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Published on Sep 03, 2020


To Study of Diffusion of Solids in Liquids.


When substances are brought in contact with each other, they intermix, this property is known as diffusion. This property of diffusion takes place very rapidly in case of gases and to a lesser extent in case of liquids whereas solids do not show this process of diffusion with each other. But what we can observe in case of solids is that the diffusion of solids in liquids takes place at very slow rate. If a solid is kept in contact with an excess of solvent in which it is soluble, some portion of the solid gets dissolved. We know that this process is known as dissolution of a solid particles into liquid, which takes place due to diffusion of solid particles into the liquid

Diffusion is also defined as the movement of particles from an area of high concentration to an area of low concentration in a given volume of fluid (either liquid or gas) down the concentration gradient. For example, diffusion molecules will move randomly between areas of high and low concentration but because there are more molecules in the high concentration region, more molecules will leave the high concentration region than the low concentration one. Therefore there will be a net movement of molecules from high to low concentration

Initially, a concentration gradient leaves a smooth decrease in concentration from high to low which will form between the two regions. As time progress, the gradient will grow increasingly shallow until the concentrations are equalized.

We know that molecules of solutes in solutions are in constant random motion due to the collisions between molecules of solute and that of solvent. Also as the temperature is raised kinetic energy of molecules increase which thereby increase the number of collisions particles of different substances move at different rates.

Eg : When different coloured inks are placed in water at constant temperature, they diffuse at different rates:

The speed of the movement of solid particles depends upon or the rate of diffusion depends upon three factors.

a. Temperature

b. Size of he particles

c. Mass of the particle

A solution is a homogeneous mixture of substances with variable composition. The substance present in the major proportion is called solvent, whereas the substance present in the minor proportion is called the solute. It is possible to have solutions composed of several solute. The process of a solute dissolving in a solvent is called dissolution. Molecules of solute and that of the solvent are in constant random motion due to the collision between molecules of solutes and that of solvent.

The components of a solution distribute themselves in a completely random manner given sufficient time. For example, a lump of sugar chopped into a glass of water dissolves, and eventually ir.olecules of sugar can be found randomly distributed throughout the water, even though no mechanical stirring has been employed. This phenomenon, called diffusion, is similar to the process of diffusion that occurs with gases. The molecules of sugar must be in constant motion in the solution. In the case of liquid solutions, the sugar molecules do not move very far before. They encounter other molecules; diffusion in a liquid is therefore less rapid than diffusion in a gas

Requirements :

CuSO4 crystals, 100 ml beaker, water


1. Take about 2g of CuSO4 crystals in 100 ml beaker.

2. Add about 50ml of water and allow it to stand for few minutes.

3. Note the rate pf development of blue colour in water.

4. Allow to stand further till it is observed that all CuSO 4 crystals disappear.

5. Note the blue colour change in water.


When solids such as copper sulphate, are brought in contact with liquids such as water intermixing of substances i.e. diffusion takes place Conclusion

The diffusion equation provides a mathematical description of diffusion. This equation is derived from Pick's laws, which states that the net movement of diffusing substance per unit area of section (the flux) is proportional to the concentration gradient (how steeply the concentration changes in space) and is toward lower concentration (Thus if the concentration is uniform three will be no met motion). The constant of proportionality is the diffusion coefficient, which depends on the diffusing spaces and the material through which diffusion occurs.

The mechanism of diffusion is "Breewnian motion" whereby a molecule makes a random walk about a central location since by kinetic the mean velocity of a particle zero if it is not subject to any external forces. Due to collisions with neighbouring molecules, the motion of the particles is characterized by a mean free path when tends to confine the particle. But since there is no potential field acting to restore a particle to it original position, it is still free to move about the vessel or liquid in which it is located

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