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## CBSE Class 9 Science Chapter 2 Notes Is Matter Around Us Pure

### Is Matter Around Us Pure Class 9 Notes Understanding the Lesson

1. Pure substance: A pure substance is defined as a material which contains only one kind of atoms or molecules. In pure substance all the constituting particles have the same chemical nature. Thus, a pure substance con¬sists of a single type of particles.

2. Pure substances are always homogeneous.

• Element: A pure substance which is made up of only one kind of atom.
• Compound: A pure substance which is made up of only one kind of molecules.

3. Mixtures: It is a form of matter in which two or more elements or compounds combine physically in any proportion by weight.

4. Types of Mixtures

• Homogeneous mixture: A mixture which has same composition throughout. Solutions are homogeneous mixtures. For example, air, sea water, grass, vinegar, etc.
• Heterogeneous mixture: A mixture which has different compositions in different parts. For example, sand, mud, iron filings, sulphur, etc.

5. Characteristics of Mixture

• Mixture may be homogeneous and heterogeneous.
• Mixture does not have a fixed melting point.
• In a mixture, the different constituents combine physically in any proportion by mass.
• The constituents of a mixture do not loose their identical property.
• Usually, no energy change take place during the formation of a mixture.

6. Solution
It is a homogeneous mixture of two or more non-reacting substances.

7. Solvent is the substance in which a solute is dissolved.

8. On the basis of the size of particles, solutions can be classified as:

• True solution
• Colloidal solution
• Suspension

9. True Solution : A homogeneous system in which the particle size is less than 1 nm. For example, sugar solution.

10. Properties of True Solution

• A solution is a homogeneous mixture.
• The particles of a solution are smaller than 1 nm in diameter. So, they cannot be seen by the naked eye.
• Because of very small particle size, they do not scatter a beam of light passing through the solution. So, the path of light is not visible in a solution.
• The solute particles cannot be separated from the mixture by the process of filtration. The solute particles do not settle down when left undisturbed that is, a solution is stable.

11. Concentration of solution
There are two methods for expressing the concentration of solution.
Saturated solution: A solution in which no more of solute can be dissolved at a given temperature is called a saturated solution.
(i) Mass by Mass percentage of a Solution =$$\frac{\text { Mass of solute }}{\text { Mass of solution }} \times 100$$

(ii) Mass by Volume percentage of a Solution $$=\frac{\text { Mass of solute }}{\text { Volume of solution }} \times 100$$

12. Unsaturated solution: A solution in which more of the solute can be dissolved at a given temperature is called an unsaturated solution.
Suspension

13. A suspension: is a heterogeneous mixture in which one substance having particle size greater than 100 nm in diameter is spread throughout another substance. For example, muddy water, dust storm, aluminium paint, etc.

14. Properties of Suspension

• A suspension is a heterogeneous mixture.
• The particles of a suspension do not pass through a filter paper. Hence, it is possible to separate them by ordinary filtration.
• The particles of a suspension settle down when a suspension is left undisturbed. Thus, a suspension is unstable.
• The particles of suspension can be seen with naked eyes or with the help of a simple microscope.
• The size of particle in a suspension is greater than 100 nm in diameter.
• A suspension is not transparent to light.

15. Colloidal solutions
A solution in which the size of particles lies in between those of true solutions and suspensions are called colloidal solutions or colloids.
Colloidal solution is heterogeneous in nature and consists of two phases:

• Dispersed phase: It is the component present in small proportion and consists of particles of colloidal dimensions (1 nm to 100 nm).
• Dispersion medium: The solvent like medium in which colloidal particles are dispersed is called dispersion medium.

16. Properties of Colloidal Solutions

(i) Heterogeneous Nature: A colloidal solution is heterogeneous in nature. It consists of two phases— dispersed phase and dispersion medium.

(ii) Filtrability: The size of the colloidal particles is less than the pores of a filter paper,, and, therefore, they easily pass through a filter paper. Colloidal particles, however, cannot pass through the parchment paper or an animal membrane or ultra-filter.

(iii) Tyndall Effect: When a strong beam of light is passed through a colloidal solution placed in a dark place, the path of the beam gets illuminated by a bluish light. This phenomenon is called Tyndall effect. The phenomenon is due to scattering of light by the colloidal particles. The same phenomenon is noticed when a beam of sunlight enters a dark room through a small slit, due to scattering of light by dust particles in the air.

(iv) Visibility: Colloidal particles are too small to be seen by the naked eye. They, however, scatter light and become visible when viewed through an ultramicroscope.

(v) Brownian Movement: When colloidal particles are seen under an ultramicroscope, the particles are found to be in constant motion in zig-zag path in all possible directions. This zig-zag motion of colloidal particles is called Brownian movement. The movement of the particles is due to the collisions with the molecules of the dispersion medium.

(vi) Diffusion: Colloidal particles diffuse from a region of higher concentration to that of lower concentration. However, because of their bigger sizes colloidal particles move slowly and hence diffuse at a slower rate.

(vii) Sedimentation or Settling: Under the influence of gravity, the solute particles tend to settle down very slowly. This rate of settling down or sedimentation can be accelerated by the use of high speed centrifuge called ultracentrifuge.

17. Common methods for the separation mixtures are:

(a) Filtration: Filtration is the process of separating solids that are suspended in liquids by pouring the mixture into a filter funnel. As the liquid passes through the filter, the solid particles remain behind on the filter.

(b) Distillation: Distillation is the process of heating a liquid to form vapour and then cooling the vapour to get back the liquid. This is a method by which a mixture containing volatile substances can be separated into its components.

(c) Sublimation: This is the process of conversion of a solid directly into vapour on heating. Substances showing this property are called sublimate, e.g., iodine, naphthalene, camphor. This method is used to separate a sublimate from non-sublimate substances.

(d) Crystallisation: It is the process of separating solids having different solubilities in a particular solvent.

(e) Magnetic separation: This process is based upon the fact that a magnet attracts magnetic components from a mixture of magnetic and non-magnetic substances. The non-magnetic substance remains unaffected. Thus, it can be used to separate magnetic components from non-magnetic components.

(f) Atmolysis: This method is based upon rates of diffusion of gases and used for their separation from a gaseous mixture.

18. Physical Change: A temporary change which includes change in the shape, size, physical states and appearance of a substance,but not its chemical composition is known as physical change. Physical change is temporary and reversible. Chemical composition of the substance remains the same.
Examples:

• Heating of sulphur
• Sublimation of camphor
• Drying of wet clothes
• Breaking of glass

19. Chemical Change: A permanent change in which the chemical substance loses its own characteristics and composition and gives one or more new substances is called a chemical change. Chemical change is generally permanent and irreversible. Chemical change gives one or more new substances as products.
Examples

• Rusting of iron
• Digestion of food
• Burning of wood
• Ripening of fruit

20. Types of Pure Substances

1. Element: An element is defined as the simplest form or the basic form of a pure substance which can neither be broken into nor built up from simpler substances by any physical or chemical changes.

Properties of metals

• They have a lustre (shine).
• They have silvery-grey or golden-yellow colour.
• They conduct heat and electricity.
• They are ductile (can be drawn into wires).
• They are malleable (can be hammered into thin sheets).
• They are sonorous (make a ringing sound when hit).
Examples of metals: Gold, silver, copper, iron, etc.

Properties of non-metals:

• They are poor conductors of heat and electricity.
• They are not lustrous, sonorous or malleable.
• They display a variety of colours.
Examples of non-metals: Hydrogen, oxygen, iodine, bromine, chlorine, etc.
Metalloids: Elements having certain properties of metals and non-metals are called metalloids.
Examples: Arsenic, germanium, antimony and bismuth.

2. Compound: A compound is a substance composed of two or more elements, chemically combined with one another in a fixed proportion. For example: Water is compound of hydrogen and oxygen elements and these elements are present in water in the ratio of
1 : 8 by mass.
Characteristics of compound:

• In a compound constituents are presents in definite proportion by mass.
• The properties of the compound are different from the properties of the constituents (elements) that make up the compound.
• The constituents of a compound cannot be separated by simple physical processes.
• A compound has a fixed melting point and boiling point.
• A compound is always homogeneous in nature.

Difference between mixture and compound

 Mixture Compound 1. In a mixture, the constituents can be present in any proportion by mass. Thus, a mixture does not have any definite formula. 1. In a compound, constituents are present in  definite proportion by mass. A compound has a definite formula. 2. A mixture shows the properties of its constituents. 2. The properties of a compound are different from the properties of its constituent elements. 3. A mixture can be separated into its constituents by physical methods such as distillation, sublimation, filtration, etc. 3. The constituent of a compound can be separated only by chemical methods. 4. Formation of a mixture is not accompanied by much energy change. 4. Formation of a compound is generally accompanied by the evolution of energy in the form of heat or light. 5. A mixture does not have a fixed melting point and boiling point. 5. A compound has fixed melting point and boiling point. 6. A mixture may be homogeneous or heterogeneous. 6. A compound is always homogeneous.

### Class 9 Science Chapter 2 Notes Important Terms

1. Matter is defined as anything that has weight and occupies space.

2. Intermolecular force is the force of attraction between the consituent particles of matter.

3. Alloys are mixtures of two or more metals or a metal and a non-metal and cannot be separated into their components by physical methods.

4. Solution is a homogeneous mixture of two or more substances. The major component of a solution is called the solvent, and the minor, the solute.

5. Colloids are heterogeneous mixtures in which the particle size is too small to be seen with the naked eyes, but is big enough to scatter light.