Is Matter Around Us Pure

Q. A solution has been prepared by dissolving 5g of urea in 95 g of water. What is mass percent of urea in the solution?

Ans. = 5%

Q. Calculate the masses of cane sugar and water required to prepare 250 g of 2i solution of cane sugar.

Ans. = mass of cane sugar = 62.5g, mass of water = 187.5g

Q. A solution contains 35 g of common salt in 300 g of water. Calculate the concentrate of the solution.

Ans. = Concentration of solution is=10.45%

Q. A solution contains 5 mL of alcohol mixed with 75 mL of water. Calculate its concentration of the solution in terms of volume percent.

Ans. = 6.25%

Suspensions

A heterogeneous mixture of insoluble particles of solute, spread throughout a solvent, is called suspension.

The particle size (diameter) in a suspension is more than 10-5 cm. The particles have a tendency to settle down at the base of solvent and can be filtered out, because their size is bigger than the size of the pores of filter paper. Following are some examples of common suspensions:

1. Muddy water, in which particles of sand and clay are suspended in water.
2. Slaked lime suspension used for white washing has particles of slaked lime suspended in water.
3. Paints in which the particles of dyes are suspended in turpentine oil.

Characteristics of Suspensions:

1. The size of particles is more than 10-5 cm in diameter and hence can be seen with unaided eye.
2. The particles of suspension can be separated from solvent by the process of filtration.
3. The particles of suspension settle down, when the suspension is kept undisturbed. The process of settling of suspended particles under the action of gravity is called sedimentation. Thus, we can say that suspensions are unstable.
4. A suspension is heterogeneous in nature.
5. An excessive amount of scattering takes place in suspensions, because of bigger size of particles. Therefore, path of light is not visible.

Colloidal Solutions or Colloids:

A neither heterogeneous solution in which the particle size is in between 10-7 cm to 10-5 cm, such that the solute particles neither dissolve nor settle down in a solvent is called colloidal solution.

Dispersed phase and dispersing medium:

In a colloidal solution, relatively large suspended particles are called dispersed phase and the solvent in which the colloidal particles are suspended is called continuous phase or dispersing medium.

Characteristics of Colloidal Solutions:

1. The size of a colloidal particle is in between 10-7 cm and 10-5 cm.
2. The particles of a colloidal solution are visible under powerful microscope.
3. The particles of a colloidal solution do not settle down with the passage of time. Therefore, colloidal solutions are quite stable.
4. The particles of a colloidal solution cannot be recovered by crystallization or evaporation. However, they can be separated by the process of centrifugation.
5. The particles of a colloidal solution can easily pass through filter paper and hence cannot be separated by filtration.
6. The particles of a colloidal solution scatter light, i.e., when strong beam of light is passed through the colloidal solution, the path of beam becomes visible.
7. Colloidal solutions are not transparent, but translucent in nature.
8. The colloidal solutions are heterogeneous in nature.

Tyndall effect:

When strong beam of light is passed through a true solution taken in a beaker placed in a dark room, the path of light through the solution is dark. But if the light is passed through a colloidal solution under conditions as above, the path of light through the colloidal solution becomes visible.

This is called Tyndall effect. The colloidal particles become illuminated because they scatter the light falling on them in all directions.