Exp2 - CHE 117L Experiment 2. Separation of Substances in a...

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CHE 117L Fall 2007 Experiment 2. Separation of Substances in a Mixture Introduction Separation is one of the important laboratory techniques used by chemists. Often, when a chemist encounters an unknown mixture, his or her objective is to first separate the mixture into its constituent parts and then to identify them. But, before we continue, we need to first define the differences among an element, a compound, and a mixture. Element: An element is the simplest type of matter, one that cannot be broken down any further into stable substances. The known elements are shown on the periodic table; the first 92 of these occur in nature. Atoms of elements cannot be created or destroyed in a chemical reaction, but only rearranged into new combinations. (Nuclear reactions are an exception to this law). Compound: A compound is a substance that is made up of different elements that are chemically combined in a definite ratio. A characteristic of a compound is that only by chemical means can it be broken down into its constituent elements. An example of a compound is water. Water is composed of hydrogen and oxygen in a two to one ratio of atoms. The properties of water are completely different from the properties of either hydrogen or oxygen. Water can only be broken down into its elements through a chemical reaction. Elements and compounds are examples of pure substances . Mixture: In a mixture, the constituent pure substances are not combined chemically, and they can be separated by physical means. Furthermore, the composition of a mixture is variable. A homogeneous mixture is called a solution. Consider the example of a solution of sucrose (table sugar) in water. Both the sucrose and the water retain their own chemical identities and properties. They may be combined in a number of different proportions. The two substances can be separated by a physical means, such as evaporation. After the separation, the two substances still exhibit the same identities and properties. Chemists make use of the differing physical properties of substances in order to separate them. Examples of some of these physical properties are solubility in different solvents, boiling point, melting point, and sublimation point. The solubility of a substance
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Exp2 - CHE 117L Experiment 2. Separation of Substances in a...

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