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Physical Properties of Gases

Properties of Gases Molecule

Physical properties of gases characterized by lack of definite volume and density and in the gaseous state, the matter has the property of filling completely any available space to uniform density. The low density, high compressibility and thermal energy much greater than the force of attraction are also important properties of the gas molecules. Among crystalline solid, liquid, and gaseous state of aggregation, the gases are allowed for the comparative study between mass, pressure, volume, and temperature. These properties of ideal gases evident to the formation of Boyle’s, Charles, Avogadro law, Gay Lussac, and Graham’s law which provide the relation between mass, pressure, volume, temperature, density, and specific heat of gas molecules for learning chemistry or physics.

The formula in which concerns among these four variables is known as ideal gas law. After knowing this experimental ideal gas laws, a theoretical model-based structure of gases developed in kinetic theory which provides the relation between pressure, volume, and velocity of gas molecules.

Boyle’s Law for Gases Properties

Boyle’s law states as, at a constant temperature, the volume of a definite mass of a gas is inversely proportional to its pressure. Therefore, the volume of a given quantity of gases, at constant temperature equilibrium with the pressure of gases. At constant temperature and 1 atm pressure, a cylinder contains 10 ml of methane or hydrogen gas. If the pressure increases to 2 atm then according to this law volume decreases to 5 ml.

Boyle’s Law of Gases Formula

According to the definition of Boyle’s law at a constant temperature

At constant T, V ∝ 1/P
∴ PV = K = constant for a gas

Therefore, the value of gas constant depends on the base properties and mass of the gas molecules. For a given mass of gas, the volume V1 at pressure P1 and V2 at pressure P2. Thus according to Boyle’s Law, P1V1 = P2V2.

Pressure Volume Graph of Gas

The relation between pressure and volume of gas can be represented by an arm of a rectangular hyperbola graph given below.

Pressure volume graph for Boyle's law in physical properties of gases

  1. The value of gas constant changes with temperature. Therefore, there will be a separate two-dimension curve for each fixed temperature. When these curves plotted at different fixed temperatures called isotherms.
  2. At constant temperature, a given mass of gas is the product of pressure and volume. If the product of pressure and volume of gas represents in y-axis and pressure represents x-axis a straight line graph obtained parallel to the x-axis.

Relation between Gas Pressure and Density

Let at a constant temperature, M mass of gas has pressure P1 at volume V2 and pressure P2 at volume V2.

According to Boyle’s law of gases formula
P1V1 = P2V2
or, P1/P2 = V2/V1

Let the density of the gas D1 at pressure P1 and D2 at pressure P2.

Therefore, D1 = M/V1 and D2 = M/V2
or, V1 = M/D1 and V2 = M/D2
∴ P1/P2 = V2/V1 = D1/D2
or, P/D = constant
or, P ∝ D

Therefore, at a constant temperature, the density of a definite mass of gases proportional to its pressure.

Relation between Gas Pressure and Volume

At constant pressure a given mass of gas, volume increases with the increasing temperature. Hence the volume of a given mass of gases at constant pressure is directly proportional to its kelvin temperature.

Charles law of Gases Properties

Charles law evident the physical properties of the volume of a fixed mass of gases at the various temperature at constant pressure. At constant pressure, each degree rise in temperature of a definite mass of gas expands 1/273.5 of its volume at 0⁰C. Let V0 = volume at 0⁰C, then 1⁰C rise of temperature the volume of the gas rise V0/273.5 ml.

It is convenient to use the absolute temperature scale on which temperature is measured in Kelvin. Hence the reading on this scale obtained by adding 273 to the celsius value.

TK = 273 + t°C
∴ Vt = (V0 × T)/273 = (V0/273) × T

Since V0 = initial volume = constant at a given pressure. Hence the above relation expressed as, Vt = K2 T, where K2 = constant. Therefore, according to this equilibrium gas formula, Charles law states as, at constant pressure, the volume of a given mass of gases is directly proportional to its kelvin temperature.

Volume Temperature Graph

A typical property of the volume of gases with a change in its kelvin temperature a straight-line isobar graph obtained. Thus the general term isobar, which means at constant pressure assigned to these plots.

Temperature Volume Properties of gases from Charles law graph

Absolute Zero Temperature of Gas Molecules

Since volume is directly proportional to its kelvin temperature. Thus the volume is theoretically zero at zero kelvin or – 273°C. This is hypothetical because the molecule of our environment has physical properties to form liquid and then solid before this low temperature of gas reached. In reality, no substance exists as gases at the temperature near kelvin zero. For the liquefaction of real gases, we must maintain critical temperature.

Relation between Gas Temperature and Density

Let at constant pressure, for M mass has volume V1 at temp T1 and V2 at temp T2.

Charles law, V1/V2 = T1/T2
Let density D1 and D2 at V2 and V1 respectively
∴ T1/T2 = V1/V2 = D2/D1
D ∝ 1/T

Therefore, at constant pressure, the density of a given mass of gases inversely proportional to its temperature.

Combined Gases Laws Properties

Charles law, V ∝ 1/P when T constant and Boyle’s law, V ∝ T when P constant in chemistry or physics. Therefore, when all the variables are taken into account the variation rule states as

V ∝ T/P
or, PV/T = constant
∴ P1V1/T1 = P2V2/T2
or, PV = KT

All gases have properties to obey the gas laws formula under all conditions of temperature and pressure but for real gases or Van der Waals gases, Boyles law, Charles law, and combined laws are approximately applicable only at low pressure and moderately high temperatures.