The three states of matter are solids, liquids, and gases. Though solids and liquids may not be found everywhere, gases are, and the atmosphere is an envelope of gases that surrounds the Earth. The molecules in gases are spread out much more than solids or liquids and are constantly moving around randomly. Gases have no fixed shape or fixed volume.
Gases can fill a container of any size or shape and will spread out to fill the whole space equally. Liquids can only fill the bottom of a container, and the shape of liquids is dependent on gravity. Less dense gases are light enough to have more freedom to move. The particles of a gas move very fast and collide into one another, causing them to diffuse, or spread out until they are distributed throughout the volume of the container.
When more gas particles enter a container, they can become compressed, and the particles exert more force on the interior volume of the container causing pressure. Several units are used to express pressure of a gas such as psi (pounds per square inch), atm (atmospheres), and others. The temperature, volume, and a number of particles also influence the pressure of the gas in a container.
Sometimes the term vapor is used instead of gas. Vapor and gas mean the same thing, but vapor is used to describe gases that are usually liquids at room temperature. For example, water, when it is in its gaseous phase may be called water vapor. In other words, it is water in a gas state. Compounds such as carbon dioxide are usually gases at room temperature too.
Because of pressure, gases can hold large amounts of energy and can be compressed with very little effort. Combinations of increased pressure and decreased temperature force gases into containers that people use every day. For example, carbon dioxide rushes out when a can of soda is opened, as the gas in a high-pressure environment, the can, move to an environment with a lower pressure.
There are many other examples of gases used every day for a wide variety of purposes. Oxygen is used for medical purposes and for welding, nitrogen is used for extinguishing fires, helium is used in balloons and for medical equipment, and argon and acetylene are also used for welding.
Other common gases include propane for heating gas grills and for heat, butane as fuel for lighters and torches, and whipped topping in a can uses nitrous oxide as a propellant which helps the cream rush out of its container. Finally, freon is a popular gas used as a coolant for air conditioners, refrigerators, and freezers.
Noble gases are those found in Group VIII on the Periodic Table of Elements which are also known as inert gases or rare gases. Noble gases are relatively nonreactive, have low boiling points, and are all gases at room temperature. Some examples of noble gases include helium, neon, argon, krypton, and radon.
In summary, gases assume the shape and volume of their container, have lower densities than their solid or liquid phases and are more easily compressed than solids or liquids. In addition, gases easily will mix completely and evenly when confined to the same volume.
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