Properties and States of Matter
Chemistry, at its heart, is the study of matter, although the focus of the study can vary among composition, structure, properties, and reactions. In these chapters, you will be introduced to matter through a discussion of properties that you can observe and study the theories that explain the properties of matter, including the kinetic theory and atomic theory.
2.1 Classifying Matter
Every sample of a given substance has the same properties because a substance has a fixed, uniform composition. An element has a fixed composition because it contains only one type of atom. A compound always contains two or more elements joined in a fixed proportion. The properties of a mixture can vary because the composition of a mixture is not fixed. Based on the size of its largest particle, a mixture can be classified as a solution, a suspension, or a colloid.
2.2 Physical Properties
Viscosity, conductivity, malleability, hardness, melting point, boiling point, and density are examples of physical properties. Physical properties are used to identify a material, to choose a material for a specific purpose, or to separate the substances in a mixture. Filtration and distillation are two common separation methods.
ONLINE RESOURCES
BrainPOP - States of Matter (4:01)
2.3 Chemical Properties
Chemical properties can be observed only when the substances in a sample of matter are changing into different substances. Three common types of evidence for a chemical change are a change in color, the production of a gas, and the formation of a precipitate. When matter undergoes a chemical change, the composition of the matter changes. When matter undergoes a physical change, the composition of the matter remains the same.
3.1 Solids, Liquids and Gases
Materials can be classified as solids, liquids, or gases based on whether their shapes and volumes are definite or variable. The kinetic theory of matter states that all particles of matter are in constant motion. There are forces of attraction among particles in all matter. The constant motion of particles in a gas allows a gas to fill a container of any shape or size. A liquid takes the shape of its container because particles in a liquid can flow to new locations. The volume of a liquid is constant because forces of attraction keep the particles close together. Solids have a definite volume and shape because particles in a solid vibrate around fixed locations.
3.2 Gas Laws
Collisions between particles of a gas and the walls of the container cause the pressure in a closed container of gas. Factors that affect the pressure of an enclosed gas are its temperature, its volume, and the number of its particles. Raising the temperature of a gas will increase its pressure if the volume of the gas and the number of particles are constant. Reducing the volume of a gas increases its pressure if temperature and number of particles are constant. Increasing the number of particles will increase the pressure of a gas if the temperature and volume are constant. The combined gas law can be expressed as: P1V1/T1 = P2V2/T2
ONLINE RESOURCES
PhET - Gases Intro
3.3 Phase Changes
Melting, freezing, vaporization, condensation, sublimation, and deposition are six common phase changes. The temperature of a substance does not change during a phase change. Energy is either absorbed or released during a phase change. The arrangement of molecules in water becomes less orderly as water melts, and more orderly as water freezes. Evaporation takes place at the surface of a liquid and occurs at temperatures below the boiling point.