# Gases - Key terms

### ABSOLUTE TEMPERATURE:

Temperature in relation to absolute zero (−273.15°C or −459.67°F), as measured on the Kelvin scale. The Kelvin and Celsiusscales are directly related; hence, Celsius temperatures can be converted to Kelvins (for which neither the word nor the symbol for "degree" are used) by adding273.15.

### ATMOSPHERE:

A measure of pressure, abbreviated "atm" and equal to the average pressure exerted by air at sea level. In English units, this is equal to 14.7 lb/in 2 , and in SI units, to 101,300 pascals.

A statement, derived by the Italian physicist Amedeo Avogadro (1776-1856), which holds that as the volume of gas increases under isothermal and isobarometric conditions, the number of molecules (expressed in terms of mole number), increases as well. Thus, the ratio of volume to mole number is aconstant.

### BAROMETER:

An instrument form easuring atmospheric pressure.

### BOYLE'S LAW:

A statement, derived by English chemist Robert Boyle (1627-1691), which holds that for gases in isothermal conditions, an inverse relationship exists between the volume and pressure of a gas. This means that the greater the pressure, the less the volume and viceversa, and therefore the product of pressure multiplied by volume yields a constantfigure.

### CHARLES'S LAW:

A statement, derived by French physicist and chemist J. A. C. Charles (1746-1823), which holds that for gases in isobarometric conditions, the ratio between the volume and temperature of a gas is constant. This means that the greater the temperature, the greater the volume, and vice versa.

### DALTON'S LAW OF PARTIAL PRES SURE:

A statement, derived by the English chemist John Dalton (1766-1844), which holds that the total pressure of a gas is equal to the sum of its partial pressures—that is, the pressure exerted by each component of the gas mixture.

### GAS:

A phase of matter in which molecules move at high speeds, and therefore exert little or no attraction toward one another.

### GAS LAWS:

A series of statements concerning the behavior of gases in response to changes in temperature, pressure, and volume. The gas laws, developed by scientists during the seventeenth, eighteenth, and nineteenth centuries, include Avogadro's law, Boyle's law, Charles's law, Dalton's law of partial pressures, Gay-Lussac's law, and Henry's law. These are summed up in the ideal gas law. The kinetic theory of gases is based on observations garnered from these laws.

### GAY-LUSSAC'S LAW:

A statement, derived by French physicist and chemist Joseph Gay-Lussac (1778-1850), which holds that the pressure of a gas is directly related to its absolute temperature. Hence, the ratio of pressure to absolute temperature is a constant.

### HENRY'S LAW:

A statement, derived by English chemist William Henry (1774-1836), which holds that the amount of gas dissolved in a liquid is directly proportional to the partial pressure of the gas above the solution. This holds true only for gases, such as hydrogen and oxygen, which don ot react chemically to liquids.

### IDEAL GAS LAW:

A proposition, also known as the combined gas law, which draws on all the gas laws. The ideal gas lawcan be expressed as the formula pV = nRT, where p stands for pressure, V for volume, n for number of moles, and T for temperature. R is known as the universal gas constant, a figure equal to 0.0821 atm · liter/mole · K.

### ISOTHERMAL:

Referring to a situation in which temperature is kept constant.

### ISOBAROMETRIC:

Referring to a situation in which pressure is kept constant.

### KINETIC ENERGY:

The energy that an object possesses by virtue of its motion.

### KINETIC THEORY OF GASES:

A set of propositions describing a gas as consisting of numerous molecules, relatively far apart in space, which interact by colliding. These collisions are responsible for the production of thermal energy, because when the velocity of the molecules increases—as it does after collision—the temperature increases as well.

### MILLIMETER OF MERCURY:

Another name for the torr, abbreviated mm Hg.

### MOLE:

The SI fundamental unit for "amount of substance." The quantity of molecules or atoms in a mole is, generallyspeaking, the same as Avogadro's number: 6.022137 · 10 23 . However, in the more precise SI definition, a mole is equal to the number of carbon atoms in 12.01 g of carbon.

### PARTIAL PRESSURE:

When two or more gases are present in a container, partial pressure is the pressure that one of them exerts if it alone is in the container. Dalton's law of partial pressure and Henry's law relate to the partial pressure of gases.

### PASCAL:

The principle SI or metricunit of pressure, abbreviated "Pa" and equal to 1 N/m 2 .

### PRESSURE:

The ratio of force to surface area, when force is applied in a direction perpendicular to that surface.

### THERMAL ENERGY:

A form of kinetic energy—commonly called "heat"—that is produced by the movement of atomic or molecular particles. The greater the motion of these particles relative to one another, the greater the thermal energy.

### TORR:

An SI unit, also known as the millimeter of mercury, that represents the pressure required to raise a column of mercury 1 mm. The torr, equal to 133 Pascals, is named for the Italian physicist Evangelista Torricelli (1608-1647), who invented the barometer.