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Home Glossary

Glossary: Gases

Glossary
absolute zero. (0 K)
The temperature at which the volume of an ideal gas becomes zero; a theoretical coldest temperature that can be approached but never reached. Absolute zero is zero on the Kelvin scale, -273.15°C on the Celsius scale, and -459.67°F on the Fahrenheit scale.

atmosphere. (atm)
A unit of pressure, equal to a barometer reading of 760 mm Hg. 1 atmosphere is 101325 pascals and 1.01325 bar.

Italian chemist and physicist Amadeo Avogadro (1776-1856) proposed a correct molecular explanation for Gay-Lussac's law of combining volumes. His work provided a simple way to determine atomic weights and molecular weights of gases.

The number of particles in one mole, equal to 6.02214199 × 1023 mol-1 (± 0.00000047 mol-1) [1998 CODATA values]

Equal volumes of an ideal gas contain equal numbers of molecules, if both volumes are at the same temperature and pressure. For example, 1 L of ideal gas contains twice as many molecules as 0.5 L of ideal gas at the same temperature and pressure.

bar.
Unit of pressure. 1 bar = 105 pascals = 1.01325 atmospheres.

barometer. Compare with manometer.
An instrument that measures atmospheric pressure. A mercury barometer is a closed tube filled with mercury inverted in a mercury reservoir. The height of the mercury column indicates atmospheric pressure (with 1 atm = 760 mm of mercury). An aneroid barometer consists of an evacuated container with a flexible wall. When atmospheric pressure changes, the wall flexes and moves a pointer which indicates the changing pressure on a scale.

Boltzmann constant. (k) Boltzmann's constant.
A fundamental constant equal to the ideal gas law constant divided by Avogadro's number, equal to 1.3805 × 10-23 J K-1.

Boyle's law.
The pressure of a ideal gas is inversely proportional to its volume, if the temperature and amount of gas is held constant. Doubling gas pressure halves gas volume, if temperature and amount of gas don't change. If the initial pressure and volume are P1 and V1 and the final pressure and volume are P2V2, then P1V1 = P2V2 at fixed temperature and gas amount.

Charles' law.
The volume of a gas is directly proportional to its temperature in kelvins, if pressure and amount of gas remain constant. Doubling the kelvin temperature of a gas at constant pressure will double its volume. If V1 and T1 are the initial volume and temperature, the final volume and temperature ratio V2/T2 = V1/T1 if pressure and moles of gas are unchanged.

Dalton's law. Dalton's law of partial pressure.
The total pressure exerted by a mixture of gases is the sum of the pressures that each gas would exert if it were alone. For example, if dry oxygen gas at 713 torr is saturated with water vapor at 25 torr, the pressure of the wet gas is 738 torr.

diffusion. diffuse. Compare with effusion.
The mixing of two substances caused by random molecular motions. Gases diffuse very quickly; liquids diffuse much more slowly, and solids diffuse at very slow (but often measurable) rates. Molecular collisions make diffusion slower in liquids and solids.

diffusion rate. rate of diffusion. Compare with effusion.
The number of randomly moving molecules that pass through a unit area per second. Diffusion rates are fastest when a large concentration difference exists on either side of the unit area. Diffusion rates increase with temperature, and decrease with increasing pressure, molecular weight, and molecular size.

effusion. effuse. Compare with diffusion and diffraction.
Gas molecules in a container escape from tiny pinholes into a vacuum with the same average velocity they have inside the container. They also move in straight-line trajectories through the pinhole.

gas. gases; vapor.
Matter in a form that has low density, is easily compressible and expandable, and expands spontaneously when placed in a larger container. Molecules in a gas move freely and are relatively far apart. "Vapor" often refers to a gas made of a substance that is usually encountered as a liquid or solid; for example, gaseous H2O is called "water vapor".

ideal gas. ideal gases; perfect gas; ideal gas law.
A gas whose pressure P, volume V, and temperature T are related by PV = nRT, where n is the number of moles of gas and R is the ideal gas law constant. Ideal gases have molecules with negligible size, and the average molar kinetic energy of an ideal gas depends only on its temperature. Most gases behave ideally at sufficiently low pressures.

ideal gas law constant. (R) ideal gas constant; universal gas constant.
A constant R equal to PV/(nT) for ideal gases, where the pressure, volume, moles, and temperature of the gas are P, V, n, and T, respectively. The value and units of R depend on the units of P, V, and T. Commonly used values and units of R include: 82.055 cm3 atm K-1 mol-1; 0.082055 L atm mol-1 K-1; 8.31434 J mol-1 K-1; 1.9872 cal K-1 mol-1; 8314.34 L Pa mol-1 K-1; 8.31434 Pa m3 mol-1 K-1.

law of combining volumes. Gay-Lussac's law.
When gases react, they do so in a definite proportion by volume, if the volumes are measured at the same pressure and temperature. For example, in the reaction N2(g) + 3 H2(g) = 2 NH3(g), 3 liters of hydrogen will react with 1 liter of nitrogen to give 2 liters of ammonia if all volumes are measured at the same temperature and pressure.

manometer. Compare with barometer.
An instrument for measuring gas pressures. A mercury or oil manometer measures gas pressure as the height of a fluid column the gas sample is able to support. Open manometers measure gas pressure relative to atmospheric pressure.

molar volume.
The volume occupied by one mole of a material. For example, the molar volume of an ideal gas at STP is 22.4 L/mol.

momentum. (p)
Momentum is a property that measures the tendency of a moving object to keep moving in the same direction. Increasing the speed of an object increases its momentum, and a heavy object will have more momentum than a lighter one moving at the same speed. For a particle with mass m and velocity v, the momentum of the particle is mv.

pascal. (Pa)
The SI unit of pressure, equal to a force of one newton per square meter. 101325 pascals = 1 atmosphere; 105 pascals = 1 bar.

pressure. (P)
Force per unit area. The SI unit of pressure is the pascal, defined as one newton per square meter. Other common pressure units are the atmosphere, the bar, and the Torr.

SATP. standard ambient pressure and temperature. Compare with STP and standard state.
Used to describe a substance at standard pressure and a temperature of 25°C (298.15 K).

standard molar volume.
The volume of 1 mole of an ideal gas at STP, equal to 22.414 liters.

standard pressure. (P° or Po)
Standard pressure is a pressure of 1 bar. Before 1982, the standard pressure was 1 atm (1 atm = 1.01325 bar).

STP. standard temperature and pressure. Compare with SATP and standard state.
Used to describe a substance at standard pressure and a temperature of 0°C (273.15 K).

torr. Torr; mm Hg. Compare with barometer and pressure.
A unit of pressure, defined so that 760 Torr is exactly 1 atmosphere. A Torr is equivalent to 1 mm Hg on barometer readings taken at 0°C; at other temperatures, the conversion from mm Hg to Torr is approximately p(Torr) = p(mm Hg) × (1 - 1.8×10-4t), where t is in °C.

van der Waals equation.
A semiempirical equation that describes the relationship between pressure (P), volume (V), temperature (T), and moles of gas (n) for a real gas. The equation is (P + n2a/V2)(V - nb) = nRT, where a and b are constants that include the effects of molecular attractions and molecular volume. a and b are usually fitted to experimental data for a particular gas.

xenon. Xe.
Element 54, a colorless, inert gas used to fill cathode ray tubes.

General Chemistry Online! Gases