The question about what things are made of started as far back as the 5th century B.C. when a Greek Philosopher, Democritus, first expressed his opinion that matter is made up of indivisible tiny particles which he called atoms. Scientists have been able to postulate the atomic theory which states that an atom is made up of sub-atomic particles namely protons, neutrons and electrons.

Why does the volume of gases expand upon heating? Why does a small volume of water, for example 1 cm3 at 100°C and standard pressure give a large volume of steam (about 1,650 cm3)? Why do we have only small amounts of hydrogen and helium in the earth’s atmosphere compared to Jupiter which has them in appreciable amounts? These and many other related questions can be answered by employing the kinetic theory.


The kinetic theory assumes that:

1) In the solid state, the molecules are more or less in fixed positions, so they hardly move.

2) In the liquid state, the movement of the molecules are less restricted and that is why a liquid flows to assume the shape of its container.

3) In the gaseous state,

  • a gas is composed of molecules that are separated from each other by large distances when compared to the molecular size;
  • gas molecules are in constant motion in random directions and in short, straight lines, colliding with one another and with the walls of the container;
  • gas molecules exert neither attractive nor repulsive forces on one another; and
  • the average kinetic energy of the molecules is proportional to the temperature (in Kelvin) and pressure.


Explanation of some Phenomena using Kinetic Theory

There are so many natural events and situations (phenomena) all around us that can accurately be explained using the kinetic theory. Some of them include:

  1. the behaviours of solids, liquids and gases under different temperatures and pressures.
  2. the large increase in the volume of a liquid (say 1 cm3) when converted to vapour.
  3. why there is only a small amount of hydrogen or helium in earth’s atmosphere compared to Jupiter’s.
  4. why substances can exist as solid, liquid or gas.
  5. why a solid has shape while liquid and gas has none.


Shape of Solids

From kinetic theory, we assume that the atoms and molecules of a solid are more or less in fixed positions as a result of strong intermolecular forces amongst them. Consequently, a solid retains a definite shape at normal (ambient) temperature and pressure.


The Volume of a Liquid increases when it turns to Vapour

Kinetic theory assumes that the molecules of a liquid are less restricted to move compared to the solid state. On the application of heat, the molecules of the liquid acquire more kinetic energy which energizes them. As a result, these energetic molecules move faster and are able to break apart to become gaseous molecules, requiring a larger space and hence, an increased volume.

This is why 1 cm3 of water at 100°C and standard pressure, gives a large volume of steam (about 1,650 cm3). This principle is used by engineers to design steam engines and internal combustion engines.


The Earth’s Atmosphere

The earth’s atmosphere contains small amount of hydrogen or helium compared to that of some other planets, say Jupiter.

The earth is considerably smaller than Jupiter and so has weaker gravitational force on matter. Helium molecule (gas) has an average velocity considerably greater than that of nitrogen or oxygen molecules and also greater than the escape velocity (1.1×10 raise to power 3 ms-1). Any object that moves with a speed equal to or greater than escape velocity can escape the earth’s gravity and go into outer space.

On the other hand, Jupiter with a mass of about 320 times greater than the Earth and hence, greater gravitational force, can retain both heavy and light gases in its atmosphere. So whether a gas can be retained or not in the Earth’s atmosphere depends strictly on its kinetic energy.


A Substance can exist as Solid, Liquid or Gas

The physical state of a substance depends on the kinetic energy of its atoms or molecules. Substances whose atoms or molecules have very low kinetic energy at ambient temperatures and pressures will normally exist as solids. Those whose molecules have somewhat higher kinetic energy than that of solids will exist as liquids; while those whose molecules have large kinetic energy will exist as gases.


Explanation of Boiling and Evaporation using Kinetic Theory

Boiling of a Liquid

When heat is applied to a given volume of a liquid, its molecules gradually acquire more kinetic energy and the temperature rises as well. If the heat supply is sustained, a time comes when the kinetic energy will be so high that the molecules will tend to separate from the mass of the liquid into the atmosphere. When the pressure exerted by the molecules (vapour pressure) is equal to or greater than that of the surroundings (say atmospheric pressure), the liquid changes rapidly and completely into vapour and is said to boil.

Each pure liquid has a characteristic boiling temperature (boiling point). Pure water has a boiling point of 100°C at standard pressure. For benzene, the boiling point is 80.1°C.


Evaporation of Liquid

Evaporation of a liquid means the escape of some molecules from the surface tension of the liquid. In principle, evaporation occurs at all temperatures above the freezing point of the liquid. The surface molecules of a liquid will normally absorb heat energy from the surroundings to increase their kinetic energy. When this kinetic energy is high enough, the surface molecules will escape into the surrounding atmosphere. The higher the surrounding temperature, the higher the rate of evaporation.


Factors that affect Evaporation

Evaporation is a phenomenon that depends heavily on the prevailing surrounding circumstances. Below are some of the factors affecting evaporation:

  1. Temperature: The prevailing temperature affects evaporation because the kinetic energy acquired by the surface molecules, ultimately comes from the surrounding temperature. The higher the surrounding temperature, the higher the rate of evaporation.
  2. Pressure: The atmospheric pressure also affects evaporation. The higher the atmospheric pressure, the lower the rate of evaporation.
  3. Air movement: Evaporation is faster when there is breeze or wind around the liquid. This is because the moving wind rapidly carries away the liquid molecules escaping from the surface, thereby encouraging more molecules to escape from the surface.
  4. Type of liquid and its vapour pressure: Vapour pressure is the amount of vapour hovering on the surface of a liquid at standard temperature. Since different liquids have different vapour pressures, the evaporation of a liquid clearly depends on its vapour pressure. Gasoline (petrol) has a higher vapour pressure than water and so will evaporate faster than water under the same conditions.


Effect of Evaporation

The ultimate effect of evaporation is cooling. Cooling occurs because the escaping surface liquid molecules take away with them the earth’s energy from the liquid body,thereby, reducing the temperature of the liquid body and hence, cooling.

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