Water h2o is made of 2 hydrogen atoms and 1 oxygen

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Unformatted text preview: tuent element. For example, water (H2O) is made of 2 hydrogen atoms and 1 oxygen atom. • Since all compounds are made of only one type of molecule, and all elements are made of only one type of atom, we can categorize compounds and elements as pure substances (commonly called simply substances) because they both have constant composition. For example, water, gold and hydrogen are pure substances because water (compound) is made of only H2O molecules, gold is made of only Au atoms and hydrogen is made of only H2 molecules. Sample Problem 1.2: How about pure sugar dissolved in pure water, is it a pure substance? No. Even though it is homogeneous, it does not have a fixed composition. Composition is not definite; it is widely variable depending on how much sugar we mix with water. Pure sugar dissolved in pure water is therefore a mixture. • When two or more compounds are combined through a physical process (e.g. mixing them together) they form a mixture. For example, whiskey & coke drink is a mixture. The compounds in the mixtures retain their own chemical properties. Mixtures have 15 Copyright © 2007 by Concise Books Publishing LLC. Visit us at www.concisechem.com to download other free chapters from "The Concise Guide to Chemistry." CHAPTER 1: INTRODUCTION TO CHEMISTRY variable composition. For example, you can make a soft whiskey & coke which has few whiskey (ethanol; C2H6O) molecules or you can have a strong whiskey & coke that will have lots of ethanol molecules. • Homogeneous mixtures are mixtures that have the same properties throughout. These usually don’t have visible parts. For example, air is composed of many different molecules: nitrogen, oxygen, water, etc. However, these molecules are evenly distributed throughout the air. Regardless of whether you sit in the front of the room or back of the room, you’re expected to breath in the same number of oxygen molecules. From the problem above, sugar dissolved in water is a mixture because the ratio of water to sugar can be changed, and is homogeneous because each component is uniformly dispersed throughout the solution. • Heterogeneous mixtures are mixtures that do not have the same properties throughout. These usually have visible parts. An example would be a soup, made of water, salt, the recipe and noodles. Not only does the ratio of each constituent vary depending on who is making the soup (mixture), but also, the noodles are always found at the bottom (heterogeneous). Sample Problem 1.3: Solving classification problems. Now, let’s put the above process into context with an example. Appropriately classify the following: sodium (Na), table salt (NaCl), wine and pen. To answer any classification question simply follow the process map below. Can you find it on the periodic table? sodium, salt, wine, pen No Yes Can you write a chemical formula for it? Element sodium: Na No Yes Does it have the same properties throughout? Compound salt: NaCl No Yes Heterogeneous Mixture Homogeneous Mixture pen wine...
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