CHEMISTRY THE CENTRAL SCIENCE

1 INTRODUCTION: MATTER AND MEASUREMENT

CHAPTER SUMMARY AND KEY TERMS

INTRODUCTION AND SECTION 1.1 Chemistry is the study of the composition, structure, properties, and changes of matter. The composition of matter relates to the kinds of elements it contains. The structure of matter relates to the ways the atoms of these elements are arranged. Aproperty is any characteristic that gives a sample of matter its unique identity. A molecule is an entity composed of two or more atoms with the atoms attached to one another in a specific way.

SECTION 1.2 Matter exists in three physical states, gas, liquid, and solid, which are known as the states of matter. There are two kinds of pure substances: elements and compounds. Each element has a single kind of atom and is represented by a chemical symbol consisting of one or two letters, with the first letter capitalized. Compounds are composed of two or more elements joined chemically. The law of constant composition, also called the law of definite proportions, states that the elemental composition of a pure compound is always the same. Most matter consists of a mixture of substances. Mixtures have variable compositions and can be either homogeneous or heterogeneous; homogeneous mixtures are called solutions.

SECTION 1.3 Each substance has a unique set of physical properties and chemical properties that can be used to identify it. During a physical change, matter does not change its composition. Changes of state are physical changes. In a chemical change (chemical reaction) a substance is transformed into a chemically different substance. Intensive properties are independent of the amount of matter examined and are used to identify substances. Extensive properties relate to the amount of substance present. Differences in physical and chemical properties are used to separate substances.

The scientific method is a dynamic process used to answer questions about our physical world. Observations and experiments lead to scientific laws, general rules that summarize how nature behaves. Observations also lead to tentative explanations or hypotheses. As a hypothesis is tested and refined, a theory may be developed that can predict the results of future observations and experiments.

SECTION 1.4 Measurements in chemistry are made using the metric system. Special emphasis is placed on SI units, which are based on the meter, the kilogram, and the second as the basic units of length, mass, and time, respectively. SI units use prefixes to indicate fractions or multiples of base units. The SI temperature scale is the Kelvin scale, although the Celsius scale is frequently used as well. Density is an important property that equals mass divided by volume.

SECTION 1.5 All measured quantities are inexact to some extent. The precision of a measurement indicates how closely different measurements of a quantity agree with one another. The accuracy of a measurement indicates how well a measurement agrees with the accepted or “true” value. The significant figures in a measured quantity include one estimated digit, the last digit of the measurement. The significant figures indicate the extent of the uncertainty of the measurement. Certain rules must be followed so that a calculation involving measured quantities is reported with the appropriate number of significant figures.

SECTION 1.6 In the dimensional analysis approach to problem solving, we keep track of units as we carry measurements through calculations. The units are multiplied together, divided into each other, or canceled like algebraic quantities. Obtaining the proper units for the final result is an important means of checking the method of calculation. When converting units and when carrying out several other types of problems, conversion factors can be used. These factors are ratios constructed from valid relations between equivalent quantities.

KEY SKILLS

• Distinguish among elements, compounds, and mixtures. (Section 1.2)

• Memorize symbols of common elements. (Section 1.2)

• Memorize common metric prefixes. (Section 1.4)

• Use significant figures, scientific notation, and SI units in calculations. (Section 1.5)

• Use dimensional analysis in calculations. (Section 1.6)

KEY EQUATIONS

Converting between Celsius (°C) and Kelvin (K) temperature scales

Converting between Celsius (°C) and Fahrenheit (°F) temperature scales

Definition of density