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PROPERTIES OF SOLUTIONS

 

What is the solution?

When two or more than two components are mixed in the fluid form, and the mixture has the same composition and same properties across the whole of it, then that mixture is called a solution.

What is a binary solution?

A homogenous mixture of 2 components is called a binary solution. It has two components, one is solute, and the other is solvent.

How will you identify a solute or a solvent in a solution?

The component which is present in the largest quantity in a solution is considered to be solvent, and therefore the other component will be solute. The physical state in which the solution is present is decided by the solvent.

Properties of solutions

  • Solutions are composed of two or more components and are homogeneous mixtures.
  • Particles of solutions are not visible to the naked eye because they have a diameter of very small size, which is 10-9 m or 1 nm.
  • They also do not perform the phenomenon of scattering of light because their particle size is too small to scatter the light passing through it, and hence, the path of the light passing through it is invisible.
  • Solutions are stable in nature, which means solute particles never settle down when they are allowed to stand for whatever span of time.
  • Also, solute particles cannot be filtered out as their particle size is too small (1 nm).

What is meant by the vapour pressure of a solution?

The pressure of any volatile substance at a given temperature is known as vapour pressure. When temperature increases, then vapour pressure increases and when the attractive forces between the particles of the solution increase, then vapour pressure decreases.

What is meant by the colligative properties of the solutions? Discuss each of them briefly, one by one.

Colligative properties of the solutions are the properties of the solutions, which depend on the number of particles of the non – volatile solute in the solution. When the number of particles of that non-volatile solute increases, then the colligative properties also increase. Following are the colligative properties:

(a). Relative lowering of vapour pressure:

As we know, the vapour pressure of the solvent is more than the vapour pressure of solution (mixture of same solvent and solute), therefore using this basic concept, we get the following relation:

qa

P0= vapour pressure of solvent

 P = vapor pressure of solution

 n1 = number of solvent moles

 n2 = number of non – volatile solute moles

i = Van't Hoff's factor

 

(b). Elevation in boiling point:

We know that the vapour pressure of liquid increases with an increase in temperature, and boiling point is the point at which the vapour pressure of liquid equals the atmospheric pressure. When we add non – volatile solute to the solvent, the resulting solution has lesser vapour pressure than the solvent.

Therefore, in order to increase the vapour pressure of the solution to such an extent that it equals the atmospheric pressure, the temperature should be slightly higher than the boiling point of the solvent.

This slight increase in boiling point is known as Elevation in boiling point.

∆Tb = i. kb. m = Tb - Tb˚

where 

j

m = molality

Elevation in boiling point

where Kb = molal elevation constant

Tb = boiling point of pure solvent

lv = latent heat of vaporization per gram

M = molar mass

 

(c). Depression in freezing point of a solution

The temperature at which the vapour pressure of the substance in its liquid phase is equal to the vapour pressure in its solid phase; that temperature is called the freezing point of a substance.

Here it will be just the opposite of Elevation in boiling point because, after the addition of a non – volatile solute to the solvent, vapour pressure decreases, and now it would be equal to that of solvent at a lower temperature. Hence, the freezing point of the solvent decreases.

∆Tf = i× k f × m = T f˚ - T f

Where

q

m = molality

Freezing point depression

 

T f = freezing point 

k f = molal depression constant

l f = latent heat of fusion per gramme

 

(d). Osmotic Pressure

Osmosis

The flow of solvent molecules from the pure solvent side (high solvent concentration) to the solution side (low solvent concentration) is called osmosis, and the pressure applied on the solution side to oppose this flow of molecules is called osmotic pressure. 

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