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SOME BASIC CHEMICAL TERMS One major goal of chemistry is to describe the properties of the many different forms of matter we encounter. Matter, the material of which the universe is composed, may be defined as anything that occupies space and has mass. Most of the materials we encounter in our daily lives, such as air, milk, and steel, are mixtures. Mixtures contain two or more substances that can be physically separated from each other. Some mixtures, such as sand mixed with gravel, are heterogeneous, in other words, we would have no trouble distinguishing gravel from sand, and the mixture could be separated by a simple mechanical procedure. Other mixtures, for example a mixture of alcohol and water are homogeneous – the mixture has the same composition throughout the entire sample and is usually called a solution. There are many different separation methods that can be used to separate such a mixture into its pure components. For example, by selective evaporation – distillation. Mixtures resemble their components. A pure substance is matter that cannot be separated into other substances by physical means. Each pure substance has its own particular set of physical properties – boiling point, freezing point, melting point, density, colour. A chemical reaction transforms one or more substances into different substances. The substances that take part in the reaction are called reactants, and those that are formed by the reaction are called products. The products of a chemical reaction usually do not resemble their reactants. Pure substances are compounds and elements. Compounds are substances that are composed of two or more elements in fixed proportions. The law of definite proportions states that a pure compound always consists of the same proportion by mass. Over ten thousand inorganic compounds are know. The ancient Greeks originated the concept that all matter is composed of a limited number of simple substances called elements. The Greeks assumed that all matter is derived from four elements: earth, air, fire, and water. Lavoisier firmly established the concept of a chemical element as a substance that resists decomposition and cannot be synthesized. At the present time, 107 elements are known. Of these, 85 have been isolated from natural sources, and the remainder have been prepared by nuclear reactions. The most abundant element in the universe as a whole is hydrogen, which is thought to constitute about 75% of the total mass of the universe. The Earth consists of a core surrounded by a mantle, and a thin crust. If the entire earth were considered the most abundant element would be iron. A. Fill in the missing words: 1. 2. 3. 4. Matter is anything that _________________ space and has mass. Mixtures are two or more _________________. Mixture of alcohol and water is usually called a _________________. The substances that take part in the reaction are called_________________, and those formed by the reaction are called ______________. 5. Pure substances are _________________ and _________________. 6. At the present time 107 _________________ are known. 7. The Earth consists of a _________________, a _________________, and a thin _________________.