NCERT Class 9 Science Chapter 2 Notes Is Matter Around Us Pure- Download PDF Notes

It is very crucial to understand whether the matter around us is pure or not. Have you ever wondered why oil does not dissolve in water, while salt dissolves in water, or sugar completely dissolves in water, while sand settles at the bottom? All these observations are linked to the concept of the purity of matter. Objects around us are categorised as either pure or a mixture.

Everything around us is matter, whether it is the air we breathe, the water we drink or the food we eat. The NCERT Notes for Class 9 Science will help you understand matter, mixture, solutions and their properties. These notes will provide the basic and detailed knowledge needed to know if the matter around us is pure. The NCERT notes are designed by our subject experts to help the students tackle easy to complex questions in the exam. The important formulas and diagrams are included to help you grasp the concepts easily.

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• NCERT Solutions for Class 9 Science Chapter 2 Is Matter Around Us Pure
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NCERT Notes Class 9 Chapter 2: Download PDF

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NCERT Notes Class 9 Chapter 2

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Matter around us

If we observe some sugar and some soil placed on two different sheets of paper with a magnifying glass, we will find that the colour, shape and size of all the particles of sugar are the same, but the soil contains particles of different colours, shapes and sizes. For example, the soil contains clay particles, some grass particles and even some dead insects, etc. Now, sugar which contains particles of only one kind is called a pure substance whereas soil which contains particles of different kinds is called an impure substance (or mixture). From this, we conclude that all the matter around us is not pure. The matter around us is of two types: pure substances and mixtures.

2.1 What is the mixture?

A mixture is a substance that consists of two or more elements or compounds not chemically combined together. For example, air is a mixture of gases like oxygen, nitrogen, argon, carbon dioxide, water vapour, etc. The various gases of the air are not chemically combined with one another. Similarly, gunpowder is a mixture of potassium nitrate, sulphur and charcoal (charcoal is a form of carbon) whereas brass is a mixture of copper and zinc. All the solutions are mixtures. For example, salt-solution (brine) is a mixture of common salt (sodium chloride) in water.

Types of Mixtures

• A homogeneous mixture
• A heterogeneous mixture

Those mixtures in which the substances are completely mixed together and are indistinguishable from one another are called homogeneous mixtures. A homogeneous mixture has a uniform composition throughout its mass. It has no visible boundaries of separation between the various constituents. A mixture of sugar in water (called sugar solution) is a homogeneous mixture because all the parts of sugar solution have the same sugar-water composition and appear to be equally sweet! There is no visible boundary of separation between sugar and water particles in a sugar solution.

Those mixtures in which the substances remain separate and one substance is spread throughout the other substance as small particles, droplets or bubbles, are called heterogeneous mixtures. A heterogeneous mixture does not have a uniform composition throughout its mass. It has visible boundaries of separation between the various constituents. The mixture of sugar and sand is a heterogeneous mixture because different parts of this mixture will have different sugar-sand compositions.

The suspensions of solids in liquids are also heterogeneous mixtures. For example, a suspension of chalk in water is a heterogeneous mixture. A mixture containing two (or more) immiscible liquids is also a heterogeneous mixture. For example, a mixture of petrol and water is a heterogeneous mixture.

2.2 What is a Solution?

A solution is a homogeneous mixture of two (or more substances). A homogeneous mixture means that the mixture is just the same throughout. Some common examples of solutions are : Salt solution, Sugar solution, Vinegar, Metal alloys (such as Brass) and Air. Salt solution is a homogeneous mixture of two substances, salt and water, whereas sugar solution is a homogeneous mixture of two substances, sugar and water. Some more examples of the solutions are : Sea-water, Copper sulphate solution, Alcohol and water mixture, etc.

Properties of a Solution

The important characteristic properties of a solution (or true solution) are as follows :

• A solution is a homogeneous mixture.
• The size of solute particles in a solution is extremely small. It is less than 1 nm in diameter (1 nanometre = 10^-9 metre).
• The particles of a solution cannot be seen even with a microscope.
• The particles of a solution pass through the filter paper. So, a solution cannot be separated by filtration.
• The solutions are very stable. The particles of solute present in a solution do not separate out on keeping.
• A true solution does not scatter light (This is because its particles are very, very small).

2.2.1 CONCENTRATION OF A SOLUTION

Based on the concentration of the solution, there are three types of solutions- diluted, concentrated and saturated.

• A dilute solution is one in which the amount of solute in a given mass is fairly minimal compared to the amount of solvent. A concentrated solution contains a large amount of solute and a saturated solution has dissolved all of the solutes it is capable of dissolving at a given temperature. In other terms, a saturated solution is one in which no more solute can be dissolved in a solution at a given temperature.
• The solubility of a solute is the amount of the solute present in a saturated solution at this temperature.
• An unsaturated solution is one in which the amount of solute in the solution is less than the saturation level.

A solution's concentration can be stated in a variety of ways-

(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
$

(iii) Volume by volume percentage of a solution

$
=\frac{\text { Volume of solute }}{\text { Volume of solution }} \times 100
$

2.2.2 WHAT IS A SUSPENSION?

A suspension is a heterogeneous combination in which the solute particles do not dissolve but stay suspended in the medium's bulk.

Properties of a Suspension

• Suspension is a heterogeneous mixture. A suspension's particles can be seen with the naked eye.

• The particles in a suspension scatter a light beam travelling across it, revealing its path.

• When a suspension is left undisturbed, the solute particles settle down, making the suspension unstable. Filtration can be used to separate them from the rest of the mixture. The suspension breaks as the particles settle down, and light is no longer scattered.

2.2.3 WHAT IS A COLLOIDAL SOLUTION?

A colloid is a type of solution in which the size of solute particles lies between that of genuine solutions and that of suspensions. A colloid is a heterogeneous combination of particles that are too tiny to be seen individually with the naked eye. Colloids are large enough to scatter a light beam passing through them, revealing their path.

The scattering of light by particles in a colloid or particles in a very fine suspension is known as the Tyndall effect.

Here, in the above figure, (a) Solution of copper sulphate does not show Tyndall effect and (b) mixture of water and milk shows Tyndall effect.

Dispersed phase- The dispersed phase of a colloid is the solute-like component of dispersed particles.

Dispersion medium- The dispersion medium is the component in which the dispersed phase is suspended.

Tyndall effect can be observed when sunlight passes through the canopy of a dense forest. In the forest, mist contains tiny droplets of water, which act as particles of colloid dispersed in air.

Aerosol- Aerosol is a colloidal solution containing a solid/liquid dispersed phase and a dispersing medium gas, such as clouds.

Foam- Foam, for example, is a colloidal solution with a dispersed phase gas and a dispersing medium solid/liquid.

Sol- Sol is a colloidal solution including a solid dispersed phase and a liquid dispersing medium. Magnesia milk and mud

2.3 Physical and Chemical Changes

There are some changes during which no new substances are formed. On the other hand, there are some other changes during which new substances are formed. So, on the basis of whether new substances are formed or not, we can classify all the changes into two groups- physical changes and chemical changes.

Physical Changes
Those changes in which no new substances are formed, are called physical changes. In a physical change, the substances involved do not change their identity. They can be easily returned to their original form by some physical process. This means that physical changes can be easily reversed. The changes in physical state, size and shape of a substance are physical changes.

Some common examples of physical changes are: Melting of ice (to form water); Freezing of water (to form ice); Boiling of water (to form steam); Condensation of steam (to form water).

Chemical Changes
Those changes in which new substances are formed, are called chemical changes. In a chemical change, the substances involved change their identity. They get converted into entirely new substances. The new substances usually cannot be returned to their original form. This means that chemical changes are usually irreversible.

Some common examples of chemical changes are: Burning of a magnesium wire, burning of paper, rusting of iron, ripening of fruits and cooking of food.

2.4 What are the Types of Pure Substances?

A pure substance is one that is made up of only one kind of particle. These particles may be atoms or molecules. So, we can also say that a pure substance is one which is made up of only one kind of atoms or molecules. For example, sulphur element is made up of only one kind of particles (called sulphur atoms), therefore, sulphur is a pure substance.

2.4.1 ELEMENTS

Robert Boyle was the first scientist to use the term element in 1661. Antoine Laurent Lavoisier (1743–94), a French chemist, was the first to establish an experimentally useful definition of an element. He defined an element as a basic form of matter that cannot be broken down into simpler substances by chemical reactions. Elements can be normally divided into metals, non-metals and metalloids.

Metals usually show some or all of the following properties:

• 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 are gold, silver, copper, iron, sodium, potassium etc. Mercury is the only metal that is liquid at room temperature.

Non-metals usually show some or all of the following properties:

• They display a variety of colours.
• They are poor conductors of heat and electricity.
• They are not lustrous, sonorous or malleable.
• Examples of non-metals are hydrogen, oxygen, iodine, carbon (coal, coke), bromine.

Metalloids are elements that have properties that are halfway between those of metals and non-metals; examples include boron, silicon, and germanium.

2.4.2 COMPOUNDS

A compound is a substance composed of two or more elements, chemically combined with one another in a fixed proportion.

Separating the Components of a Mixture

• To separate distinct components from a mixture, many separation procedures are used.
• Separation enables the study and use of distinct components of a mixture.
• Simple physical procedures like handpicking, sieving, and filtering that we employ in our daily lives can be used to separate heterogeneous mixtures into their constituents.

Previous Year Questions of Class 9 Science Chapter 2

Question: Which of the following mixtures shows the Tyndall effect?

(1) Salt solution

(2) Copper sulphate solution

(3) Milk

(4) Air

Answer: Tyndall effect is the scattering of light by particles in a colloid. Milk is a colloid (liquid in liquid), so it scatters light.

Hence, the correct answer is option (3).

Question: Which of the following is a homogeneous mixture?
A. Air
B. Oil and water
C. Sand and iron filings
D. Water and chalk powder

Answer:

Air is a homogeneous mixture of gases, where the components are uniformly mixed and not visibly separate.

Hence, the correct answer is option (1).

Question: A mixture of ammonium chloride and common salt is best separated by:

(a) Filtration

(b) Sublimation

(c) Distillation

(d) Crystallisation

Answer: Ammonium chloride sublimes, while common salt does not. Sublimation is the only technique that separates a sublimable component from a non-sublimable one.

Hence, the correct answer is option (b).

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