Physics I in Physics 2 at University of North Carolina - Charlotte.
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Date Created: 01/13/16
What is Chemistry ? Chemistry is defined as the study of matter and its properties. Matter is defined as everything that has mass and occupies space. Although these definitions are acceptable, they do not explain why one needs to know chemistry. The answer to that query is that the world in which we live is a chemical world. Your own body is a complex chemical factory that uses chemical processes to change the food you eat and the air you breathe into bones, muscle, blood, and tissue and even into the energy that you use in your 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 carrying substances such as vinyl chloride, sulfuric acid, and ammonia. 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 the package contains, such as preservatives, and the nutritional content in terms of vitamins, minerals, fats, carbohydrates, and proteins. Everyday life is besieged with chemicals. In beginning the study of chemistry, it is unwise to start with topics as complex as the latest miracle drug. We will begin with the composition of matter and the different kinds of matter. We can then talk about the properties of the different types of matter and the changes that each can undergo. You will learn that each of these changes is accompanied by an energy change and learn the significance of these energy changes. The Kinds of Matter Chemistry is defined as the study of matter. In this introductory text we will not study all types of matter. Rather, we will concentrate on simple substances, the properties that identify them, and the changes they undergo. Pure Substances 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 a 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, some of which we have listed. The properties we have described hold true for any sample of baking soda. These properties are the kinds in which we are interested. A note about the term pure; in this text, the word pure means a single substance, not a mixture of substances. As used by the U.S. Food and Drug Administration (USFDA), the term pure 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. Mixtures 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. One type of chromiumnickel 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, foodprocessing equipment, and kitchenware 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. You can often tell from the appearance of a sample whether it is a mixture. For example, if river water is clouded with mud or silt particles, you know it is a mixture. If a layer of brown haze lies over a city, you 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, you 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. Figure 1.2 shows the relationships between different kinds of matter. The Properties of Matter Each kind of matter possesses a number of properties by which it can be identified. In Section 1.2A , we listed some of the properties by which the pure substance baking soda 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. The decomposition of baking soda on heating is a chemical property. You can observe the decomposition of baking soda, but, after you make this observation, you no longer have baking soda. Instead you have carbon dioxide, water, and sodium carbonate. A physical change alters only physical properties, such as size and shape. A chemical change alters chemical properties, such as composition (see Figure 1.3). Physical Change Chemical Change FIGURE 1.3 Physical and chemical properties of matter. Breaking a stick physically changes its size but not its composition. Burning wood changes it chemically, turning it into other substances. 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. A mixture of salt and sand can be separated because salt dissolves in water but sand does not. If we add water to a saltsand 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. Solubility is a physical property. Pure substances, on the other hand, can be separated into their components only by chemical changes. 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. You are familiar with water as a solid (ice), a liquid, and a gas (steam) (Figure 1.4). You have seen wax as a solid at room temperature and a liquid when heated. You have probably seen carbon dioxide as a solid (dry ice) and been 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. Solid Water Liquid Water Gaseous Water (ice) (steam) FIGURE 1.4 The three physical states of water: ice (solid), water (liquid), and steam (gas). 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. The Law of Conservation of Mass The Law of Conservation of Mass states that matter can be changed from one form into another, mixtures can be separated or made, and pure substances can be decomposed, but the total amount of mass remains constant. We can state this important law in another way. The total mass of the universe is constant within measurable limits; whenever matter undergoes a change, the total mass of the products of the change is, within measurable limits, the same as the total mass of the reactants. The formulation of this law near the end of the eighteenth century marked the beginning of modern chemistry. By that time many elements had been isolated and identified, most notably oxygen, nitrogen, and hydrogen. It was also known that, when a pure metal was heated in air, it became what was then called a calx (which we now call an oxide) and that this change was accompanied by an increase in mass. The reverse of this reaction was also known: Many calxes on heating lost mass and returned to pure metals. Many imaginative explanations of these mass changes were proposed. Antoine Lavoisier (17431794), a French nobleman later guillotined in the revolution, was an amateur chemist with a remarkably analytical mind. He considered the properties of metals and then carried out a series of experiments designed to allow him to measure not just the mass of the metal and the calx but also the mass of the air surrounding the reaction. His results showed that the mass gained by the metal in forming the calx was equal to the mass lost by the surrounding air. With this simple experiment, in which accurate measurement was critical to the correct interpretation of the results, Lavoisier established the Law of Conservation of Mass, and chemistry became an exact science, one based on careful measurement. For his pioneering work in the establishment of that law and his analytical approach to experimentation, Lavoisier has been called the father of modern chemistry. Note that this step forward, like so many others in science, depended on technology in this instance, on the development of an accurate and precise balance (see Figure 1.5). FIGURE 1.5 Lavosier's apparatus for heating mercury in a confined volume of air (after a drawing by Mme. Lavoisier).