Page 14 - Theory and Problems of BEGINNING CHEMISTRY

CHAP. 1] BASIC CONCEPTS 3

Except for reactions in which the quantity of matter is changed, as in nuclear reactions, the law of conservation
of energy is obeyed:
Energy can be neither created nor destroyed (in the absence of nuclear reactions).
In fact, many chemical reactions are carried out for the sole purpose of converting energy to a desired form.
For example, in the burning of fuels in homes, chemical energy is converted to heat; in the burning of fuels
in automobiles, chemical energy is converted to energy of motion. Reactions occurring in batteries produce
electrical energy from the chemical energy stored in the chemicals from which the batteries are constructed.

1.4. PROPERTIES
Every substance (Sec. 1.5) has certain characteristics that distinguish it from other substances and that may
be used to establish that two specimens are the same substance or different substances. Those characteristics that
serve to distinguish and identify a specimen of matter are called the properties of the substance. For example,
water may be distinguished easily from iron or gold, and—although this may appear to be more difficult—iron
may readily be distinguished from gold by means of the different properties of the metals.

EXAMPLE 1.1. Suggest three ways in which a piece of iron can be distinguished from a piece of gold.
Ans. Among other differences,
1. Iron, but not gold, will be attracted by a magnet.
2. If a piece of iron is left in humid air, it will rust. Under the same conditions, gold will undergo no appreciable
change.
3. If a piece of iron and a piece of gold have exactly the same volume, the iron will have a lower mass than the
gold.

Physical Properties
The properties related to the state (gas, liquid, or solid) or appearance of a sample are called physical
properties. Some commonly known physical properties are density (Sec. 2.6), state at room temperature, color,
hardness, melting point, and boiling point. The physical properties of a sample can usually be determined
without changing its composition. Many physical properties can be measured and described in numerical terms,
and comparison of such properties is often the best way to distinguish one substance from another.

Chemical Properties
A chemical reaction is a change in which at least one substance (Sec. 1.5) changes its composition and its
set of properties. The characteristic ways in which a substance undergoes chemical reaction or fails to undergo
chemical reaction are called its chemical properties. Examples of chemical properties are flammability, rust
resistance, reactivity, and biodegradability. Many other examples of chemical properties will be presented in this
book. Of the properties of iron listed in Example 1.1, only rusting is a chemical property. Rusting involves a
change in composition (from iron to an iron oxide). The other properties listed do not involve any change in
composition of the iron; they are physical properties.

1.5. CLASSIFICATION OF MATTER
To study the vast variety of materials that exist in the universe, the study must be made in a systematic manner.
Therefore, matter is classified according to several different schemes. Matter may be classified as organic or
inorganic. It is organic if it is a compound of carbon and hydrogen. (A more rigorous definition of organic must
wait until Chap. 18.) Otherwise, it is inorganic. Another such scheme uses the composition of matter as a basis
for classification; other schemes are based on chemical properties of the various classes. For example, substances
may be classified as acids, bases, or salts (Chap. 8). Each scheme is useful, allowing the study of a vast variety
of materials in terms of a given class.