Chemistry The Central Science
Chapter 1, Introduction: Matter and Measurement
The Study of Chemistry
Chemistry is the study of matter and the changes that matter undergoes. Matter is made up of almost
infinitesimally small building blocks called atoms. Atoms can combine together to form molecules.
Molecules of a few familiar substances are represented here. In later chapters you will learn more about
how atoms combine to form molecules, and how molecules have the shapes and properties that they
do.
Classification of Matter
Matter can exist in one of three states of matter: a gas, a liquid, or a solid. A gas is highly compressible
and will assume both the shape and the volume of its container. A liquid is not compressible and will
assume the shape but not the volume of its container. A solid also is not compressible, and it has a fixed
volume and shape of its own.
Matter can also be classified according to its composition. Most of the matter that we encounter exists
in mixtures, which are combinations of two or more substances. Mixtures can be homogeneous or
heterogeneous.
Mixtures can be separated into pure substances, and pure substances can be either compounds or
elements.
A familiar example of a mixture is salt water. A sample of salt water has the same composition
throughout. It can be separated into pure substances—water and ordinary table salt—by a physical
process, such as distillation.
Oxygen and hydrogen are elements. When water is separated into its constituent elements, the relative
amounts of those elements are always the same. Water is 11 percent hydrogen and 89 percent oxygen
by mass. This is an example of the law of constant composition, also known as the law of definite
proportions. Salt can also be separated into its constituent elements, sodium and chlorine, by
electrolysis. Sodium chloride also has a constant composition, as do all pure substances. It is 39 percent
sodium and 61 percent chlorine by mass.
Properties of Matter
Different types of matter have different distinguishing characteristics that we can use to tell them apart.
These characteristics are called physical properties and chemical properties. Physical and chemical
properties may be intensive or extensive. Intensive properties such as density, color, and boiling point
do not depend on the size of the sample of matter and can be used to identify substances. Extensive
properties such as mass and volume do depend on the quantity of the sample.
Physical properties are those that we can determine without changing the identity of the substance we
are studying. For instance, we can observe or measure the physical properties of sodium metal. It is a
soft, lustrous, silver-colored metal with a relatively low melting point and low density. Hardness, color,
melting point and density are all physical properties. Figure 7.15 shows a chunk of metallic sodium,
which is soft enough to be cut with a knife.
Chemical properties describe the way a substance can change or react to form other substances. These
properties, then, must be determined using a process that changes the identity of the substance of
interest. One of the chemical properties of alkali metals such as sodium and potassium is that they react
with water. To determine this, though, we would have to combine an alkali metal with water and
observe what happens.
The changes undergone by sodium and potassium when they react with water are chemical changes,
also known as chemical reactions. Matter can also undergo physical changes in which the chemical
identity of the matter does not change. One example of a physical change is the melting of a solid. When
ice melts, it changes from a solid state to a liquid state, but its chemical identity (H2O) is unchanged. All
changes of state are physical changes.
Units of Measurement
Uncertainty in Measurement
Two terms are used to describe the quality of measurements: precision and accuracy. Precision is a
measure of how closely individual measurements agree with one another. Accuracy refers to how
closely individually measured numbers agree with the correct or "true" value.
In order to convey the appropriate uncertainty in a reported number, we must report it to the correct
number of significant figures. The number 83.4 has three digits. All three digits are significant. The 8 and
the 3 are "certain digits" while the 4 is the "uncertain digit." As written, this number implies uncertainty
of plus or minus 0.1, or error of 1 part in 834. Thus, measured quantities are generally reported in such a
way that only the last digit is uncertain. All digits, including the uncertain one, are called significant
figures.
Guidelines
1. Nonzero digits are always significant–457 cm (3 significant figures); 2.5 g (2 significant
2.
3.
4.
5.
6.
7.
figures).
Zeros between nonzero digits are always significant–1005 kg (4 significant figures); 1.03
cm (3 significant figures).
Zeros at the beginning of a number are never significant; they merely indicate the
position of the decimal point–0.02 g (one significant figure); 0.0026 cm (2 significant
figures).
Zeros that fall at the end of a number or after the decimal point are always significant–
0.0200 g (3 significant figures); 3.0 cm (2 significant figures).
When a number ends in zeros but contains no decimal point, the zeros may or may not
be significant–130 cm (2 or 3 significant figures); 10,300 g (3, 4, or 5 significant figures).
When multiplying or dividing measured numbers, the answer must have the same
number of significant figures as the measured number with the fewest significant
figures.
When adding or subtracting, the answer can have only as many places to the right of the
decimal point as the measured number with the smallest number of places to the right
of the decimal point.
Dimensional Analysis
If you don’t remember this from Chemistry I or physics, you made your science department cry a little.