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Colloidal Solutions, Suspensions And True Solutions

Chemical Bonding Assignment / Homework Help
Soluble substances can be of two types they are the crystalloids and colloids.

Substances which can easily be obtained in the crystalline form and whose solution can diffuse rapidly through a vegetable membrane are termed as crystalloids. Ex: sugar, urea, salts etc.

Substances which show little or no tendency to diffuse through the membrane are termed as colloids. Ex: gelatin, albumin, glue, gums, starch.

However, certain crystalloids such as sulphur, carbon and salts can be converted into colloidal solutions. Furthermore, substances such as gelatin, starch, gum etc possess a crystalline structure and are found to behave as crystalloid at one time colloid at another time. Ex: soaps show crystalloid character in alcohol in which they are freely soluble and colloidal character in water in which they are sparingly soluble. Thus, substances cannot be classified as crystalloids and colloids. It is only the nature in which a particular substance exists under suitable conditions.

What is a colloidal solution?

Substances such as sugar, salt when put in water get dissolved completely and form a homogenous solutions. This means that the molecules of sugar are dispersed uniformly in water. The solute particles in this case are of molecular size, invisible and do not settle on standing. Such solutions are termed as true solutions. There are substances such as sand when put in water does not dissolve and settle down on standing for some time. The particles are large enough and are visible to naked eye. They can be very easily separated from water. Such solutions are termed as suspensions and are heterogenous in nature. In between, there are particles which are bigger in size than molecules and smaller than suspension particles and are visible under a powerful microscope. Such particles belong to a state termed as colloidal state. These colloidal particles when placed in water form colloidal solutions.

Size of colloidal particles:

The size of colloidal particles ranges from 2000 Aο to 10 Aο. As size of the colloidal particles s very small, they appear homogenous when placed in solutions. But, colloidal solutions appear hetergenous under a microscope. However, they do not settle down like suspensions.

Hence, they cannot be separated by ordinary filtration. They can be made to settle down by the centrifugation and then filtered using special filters.


Comparison of true solutions, suspensions and colloidal solutions:

S.No

True solutions

Colloidal solutions

Suspensions

1

Particle size is of the order of molecular size i.e 10Aο

Particle size ranges from
10Aο - 2000Aο .

Particle size is greater than 2000Aο.

2

Particles are invisible under all circumstances i.e. even under a microscope.

Particles can be observed under a powerful microscope.

Particles are visible under a microscope. In case of particles such as sand they are visible even to naked eye.

3

True solutions pass easily either through a filter paper or even a parchment membrane.

Colloidal solutions pass easily through filter paper but slowly through a parchment membrane.

Suspensions cannot pass through either filter paper or parchment membrane.

 

4

Do not scatter light.

These particles can scatter light. A phenomenon known as Tyndall effect.

Do not show Tyndall effect.


Solved problems
  • The size of a colloidal particle is
    • >200 nm
    • <1nm
    • 1nm -200nm
    • None of the above.
    Ans: c
  • 2. Which of the following is not true in case of colloidal solutions? simple warming and shaking are termed as
    • It can be coagulated
    • It carries charge
    • Particles can be seen under any ordinary microscope
    • It passes through a permeable membrane. .
    Answer: c
  • Colloids represent a ----------of matter but not a class of substances.
    • State
    • Type
    • None
    • Both a and b
    Answer: a
  • Which of the following is true?
    • True solutions can scatter light.
    • Colloidal solutions can scatter light.
    • Suspensions can scatter light.
    • None.
    Answer: b
  • The phenomenon of scattering light by colloidal particles is known as
    • Reflection
    • Refraction.
    • Tyndall effect
    • Effect of light scattering.
    Answer: c

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