Why Does Air Expand On Heating

You have probably witnessed smoke gushing out of a fire or vapours rising up while cooking. Ever wondered what is the science behind it? We know everything is pulled down by gravitation but why does hot air rise up defying gravity?

Everything is made up of molecules, gases are no exception. Normal air is a mixture of various gas molecules. In gaseous states, the molecules are far apart from each other compared to solid or liquid states. The cohesive attraction between gaseous molecules is significantly low, as such the molecules have a high degree of freedom. In this article, we will study the heating of air concept in detail.

What Happens When Air is Being Heated?

When heat is supplied, each molecule gets excited by absorbing the heat energy and begins to wander more vigorously; the limited attraction between the molecules is overcome and the molecules spread over a larger space. This results in the expansion of air on heating. The expansion due to heat is called thermal expansion.

Hot air, therefore, tends to occupy a large volume, and it becomes less dense. Low density makes the gases light and hence, it rises up. Heating gives the gas molecules the necessary energy for its upward drift.

Convection Current

The flow of fluids due to temperature or density difference within the material is called convection current. In nature, it is called free convection or natural convection. The phenomenon is very crucial in the atmosphere for weather patterns. High temperature heats up the air at the surface of the earth, lowering the density while the air at layers above the surface is still cold and has a higher density. Gravity pulls down the high-density cold air while the hotter low-density air (which is lighter too) rises up. As the hot air reaches higher altitude, it cools down.

Lowering of temperature saps the molecules of its energy and the intermolecular attraction brings them closer, resulting in a contraction and increase in density. While the air below heats up, the process is repeated. This circulation of hot and cold air is convection.

Effects of Convection Current

Convection current helps in the thermal circulation of earth's atmosphere. It drives the large-scale drift of air across the globe. Heating of the surface air is caused by incident sunlight, which is highest in the equator while at the poles it is lowest and hence forms an extremely cold climate. The cold wind flows from the poles towards the equator due to convection.

Another phenomenon resulting from convection is the sea breeze. Sea water warms slower than land, as a result relatively cold air from the sea/ocean flows towards the land as the air above the land is hotter and lighter and thus rises up making way for the cool sea breeze.

The phenomenon we spoke about at the beginning — the rise of smoke due to combustion and vapour during cooking are also due to convection.

Expansion and Contraction of Matter

Since all matter is composed of molecules, the effect of heat is the same for all. All matter tends to expand when heated. In solid, the molecules are most compactly packed and have the lowest degree of freedom. The expansion of matter is least in solid followed by liquid and gas. Similarly, when temperature is lowered, that is, a substance is cooled, the average motion of molecules is reduced and the molecular attraction takes over causing a shrinkage of space between the molecules. This is a contraction.

This phenomenon is best demonstrated by the change of the state of water. When water is heated, the liquid molecules acquire energy and fly off - we see vapours. When the liquid is cooled, the molecules condense and assume a solid structure - ice!

Just like heating causes expansion, cooling causes contraction; contraction results in lowering of volume and an increase in density.

Air Expansion on Heating Examples

• Tyres burst during hot summer days, due to build up pressure inside the tube by the expanding air.

• Hot air balloons rise up because heating causes the air inside the balloons to rise up.

Interesting Facts

• All gases expand to the same volume on heating to the same degree.

• Ice, although solid, has less density than liquid water.

• Substances which contract with heating are called Negative Thermal Expansion (NTE) materials. Although such material exists ‘air contracts on heating’ is still inconceivable.

Key Features

• All matters are composed of molecules.

• Heating causes the expansion of matter while cooling causes contraction.

• Heating of air cause it to expand and the warm air rises.

• Circulation of fluid due to temperature or density difference is called convection.

• Convection helps in thermal distribution and circulation of atmosphere.