Revision Notes For Chapter 2 - Is Matter Around Us Pure?

Here we are providing class 9 science notes for revision. Students should be very serious in Class 9 because it is a gateway to higher education. 

We hope that this NCERT Notes For Class 9 Science Chapter 2 will help the students to get good scores.



CBSE NCERT Notes For Class 9 Science Chapter 2 - Is Matter Around Us Pure?


Class 9th Science NCERT Notes


You can find CBSE NCERT Notes For Class 9 Science for other chapters here.

TOPIC-1 Elements, Compounds, Mixtures, Heterogeneous and Homogeneous Mixtures


Matter can be classified as pure substances or mixtures.

♦ A pure substance may either contain constituent particles of only one kind or of different kinds. A pure substance has a fixed composition.

♦ An element is a basic form of matter which cannot be broken down into simpler substances by any physical or chemical means.

♦ Elements can be broadly classified as metals, non-metals, and metalloids.

Metals are one category of elements that have luster. They conduct heat and electricity. They are sonorous.
They are malleable and ductile.

♦ Non-metals do not have luster, are not sonorous, and are bad conductors of heat and electricity.

Metalloids are elements having properties intermediate between those of metals and non-metals.

♦ A compound is a pure substance composed of two or more elements chemically combined in a fixed proportion.

It can be broken down into simpler substances by chemical or electrochemical methods.

Properties of compounds: are different from those of its constituent elements, whereas a mixture shows the properties of its constituent elements or compounds.

♦ A mixture contains two or more elements or compounds which are mixed together in any proportion. From a mixture, no new compound is formed. 

A mixture shows the properties of the constituent substances.

♦ Mixtures are classified as homogeneous and heterogeneous mixtures.

♦ Mixtures whose components mix completely with each other to make a uniform composition are called homogeneous mixtures.

♦ A heterogeneous mixture has a non-uniform composition.

Alloys are a mixture of two or more metals or a metal and a non-metal and cannot be separated by physical methods.

♦ The ability of a substance to dissolve in another substance is called solubility.

♦ A homogeneous mixture of two or more substances is called a solution.

♦ The component of a solution present in small quantities is called a solute.

♦ The component of a solution present in large quantity is called a solvent.

♦ Particles of a solution are smaller than 1nm in diameter. They cannot be seen by naked eyes.

♦ Particles of solution do not scatter a beam of light.

♦ Solute particles cannot be separated from the mixture by filtration.

♦ Solution with high solute concentration is called a concentrated solution and those with low concentration are called a dilute solution.

♦ The concentration of a solution is the amount of solute present in a given amount (mass or volume) of solvent or solution. 

The concentration of a solution = Amount of solute Amount of solution

♦ Percentage by mass is one method of expressing the concentration of solution.

♦ There are two kinds of heterogeneous mixtures :– colloids and suspensions.

♦ Colloids are mixtures with particle sizes from 1nm to 100 nm.

♦ The component of colloid present in small amount is called the dispersed phase.

♦ The medium in which colloidal particles disperse or suspend themselves is called dispersion medium.

Brownian movement: In a colloidal system particles are always suspended and do not settle down. This constant colliding of the particles in continuous motion is called Brownian movement.

Tyndall effect: Scattering of a beam of light when light is passed through a colloidal solution is called the Tyndall effect.

♦ Colloids are classified according to the state (solid, liquid, or gas) of the dispersed medium or dispersing medium and the dispersed phase.

♦ Colloid in which the dispersed medium is a liquid and the dispersed phase is solid is called a solution.

♦ A colloid in which both dispersed phase and dispersed medium are in the liquid state is called an emulsion.

Aerosol: A colloid in which the dispersed phase is either liquid or a solid and dispersed medium is gas is called as an aerosol.

♦ A suspension is a heterogeneous mixture in which the solute particles do not dissolve but remain suspended throughout the bulk of the medium. 

Particles of suspension are visible to the naked eye. Suspensions are heterogeneous mixtures with particles that have a size greater than 1000 nm.

All these topics are very important. You should write this NCERT Notes For Class 9 Science Chapter 2.

TOPIC-2 Separation Techniques, Physical and Chemical Change

Quick Revision Notes

♦ Heterogeneous mixtures can be separated by simple physical methods like handpicking, sieving, filtration etc.

♦ Magnetic impurities can be separated from non-magnetic impurities by magnetic separation.

♦ Volatile compounds can be separated from non-volatile compounds by sublimation.

♦ Mixtures of two or more liquid components can be separated by simple or fractional distillation or by using a separating funnel.

♦ Simple distillation is used for separating liquids having a difference in boiling points of more than or equal to 25° C.

♦ Fractional distillation is used for separating liquids having a difference in boiling points of less than 25°C.

♦ Liquids that are immiscible in each other differ in their densities and so can be separated by making use of a separating funnel.

Chromatography is used for the separation of those solutes which dissolve in the same solvent.

♦ Chromatography is a method of separating and identifying various components in a mixture, which are present in small trace quantities.

♦ A mixture containing two solid substances out of which one is soluble in a particular solvent and the other is insoluble can be separated by dissolving the soluble constituent in a suitable solvent and then separating the insoluble substance through filtration.

♦ The change in which the shape, size, appearance, or state of a substance may alter but its chemical composition remains the same is called a physical change. In a physical change, no new substance is formed.

♦ Any change that involves the formation of a new substance and leads to a transformation of chemical identity is called a chemical change.

♦ Chemical changes are usually accompanied by heat exchanges. Chemical changes are permanent changes that are usually irreversible.

♦ Applications of crystallizationPurification of salt from seawater and separation of crystals of alum from impure samples.

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