Introduction:

We are familiar that matter is made up of tiny particles, known as atoms or molecules. No one has ever seen an atom or a molecule. None of the greatest microscopes known to us can assist us see such particles.

The Matter exists in three states. These states are: solid, liquid and gas. Ice, water and steam are good illustrations of these states. Let us consider a state such as the solid state. The particles in the solid are tightly joined altogether by forces of cohesion.

The forces holding the particles of a solid limit their movement, in such a way that they are held in fixed positions. Solids encompass definite shapes and volumes and are extremely difficult to compress. Liquids are hard to compress, have no definite shape however posses definite volumes. A gas occupies the whole volume of the container, consists of no definite shape and is extremely compressible.

The Kinetic Theory:

The term 'kinetic' explains motion or movement. The kinetic energy of a body is the energy it comprises as a result of its motion. The higher the velocity of a body, the higher its kinetic energy.

The postulate of the kinetic theory:

The main postulate of the kinetic theory is this, that the particles of matter (that is, atoms or molecules) are continually moving and therefore possess kinetic energy. In any particular sample of matter, a few molecules encompass very high energies whereas some encompass very low kinetic energies. The average kinetic energy of the particles rises with increasing temperature of the matter.

The kinetic theory and practical proofs:

1)  Measurement of molecular velocities by the help of scientific instruments:

Scientists have been capable to measure the velocities of molecules and detect their correct positions at definite times.

2)  Natural phenomena:

a) Brownian motion: This is the jagged or irregular movement of suspended particles caused via constant collision with molecules of the suspending medium. The suspension of sulphur powder in water whenever viewed under a microscope will illustrate Brownian motion. Brown was the first scientist to notice this behavior, therefore the name Brownian motion.

b) Diffusion: Diffusion illustrates the movement or spread of substances from an area of high to low concentration. Diffusion takes place in solids, liquids and gases. A drop of liquid bromine in a closed jar of air vaporizes and spreads uniformly all through the jar. A crystal of a soluble colored solid whenever dropped in water will after sometime color the whole volume of water.

Example: CuSO₄ .5H₂ 0 (blue in color). Diffusion is fastest in gases and slowest with solids.

c) Osmosis: This is a special kind of diffusion that comprises movement of water particles. The swelling of bean seed in water is an illustration of osmosis.

All the above proofs confirm that the particles of matter are in motion as postulated via the kinetic theory.

The Kinetic Theory and the States of Matter:

The particles of matter in a solid are closely packed and firmly held altogether via strong forces of attraction. Due to the strong cohesive force, the particles are held in fixed positions and can only rotate or vibrate about a mean position. This illustrates why solids have definite shapes and volumes and are extremely difficult to compress example: iron, ice, candle and so on.

In liquid state, the particles are further apart than in the solid. The kinetic energy of the liquid particles is higher and particles are not fixed in positions. There is certain motion which lets the liquid to maintain a fixed volume however no fixed shape example: Ethanol, water and kerosene.

In the gaseous state, the particles are in constant arbitrary motion in all directions at very high velocities. There is virtually no force of attraction among the particles describing why a gas diffuses freely filling all available space. A gas consists of no definite shape or volume. The large empty spaces between the gas particles illustrate why gases are very compress.

Fig: Model of the three states of matter

Kinetic Theory and Change of State:

We are familiar that solid particles are fixed in positions by strong forces. On the other hand gas molecules are in perpetual and arbitrary motion at very high velocities due to lack of cohesive forces among the particles. Whenever solid matter is heated, the average kinetic energy of the particles rises and changes in the nature of matter takes place from solid-liquid-gas.

Melting and melting point:

Heating raises the average kinetic energy of the solid particles. At a temperature characteristic of a specific solid, the particles which are fixed in position get sufficient energy (that is, kinetic energy) to overcome the cohesive force keeping them in fixed positions. Therefore the solid steadily change to the liquid form.  The temperature at which this occurs is termed as the melting point of the solid and the phenomenon is termed as melting.  The melting point is the feature of solid and is often employed as a criterion of purity for the solid substance. A pure solid will encompass a sharp melting point that is, the temperature of the solid or liquid mixture remains constant till the melting procedure is completed.

Vaporization and boiling point:

The heating of liquid matter leads to rise in the average kinetic energy of liquid particles. As the temperature rises, the particles obtain adequate energy to overcome the cohesive energy of the liquid state. The particles become free, move more arbitrarily independent of one other. The liquid has steadily been turned to gas (vapor). This is known as vaporization.  The temperature at which there is massive vaporization from in the bulk of the liquid is the boiling point.  At boiling point, vapor molecules escape from the inside of containing vessel to the surrounding space. The temperature of the vapor or liquid mixture remains constant till all the liquid is vaporized. The boiling point is as well a criterion of purity for liquid substances. Again pure liquids encompass a sharp boiling point. For illustration the boiling point of water is 100C however vaporization can occur beneath that temperature.

Evaporation:

There is a decrease in liquid volume as an outcome of loss of vapor molecules throughout vaporization. This is known as evaporation. Evaporation is most fast at boiling as the liquid particles encompass maximum kinetic energies. Evaporation as well takes place at temperatures beneath the boiling point. This is most likely whenever a liquid sample is positioned in an open container. The high energy particles on the liquid surface can vaporize to the surrounding space. The loss of high energy particles from the liquid surface will outcome in a decrease in the liquid volume and also a decrease in the average kinetic energy of the liquid sample.

Sublimation:

Some substances (example: ammonium chloride and solid carbon (iv) oxide) don't melt if they are heated however change directly from a solid to a gas. If the gas cools it returns directly to the solid state. This method is termed as sublimation and is a helpful process for separating a mixture of substances when only one of the substances sublimes, example: ammonium chloride and sodium chloride.

Fig: States of matter and their Interconversions

Heating and Cooling Graphs:

These are the plots of temperature change (that is, average kinetic energy change) having the time of heating or cooling. The figure below illustrates the graph obtained whenever a sample of ice is heated by a steady source of energy. This is a typical heating graph.

Fig: Heating and Cooling Graphs

Whenever a change of state is taking place example: solid-liquid or liquid-vapor gas, the temperature remains constant in spite of the continue supply of heat. This energy which is not employed to increase the temperature is known as the latent heat of vaporization and the latent heat of fusion at the boiling and melting points correspondingly. The latent heat is employed to supply the particles energy to overcome the cohesive forces in the liquid or solid state. If the substance cools the reverse changes take place. As the vapor condenses and the liquid freezes the latent heats are evolved.

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