Course Outline

General Chemistry

03/24/07

A sample of matter can be classified according to its phase or state. The most common states of matter are solid, liquid, gas. A solid is formed at low temperatures and/or high pressures. It is rigid, because it has a definite shape and volume. A liquid is formed at intermediate temperatures and pressures. A liquid is able to flow since it does not have a definite shape, but it is not compressible, since it does have a definite volume. A gas is formed at high temperature and/or low pressures. A gas doesn't have a definite shape or volume; instead, it expands to fill its container. Substances change from one state to another as pressure and temperature change. For example, water is a liquid at room temperature and pressure. When it gets colder than 0 C (at normal pressure), water becomes the solid we commonly call ice. When water gets hotter than 100 C (at normal pressure), it becomes the gas we commonly call steam. The properties of an element or compound in any of its states of matter are a result of the organization of its atoms or molecules. Interactions between groups of molecules give a sample of matter its shape and/or volume. When these interactions are missing, the molecules separate and expand to fill their container.

Another state of matter we often see is plasma. Like a gas, a plasma does not have definite shape or volume. However, plasma is made up of high energy charged particles. An example of a plasma is a flame.

Substances tend to form gases at high temperatures and liquids at high pressures. If both temperature and pressure are high enough, the substance may form a supercritical fluid, a state of matter which is neither liquid nor gas but has some properties of both states.  A supercritical fluid has no surface tension and very low viscosity (like a gas) yet it has a density similar to that of a liquid and makes a good solvent. Supercritical carbon dioxide is an important cleaning agent in the semiconductor industry.

For more information about the states of matter and phase changes, see the  "Kinetic Molecular Theory and the States of Matter" section in the Chemistry Web book.

Concept Check: Besides water, what other common household chemical have you observed in all three states?

Answer: Wax is one example. Candle wax is a solid at room temperature. When you light a candle, a puddle of melted candle wax forms near the wick. Wax vapor (a gas) is the fuel that burns to keep the candle alight. You can't see it, but you should recognize the smell.

© Copyright 2006, Kelley Whitley, ChemProfessor. All rights reserved.

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