Thermal expansion is a phenomenon that all states of matter (solids, liquids & gases) undergo when subjected to increased temperature. It refers to an object’s tendency to expand or contract in length, density, area or volume as a result of heat. On getting heat energy, the potential energy stored inside atoms gets converted into kinetic energy. Thus, the kinetic energy of a substance increases when it is heated. There are three types of thermal expansion:
The thermal expansion of gases is a change in their volume in response to a change in temperature. Heating a gas increases the kinetic energy of the particles, causing the gas to expand. The thermal expansion of gases is larger than that for solids and liquids. The particles in a gas are farther apart than those in a liquid or solid. This means that they have more room to move around when they are heated. The distance between each particle increases as the kinetic energy increases.
The distribution of molecules in gases differs greatly from the distribution of molecules in liquids and solids. Therefore, the behaviour of gas molecules is determined by certain properties and laws that the gas molecules obey.
The five properties of gases are
The volume expansion coefficient of a gas is the fractional change in volume per unit change in temperature at constant pressure. At ordinary temperatures, gases expand more than solid and liquid. The coefficient of gas expansion is dependent on temperature. For an ideal gas, the coefficient of volume expansion at constant pressure can be found from the ideal gas equation:
PV = μRT
At constant pressure
PΔV = μR ΔT
ΔV/V = ΔT/T
i.e. αv = 1/T for ideal gas
At 0 °C, αv = 3.7 × 10-3 K–1, which is much larger than that for solids and liquids.
Where V is the volume, n is the number of moles of gas, R is the gas constant, αv is the coefficient of expansion and T is the absolute temperature.
We have already studied that the behaviour of gases is affected by certain properties like temperature, pressure, volume and the amount of gas present. How their change is governed by a set of equations known as the Gas Laws. The gas laws deal with how gases behave with respect to pressure, volume, temperature and amount. Let’s take a look at these Gas Laws.
As per Boyle’s law, a change in the volume occupied by a gas (at constant quantity and temperature) will result in a change in the pressure exerted by it. In other words, the product of the initial pressure and the initial volume of a gas is equal to the product of its final pressure and final volume (at constant temperature and number of moles).
Boyle’s Law Equation: P1V1 = P1V2
Where
Boyle’s law equation is useful in determining the rise in pressure exerted by a gas on the walls of a container whose volume is reduced. It should be noted that its quantity and absolute temperature remain constant.
Charles’s law says that at a constant pressure of the gas, any change in the volume of the gas is directly proportional to the temperature of the gas (Kelvin scale). In other words, if you increase the temperature of a gas by 1 kelvin, its volume will increase by a definite amount. This is also known as the law of volumes.
Charles’s Law Equation: V2/V1=T2/T1 or V1T2 = V2T1
Where,
It can be inferred from Charle’s law equation that the absolute temperature of a gas is directly proportional to its volume.
In the Ideal Gas Law, the pressure, volume and temperature are directly proportional to each other. The relationship between the variables is given as PV = NRT. Here, P is the pressure of the gas, V is the volume of gas, T is the temperature and N is the number of moles of gas. R is a constant value that depends on the units used and this constant value is also known as the universal gas constant.
Formula: PV = nRT
Where
Thermal expansion is a concept that most people witness or experience, whether they realise it or not. After reading this article, you may have a better understanding of how air and other gases change in volume when exposed to a range of temperatures.
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