Chapter 18: Classification of Matter

Section 1: Composition of Matter

Pure Substances * Every material has its own properties. The properties of materials can be used to classify them into general categories. * Substance: type of matter with a fixed composition * A substance can be either an element or a compound. * Element: If all the atoms in a substance have the same identity * About 90 elements are found on Earth. * All the atoms of an element are alike. * Compound: a pure substance in which the atoms of two or more elements are combined in a fixed proportion. * Chlorine gas and sodium metal combine dramatically in the ratio of one to one to form sodium chloride. * A particle consisting of two or more atoms that are bonded together is called a molecule. * A molecule is a basic unit of a molecular compound.
Mixtures * A mixture is a material made up of two or more substances that can be easily separated by physical means. * The number of mixtures that can be created by combining substances is unlimited * Mixtures do not always contain the same proportions of the substances that make them up. * Heterogenous Mixture: A mixture in which different materials can be distinguished easily * Heterogeneous mixtures can be hard to detect. * Homogeneous Mixture: contains two or more gaseous, liquid, or solid substances blended evenly throughout. * Solution: a homogeneous mixture of particles so small that they cannot be seen with a microscope and will never settle to the bottom of their container. * Solutions remain constantly and uniformly mixed. * All matter can be divided into substances and mixtures. * Colloid: a type of mixture with particles that are larger than those in solutions but not heavy enough to settle out. * One way to distinguish a colloid from a solution is by its appearance. * Fog is a colloid composed of water droplets suspended \n in air. * Tyndall Effect: The scattering of light by colloidal particles * Suspension: a heterogeneous mixture containing a liquid in which visible particles settle. * River deltas are large scale examples of how

a suspension settles.

Section 2: Properties of Matter

Physical Properties * The abilities to stretch and bend are physical properties. * Physical Property: Any characteristic of a material that you can observe without changing the identity of the substances that make up the material * Appearance is the most obvious physical property. * Some physical properties describe the behavior of a material or a substance. * Every substance has a specific combination of physical properties that make it useful for certain tasks. * The best way to separate substances depends on their physical properties.
Physical Changes * When a substance freezes, boils, evaporates, or condenses, it undergoes physical changes. * Physical Change: A change in size, shape, or state of matter * Iron is a substance that can change states if it absorbs or releases enough energy; at high temperatures, it melts. * Heating iron raises its energy level and changes its color. These energy changes are physical changes because the substance is still iron. * One separation method, which uses the property of boiling point, is distillation. * Distillation: The process used for separating substances in a mixture by evaporating a liquid and recondensing its vapor * Distillation can easily separate liquids from solids dissolved in them. The liquid is heated until it vaporizes and moves up the column. Then, as it touches the water-cooled surface of the condenser, it becomes liquid again.
Chemical Properties and Changes * The tendency of a substance to burn, or its flammability, is an example of a chemical property. * Chemical Property: a characteristic of a substance that indicates whether it can undergo a certain chemical change. * Many medicines are stored in dark bottles because they contain compounds that can change chemically if they are exposed to light.
Detecting Chemical Changes * Chemical Change: A change of one substance to another * In some chemical changes, a rapid release of energy—detected as change of heat, light, and/or sound production—is a clue that changes are occurring. * Burning and rusting are chemical changes because new substances form. * The solid forming from two liquids is another sign that a chemical reaction has taken place. * You might separate substances using a chemical change when cleaning tarnished silver.
Weathering - Chemical or Physical Change * The forces of nature continuously shape Earth’s surface. * Weathering can involve physical or chemical change. * Flowing water shaped and smoothed these rocks in a physical process. * Both chemical and physical changes shaped the famous White Cliffs of Dover lining the English Channel.
The Conservation of Mass * Wood is combustible, or burnable. * Not only is no mass lost during burning, mass is not gained or lost during any chemical change. * Law of Conservation of Mass: the mass of all substances that are present before a chemical change equals the mass of all the substances that remain after the change. \n