Page 15 - Theory and Problems of BEGINNING CHEMISTRY

4 BASIC CONCEPTS [CHAP. 1

In the method of classification of matter based on composition, a given specimen of material is regarded as
either a pure substance or a mixture. An outline of this classification scheme is shown in Table 1-1. The term
pure substance (or merely substance) refers to a material all parts of which have the same composition and that
has a definite and unique set of properties. In contrast, a mixture consists of two or more substances and has
a somewhat arbitrary composition. The properties of a mixture are not unique, but depend on its composition.
The properties of a mixture tend to reflect the properties of the substances of which it is composed; that is, if the
composition is changed a little, the properties will change a little.
Table 1-1 Classification of Matter
Based on Composition

Substances
Elements
Compounds
Mixtures
Homogeneous mixtures (solutions)
Heterogeneous mixtures (mixtures)

Substances
There are two kinds of substances—elements and compounds. Elements are substances that cannot be
broken down into simpler substances by ordinary chemical means. Elements cannot be made by the combination
of simpler substances. There are slightly more than 100 elements, and every material object in the universe
consists of one or more of these elements. Familiar substances that are elements include carbon, aluminum, iron,
copper, gold, oxygen, and hydrogen.
Compounds are substances consisting of two or more elements chemically combined in definite proportions
by mass to give a material having a definite set of properties different from that of any of its constituent elements.
For example, the compound water consists of 88.8% oxygen and 11.2% hydrogen by mass. The physical and
chemical properties of water are distinctly different from those of both hydrogen and oxygen. For example,
water is a liquid at room temperature and pressure, while the elements of which it is composed are gases under
these same conditions. Chemically, water does not burn; hydrogen may burn explosively in oxygen (or air). Any
sample of pure water, regardless of its source, has the same composition and the same properties.
There are millions of known compounds, and thousands of new ones are discovered or synthesized each year.
Despite such a vast number of compounds, it is possible for the chemist to know certain properties of each one,
because compounds can be classified according to their composition and structure, and groups of compounds
in each class have some properties in common. For example, organic compounds are generally combustible in
excess oxygen, yielding carbon dioxide and water. So any compound that contains carbon and hydrogen may be
predicted by the chemist to be combustible in oxygen.
Organic compound + oxygen −→ carbon dioxide + water + possible other products

Mixtures
There are two kinds of mixtures—homogeneous mixtures and heterogeneous mixures. Homogeneous mix-
tures are also called solutions, and heterogeneous mixtures are sometimes simply called mixtures. In heteroge-
neous mixtures, it is possible to see differences in the sample merely by looking, although a microscope might be
required. In contrast, homogeneous mixtures look the same throughout the sample, even under the best optical
microscope.

EXAMPLE 1.2. A teaspoon of salt is added to a cup of warm water. White crystals are seen at the bottom of the cup. Is
the mixture homogeneous or heterogeneous? Then the mixture is stirred until the salt crystals disappear. Is the mixture now
homogeneous or heterogeneous?
Ans. Before stirring, the mixture is heterogeneous; after stirring, the mixture is homogeneous—a solution.