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Print Lab Manual Identifying Unknown Substances From Characteristic Properties Figure 1. Sample Driver License Have you ever wondered why there is so much information on a driver license such as the one in Figure 1 ? An identification card such as a driver license requires more than one characteristic to distinguish one person from another. For example, many people have the same hair color, eye color, or height, but fewer have all three of the same characteristics. The more characteristics described, the fewer people fit the description. This allows a person to be quickly identified. Substances are identified in much the same way by using characteristics, or properties, to distinguish them. Background Physical properties are properties that can be observed or measured without changing the identity of a substance. Examples of physical properties include melting point, boiling point, density, and solubility. Chemical properties relate to a substance's ability to undergo changes that transform it into different substances. Examples of chemical properties include flammability, corrosion, and heat of combustion. For example, a chemical property of iron is that it forms rust when reacting with oxygen. The chemical and physical properties of a substance can be categorized into two groups: Extensive properties depend on the amount of a substance. Examples of extensive properties are mass and volume. The larger the quantity of a substance, the larger the value of an extentive property is. Some extensive properties are visible to the naked eye, such as thickness, whereas others, such as energy, are not. Intensive properties are independent of the amount of a substance. Examples of intensive properties are boiling point, melting point, and density. For example, no matter how much water is present, its boiling point remains 100 C at a pressure of 1 atm. Because intensive properties are not dependent upon the amount of a sample, they can be used to identify unknown substances. Figure 2 and Figure 3 contain intensive property values for a series of solid and liquid compounds, respectively. Name Formula Density (g/mL) Solubility in Water (g/100 mL) Melting Point ( C) Boiling Point ( C) silver nitrate AgNO 3 5.35 245 212 440 BHT C 15 H 24 O 1.05 0.00006 71 265 ascorbic acid C 6 H 8 O 6 1.65 40 190 decomposes
calcium carbonate CaCO 3 2.71 0.0013 1339 decomposes calcium sulfate CaSO 4 2.96 0.21 1450 decomposes cobalt (II) chloride CoCl 2 3.36 53 735 1049 zinc nitrate Zn(NO 3 ) 2 2.07 120 110 decomposes Figure 2. Intensive Properties of Solid Compounds Name Formul a Density (g/mL) Boiling Point ( C) acetic acid C 2 H 4 O 2 1.05 117.9 acetonitrile C 2 H 3 N 0.79 81.6 cyclohexane C 6 H 12 0.78 80.7 cyclopentylamin e C 5 H 11 N 0.86 107.0 ethanol C 2 H 6 O 0.79 78.3 Figure 3. Intensive Properties of Liquid Compounds

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