A gas is a state of matter, the others being solids and liquids. Gases are distinguished from the other states by the amount of energy the constituent molecules have. With gases the molecules making up the gas have virtually completely free movement within their container. If the volume is increased then the gas expands to fill the new volume. As this happens the pressure that the gas is exerting on the walls of the chamber decreases as there are fewer molecules colliding with a given area of the containers walls. A gas is distinguished from a vapor in that a gas is above the critical point at which the liquid boils.
There are several laws and equations that describe the behavior of gases under specified conditions, collectively they are known as the gas laws.Boyle's law describes the behavior of a gas when the temperature is kept constant. At constant temperature the volume of a given mass of gas is inversely proportional to the pressure. This law works very well at low pressures but at high pressures the predictions become less accurate. This failure of Boyle's law is due to the fact that the molecules of which the gas are composed are being pressed so close together that they are interacting. These interactions are attractions and repulsions due to intermolecular forces and the fact that the molecules have a definite size.
Charles's law describes the behavior of a gas at a constant pressure. At constant pressure the volume of a given mass of gas is directly proportional to the absolute temperature, that is the temperature expressed in Kelvin. A third law, the constant volume law, states that when the volume is kept constant the temperature is inversely proportional to the pressure of the gas.
These laws are combined together to give the ideal gas equation. The ideal gas equation can be written as PV = nRT where P is the pressure, V is the volume, n is the number of moles of gas, T is the temperature of the gas (in Kelvin), and R is the universal gas constant. This equation can be used to calculate the behavior of a gas as conditions are altered.
There are several other laws pertaining to gases including Dalton's law of partial pressures. In a mixture of gases each gas (assuming there is no reaction between the gases) exerts the same pressure on the container walls as if it were the only gas present. As a consequence of this the total pressure in a container of a mixture of gases is the sum of the individual pressure that each gas would exert. Graham's Law concerns itself with the effusion of two or more gases through a hole in a container. This law states that the rate of effusion of a gas is inversely proportional to the square root of the molar mass of the gas.Avogadro's law is an explanation of the relationship between the quantity of a gas and its volume. At a constant temperature and pressure the volume of a gas will increase if the quantity of gas is increased. Avogadro also worked out that identical volumes of gas under the same conditions contained the same number of molecules.
Gases are a high energy state, compared to liquids and solids. The molecules are more free to move and expand to fill the space into which they are placed. The change from liquid to gas is boiling and some chemicals can change directly from solid to gas in a process known as sublimation.
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