D1.1 Substances and Chemical Reactions
A fundamental aspect of chemistry is that substances change into other substances. A process in which one or more substances changes into one or more different substances is called a chemical reaction. This course aims to enhance your understanding of substances and chemical reactions. For a specific substance, what properties can you expect? Given one or two substances, is a reaction likely? When a reaction occurs, what new substances are formed? How is energy related to chemical reactions? Can we make new substances that have properties we want (such as alleviating disease)? Ability to answer such questions is valuable in a broad range of fields, from physics to pharmacy.
Activity: Chemical Reactions
Here are videos of some chemical reactions. Watch each reaction carefully and write down your observations.
Reactions of copper with air and of copper oxide with hydrogen:
Reaction of lead nitrate aqueous solution with potassium iodide solution:
Reactions of Li, Na, and K with air and of Li, Na, K, Rb, and Cs with water (if this video does not load for you, you can also watch it here):
For each chemical reaction, write a few sentences in your class notebook describing the reaction: How do you know that a reaction occurred? What are similarities and differences among the reactions you observed? How do temperature and other variables affect the reactions?
Watch videos and write in your notebook. Then left-click here for our observations.
In the video of copper reacting in air a change in color to black shows that a reaction has occurred. The dark color indicates a different substance has been formed from the copper. The color disappears when hydrogen gas passes over the new substance, indicating that the new substance has reacted with the hydrogen. One product of this second reaction appears to be copper, because the original color is restored. At high temperature the reaction forming the dark color and the reaction removing it are quite rapid, but at room temperature the reactions are much slower.
In the video of lead nitrate and potassium iodide, a pale yellow solution poured into a colorless solution produces a bright yellow solid–a different substance from the reactants. The color change and the fact that a solid is produced from two solutions is evidence of a reaction. No information is given about temperature, but it seems reasonable to assume room temperature The reaction occurs very quickly–as soon as the solutions are mixed–but it takes longer for the yellow solid (precipitate) to sink to the bottom of the remaining solution, which is colorless or nearly so.
In the third video, a change in the luster and color of the surface of Li, Na, and K indicates that a reaction has occurred, presumably with air and at room temperature. The reactions are similar for all three metals but they become faster going from Li to K. These metals also react rapidly with water at room temperature, as do two more substances, Rb and Cs. For Li, Na, and perhaps K, it is clear that a gas is released and there is a sizzling noise. With K the gas catches fire and the flame has a pink color (another, different, chemical reaction). The last two metals cause explosions. All metals disappear, leaving behind a solution. Any of these observations gives evidence that a chemical reaction has occurred. These reactions all begin as soon as the metal comes in contact with the water. They become more vigorous in the order Li < Na < K < Rb < Cs.
Comparing the reactions of Li, Na, K, and Cu with air, we notice that the the first three substances react rapidly at room temperature but Cu requires a higher temperature for rapid reaction.
Saying that one substance changes into another is not very precise without a definition of what a substance is. In chemistry, a substance is matter that, when purified, has specific, characteristic properties and composition. For example, all samples of pure copper have an orange, lustrous surface; all conduct electricity equally well; all react with dry air to produce a black substance; and all consist solely of copper. The American Chemical Society currently lists more than 160 million chemical substances in a data base. New substances are being synthesized every day.
Because there are so many chemical substances, it is useful to develop categories and classifications to help guide our thinking. The most important is that chemical substances are composed of only a few chemical elements (118 of which have been discovered so far). A chemical element is a substance that cannot be changed by chemical reaction into two or more different substances. Elements combine to form chemical compounds, substances that can be decomposed by chemical reactions into two or more new substances. If you know something about the chemical elements and how they combine to form chemical compounds, you can predict properties of a wide range of substances and often predict which substances are likely to react to form what products.
Another useful classification is to divide the elements into metals and nonmetals. Metals conduct electricity as both solids and liquids, have lustrous surfaces when pure, can be pounded into different shapes and drawn into wires, and conduct heat well. Nonmetals have very small electrical conductivity and have a broad range of other properties. Six elements, the metalloids, have properties intermediate between metals and nonmetals.
Chemical reactions and the classifications of substances discussed so far are based on what are called macroscopic observations. Macroscopic refers to things large enough to be seen and manipulated in a laboratory (or anywhere else). Enhancing your understanding of the macroscopic world is a goal of this course.
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