To ensure that you understand the material in this chapter, you should review the meanings of the bold terms in the following summary and ask yourself how they relate to the topics in the chapter.

Chemistry is the study of matter, which is anything that has mass and takes up space. Chemistry is one branch of science, which is the study of the natural universe. Like all branches of science, chemistry relies on the scientific method, which is a process of learning about the world around us. In the scientific method, a  hypothesis is tested through experiment and measurement. Current scientific knowledge can be summarized via theories, laws, and models.

Matter can be described in a number of ways. Physical properties describe characteristics of a sample inherant in the sample: size, shape, color, density, melting point, boiling point, and solubility, while chemical properties describe how a sample of matter changes its chemical composition by means of chemical reactions. A substance is any material that has the same physical and chemical properties throughout. An element is a substance that cannot be broken down into chemically simpler components and contains only one type of atom. A substance that consists of atoms of two or more elements chemically bonded in a set ratio and that can be broken down into simpler chemical components is a compound. The smallest chemically identifiable piece of a compound is a molecule (for covalent compounds) or formula unit for ionic componds). Two or more substances combine physically to make a mixture. If the mixture is composed of discrete regions that maintain their own identity, the mixture is a heterogeneous mixture. If the mixture is so thoroughly mixed at the molecular level that the different components are evenly distributed throughout, it is a homogeneous mixture. Another name for a homogeneous mixture is a solution. Substances can also be described by their phase: solid, liquid, or gas.

Scientists learn about the universe by making measurements of quantities, which consist of numbers (how many), units (of what), and uncertainty (how many decimal places are known). The numerical portion of a quantity can be expressed using scientific notation, which is based on powers, or exponents, of 10. Large numbers have positive powers of 10, while numbers less than 1 have negative powers of 10. The level of uncertainty of a measurement is expressed through proper use of significant figures, which are all the known digits of a measurement plus the first estimated digit. When measuring, the correct number of digits to report is based on reporting all certain digits and one estimated digit, in other words, one place beyond the measuring tool’s smallest division. The number of figures to report in the result of a calculation based on measured quantities depends on the numbers of significant figures in those quantities. For addition and subtraction, the number of significant figures is determined by decimal place; for multiplication and division, it is decided by the number of significant figures in the original measured values. Nonsignificant digits are dropped from a final answer in accordance with the rules of rounding.

Chemistry uses SI, a system of units based on seven basic units. The most important ones for chemistry are the units for length, mass, amount, time, and temperature. Basic units can be combined with numerical prefixes to change the size of the units. They can also be combined with other units to make derived units, which are used to express other quantities such as volume, density, percent, or energy. A formal conversion from one unit to another uses a conversion factor, which is constructed from the relationship between the two units. Numbers in conversion factors may affect the number of significant figures in a calculated quantity, depending on whether the conversion factor is exact. Conversion factors can be applied in separate computations, or several can be used at once in a single, longer computation.