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What are the Two Evidence of Molecular Motion of Matter?
For centuries, scientists have studied the structure of matter—how it is formed and which are its smallest entities, possessing properties identical to that of the matter itself. In the early nineteenth century, Dalton and Avogadro for the first time proposed the theory of molecular structure of matter. A molecule is the smallest entity and matter is composed of molecules having all the chemical properties of the matter itself.
The physical quantities related to molecules are: number of molecules in a material body, molecular velocities, intermolecular distances, intermolecular forces, etc. These are the internal microscopic properties of a body. Unfortunately, these properties cannot be measured directly through experiments. As a result, we have to start with certain basic assumptions (postulates) about the behaviour of molecules.
This gives us a picture about how molecules behave in a body. This is known as the molecular model. The next step is the application of Newton’s laws of motion on the molecules. Now, the experimentally determined properties of a body as a whole, like pressure, temperature, internal energy, are expected to be intimately related to the molecular model. So Newton’s laws should give us expressions leading to these bulk properties. This is essentially the object of the kinetic theory of matter. In short, the subject of study in which theoretical expressions of the bulk properties of a body are obtained from the application of Newton’s laws of motion on the internal molecular behaviour is called the kinetic theory of matter.
Naturally, the values from the theoretical expressions of kinetic theory should match with the experimental values. The formulas obtained from kinetic theory should be identical to the experimentally obtained thermodynamic formulas. This is actually the pre-condition for the sucess of kinetic theory. This condition is beautifully obeyed in the case of gases, leading to the very successful and advanced theory of the kinetic theory of gases. But this is not so in case of liquids or solids. Partially successful molecular models exist for solids, but almost none so far has been developed for liquids.
Evidence of molecular motion: Molecules cannot be observed directly, but some natural phenomena clearly indicate the existence of molecular motion.
1. Diffusion: Let a gas jar filled with hydrogen gas be held upside down on another gas jar filled with carbondioxide gas. Now the lids are removed. After an interval of time, it will be observed that the two gases will produce a homogeneous mixture in the two jars, ignoring gravitation. This phenomenon is called diffusion of the two gases. Clearly, molecular motion is evident in this phenomenon. Though carbon dioxide is heavier than hydrogen, CO2 molecules move up and the hydrogen molecules move down to produce the mixture. The molecules of a gas randomly move at different velocities in all directions. So the molecules of the two gases mix and produce a homogeneous mixture.
Diffusion takes place in liquids and solids also. If a few granules of copper sulphate are dropped at the bottom of a container filled with water, the blue colour gradually spreads throughout the whole volume of water. The density of copper sulphate solution is more than the density of water. But the solution moves up ignoring gravitation and after an intei-val of time, the whole mixture turns blue. It is an example of the motion of copper sulphate molecules diffusing into water. In a similar manner, solid phosphorus or boron can be diffused at high temperature into solid silicon crystal to produce extrinsic semiconductors.
In general diffusion can be defined as the phenomenon by virtue of which movement of molecules occur from a region of higher concentration to a region of lower concentration in a mixture till a homogeneity of concentration is established.
2. Vaporisation and vapour pressure: Liquid molecules are in motion inside the liquid. They move randomly at different velocities. Some molecules rise to the liquid surface with sufficient kinetic energy and overcome the attractions of other molecules inside the liquid. As a result, they may escape from the liquid. This phenomenon is called evaporation. Again, kinetic energies of molecules may be increased by applying heat. Hence, more molecules may escape from the liquid and vaporisation may occur.
If a liquid is enclosed in a container, molecules leaving the liquid move randomly above the liquid surface. They collide with each other and hit the surface again and again. Some molecules may enter the liquid again. This leads to the vapour pressure corresponding to the vapour above the liquid surface. At a particular higher temperature, the kinetic energies of the molecules of the liquid become very high. In comparison, the potential energies due to intermolecular attractions become negligible. So the molecules are effectively free and all of them try to come out of the liquid at the same time. This phenomenon is called boiling of the liquid.
3. Expansion of gas: A gas spreads throughout the whole volume of its container. If the volume of the container increases, the gas spreads again to occupy the whole volume. This shows the property of random and unrestricted motion of the gas molecules.
4. Brownian motion: Very small, but still visible particles are often present as impurities in a liquid or in a gas. Observations through microscopes show that these particles move in a very random manner in all possible directions. This is known as Brownian motion.
This phenomenon can be explained from the concept of molecular motion. Molecules inside a liquid or a gas move randomly in all directions and collide time and again with small foreign particles (called Brownian particles). These collisions are directly responsible for the Brownian motion.