This article brings you the CBSE Class 9 Science notes on chapter 2 ‘Is Matter Around Us Pure’ (Part-II). This part is a continuation of CBSE Class 9 Science notes on chapter 2 ‘Is Matter Around Us Pure’ (Part-I). In Part-I, the main topics explained were: Matter and its Types; Elements; Compounds; Metals, Non-metals and Metalloids; Mixture and its Types; Solution and its Types; Solubility and Concentration of Solution.
Main topics covered in this part of CBSE Class 9 Science Chapter Is Matter Around Us Pure Notes, are:
- Suspension - Definition and properties
- Colloid - Types and properties
- Tyndall Effect
- Physical change
- Chemical change
- Methods for separation of mixtures
Also check: CBSE Class 9 Study Material 2020-2021
These chapter notes are prepared by the subject experts to discuss every important topic from the chapter. In between these notes you can try the questions asked from the discussed set of topics. These questions will help you to track your preparation level and get a hold on the subject.
Key notes for Chapter- Is Matter Around Us Pure, are:
The heterogeneous mixture in which solids are dispersed in a liquid are called suspensions. For example, chalk-water mixture, muddy water, flour in water, etc.
A suspension is a heterogeneous mixture in which the solute particles do not dissolve but remain suspended throughout the bulk of the medium.
Properties of a Suspension:
(i) It is a heterogeneous mixture.
(ii) The particles of a suspension can be seen by naked eyes.
(iii) The particles of a suspension scatter a beam of light passing through it and make its path visible.
(iv) A suspension is unstable.
(v) Constituents of a suspension can be separated by the process of filtration.
Colloid or Colloidal solution:
Solutions in which the size of particles lies in between those of true solutions and suspensions are called colloidal solutions or simply colloids. For example, Milk, smoke and starch solution, etc.
Properties of Colloids:
(i) A colloid is a heterogeneous mixture.
(ii) The size of particles of a colloid is too small to be individually seen by naked eyes.
(iii) The particles of a colloidal solution are big enough to scatter a beam of light passing through it.
(iv) A colloid is quite stable.
(v) Colloidal particles cannot be separated by the process of filtration.
Tyndall Effect: The scattering of light by the colloidal particles is known as Tyndall effect.
It can be observed when:
- A fine beam of light enters a room through a small hole.
- Sunlight passes through the canopy of a dense forest.
Note: The colloidal particles are not the true solutions as their particles are big enough to show the Tyndall effect, whereas the particles of a true solution do no show Tyndall effect.
The components of a colloidal solution:
(i) Dispersed Phase: The solute-like component or the dispersed particles in a colloid form the dispersed phase.
(ii) Dispersion Medium: The component in which the dispersed phase is suspended is known as the dispersing medium.
Types of Colloids:
Different types of colloids are discussed in the table given below:
Those changes in which only physical properties of the substances change but no new substances are formed are called physical changes.
For example - Melting of ice to form water, breaking of glass, preparing a solution etc.
Those changes in which new substances are formed are called chemical changes and chemical properties of a substance gets changed in a chemical change.
For example - Rusting of iron, bumning a piece of paper etc.
Difference between Physical and Chemical changes:
Here only the physical properties like state, shape or size of a substance change.
It brings about a change in the chemical properties of a substance.
There is no change in chemical
There is always a change in chemical
No new substance is formed.
A new substance is always formed.
It is temporary and hence reversible.
It is permanent and hence irreversible.
Try the following questions:
Q1. How will you distinguish a colloid from a solution?
Q2. What is effect of temperature on the solubility of solids in liquids?
Q3. Sea water can be classified both as a homogeneous as well as a heterogenous mixture. Comment?
Q4. Distinguish between the following as physical changes and chemical changes?
(a) Burning of a magnesium wire
(b) Rusting of iron
(c) Condensation of steam
(d) Glowing of an electric bulb
The separation of components of a Mixture:
Some of the important methods used to separate different mixtures are explained below::
Evaporation is a process which is used to separate a solid substance dissolved in liquid. It is based on the fact that liquids vaporize easily whereas solids do not.
Applications of Evaporation:
- Obtaining salt from see water.
It is a method of separating the suspended particles of a substance from a liquid in which the mixture is rotated at high speed in a centrifuge.
This method is useful in case the suspended particles in a liquid are too small too be retained by filter paper.
Principle of Centrifugation:
When a mixture is rotated very fast, the denser particles are forced to go to the bottom of the centrifuge and the lighter particles stay at the top.
Applications of Centrifugation:
- Used in dairies to separate cream from milk.
- Used in washing machines to squeeze out water from wet clothes.
(3) Separating funnel
It is used to separate a mixture of two immiscible liquids, like oil and water.
Principle of Separating funnel :
When a mixture of two immiscible liquids is kept in a separating funnel, the liquids separate out in layers depending on their densities with the heavier forming the top layer.
Applications of Separating funnel:
- To separate mixture of oil and water.
- In the extraction of iron from its ore where the lighter slag (molten waste material) is removed from the top by to leave the molten iron at the bottom of the blast furnace.
This method is used to separate those components from a mixture which can sublime on heating.
For example: Ammonium chloride, camphor, naphthalene etc, can be separated from their mixture by sublimation
Applications of Sublimation:
- Iron can be separated from a mixture of iron fillings and camphor (volatile).
- Common salt can be separated from a mixture of salt and ammonium chloride (volatile).
This method is used to separate two or more dissolved solids which are present in a solution in very small quantities.
Principle of Chromatography:
This method of separation is based on the fact that though two substances are dissolved in the same solvent but their solubilities can be different. The component which is more soluble in, rises faster and gets separated from the mixture.
Applications of Chromatography:
- To separate colours in a dye by paper chromatography.
- To separate drugs from blood.
This method is used to separate a mixture solid in a liquid. It is the process of heating the liquid to form vapour, and then cooling the vapour to get back liquid.
It is used for the separation of components of a mixture containing two miscible liquids that boil without decomposition and have sufficient difference in their boiling points.
Principle of Distillation:
The volatile liquid evaporates on heating which can be recovered by cooling its vapours by the process of condensation.
Applications of Distillation:
- It is used to remove salt from sea water to obtain drinking water.
(7) Fractional distillation
It is the process of separating two or more miscible liquids by distillation, the distillate being collected in fractions due to boiling at different temperatures.
The apparatus used in this process is similar to that for simple distillation except a fractionating column which is fitted in between the distillation flask and the condenser. A simple fractionating colunrn is a tube packed with glass beads. The beads provide surface for the vapours to cool and condense repeatedly.
Principle of Fractional Distillation:
In a mixture of two or more miscible liquids, the separation of various liquids depends on their boilibg points. The liquid having lower boiling point boils first and can be obtained first from the fractionating column than the liquid having higher boiling point.
Applications of Fractional Distillation:
- It is used to separate a mixture of miscible liquids like alcohol-water mixture.
- It is used to separate cruid oil ‘petroleum’ into useful fractions like kerosene, petrol, diesel, etc.
- It is used to separate different gases of the air by taking the liquid air.
(8) Crystallisation :
Crystallisation is a process used to separates a pure solid in the form of its crystals from a solution. The process involves cooling a hot, concentrated solution of a substance to obtain crystals.
Applications of Crystallisation:
- Purification of common salt obtained from sea water.
- To obtain crystals of alum (phitkari) from impure samples.
- To obtain pure copper sulphate from an impure sample.
Try the following questions:
Q1. Name the method of separation that you will apply for separation of the following mixtures?
(a) Amnonium chloride from a powdered mixture of sodium chloride and Ammonium chloride.
(b) Butter from cream.
(c) Oil from water.
Q2. Which of the following will show Tyndall effect:
(a) Salt solution
Q3. How will you separate a mixture of iron filings, chalk powder and common salt?
Q4. Discuss the process of purification of impure copper sulphate by crystallization.
Q5. Explain how nitrogen, oxygen and argon gases are separated from air?