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Figure 15 - from "Foto power dusting"

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arrow_backarrow_forwardSublimation The direct phase change from a solid to a gas.

Sublimation The direct phase change from a solid to a gas.

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Figure 1
FIGURE 1 Involvement of various individuals in an investigation.
FIGURE 3 The reactivity of baking soda with vinegar is a chemical property. FIGURE 2 One physical property of this sulfur powder is its bright yellow color. Recall that physical properties are properties of a substance that can be de- scribed or displayed without requiring a chemical change. For example, sulfur is yellow (see Figure 2), iron is malleable (able to be hammered into sheets), cocaine is a white solid, and the density of a glass fragment broken from a win- dowpane at a crime scene is approximately 2.5 g/mL. Although the exact manner in which the physical and chemical properties are analyzed for each substance differs, the analyses are all based on the principles of the scientific method. The scientific method begins with observations. Scien- tists attempt to organize observations and look for trends or patterns. When the scientists find what appears to be a relationship among the observations, they suggest a hypothesis (an educated guess) that tentatively explains what is being observed. A plan is devised to test the hypothesis. Ultimately, the plan is carried out and further observations are made. If the new observations con- tradict the original hypothesis, a new hypothesis is suggested and tested. How- ever, if the new observations validate the original hypothesis, the scientists often choose to devise a subsequent plan to further validate the hypothesis. This cycle, as illustrated in Figure 4, continues until the hypothesis has been sufficiently validated. For example. when an wnknown sibstance is submitted to a crime lahora-
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Chemical properties are properties of a substance that can be described through a chemical change only. Chemical changes require a chemical reac- tion to occur between reactants, generating new products. The chemical properties of a substance are described by the reaction that occurs and the products that are formed. For example, a chemical property of baking soda (sodium bicarbonate) is its reactivity with vinegar (acetic acid) to produce car- bon dioxide bubbles, as shown in Figure 3. This reaction also describes a chemical property of vinegar—its reactivity with baking soda. A chemical property of cocaine is its reactivity with cobalt thiocyanate, which produces a blue-colored product. This chemical property of cocaine, in conjunction with its physical properties (a white, fluffy powder), help investigators identify cocaine.
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FIGURE 4 The scientific method.
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Jacobo Ustariz
Jacobo Ustariz
Foto power dusting
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