Class 9 Science Chapter 2 Is Matter around us Pure? NCERT Notes

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Chapter 2 Is Matter around us Pure? Class 9 Science CBSE NCERT Notes

What is a pure substance?

When a scientist says that something is pure, it means that all the constituent particles of that substance are the same in their chemical nature. For example, a pure substance consists of a single type of particles.

What is mixture?

As we look around, we can see that most of the matter around us exist as mixtures of two or more pure components, for example, sea water, minerals, soil etc. are all mixtures. Mixtures are constituted by more than one kind of pure form of matter.

Classification of matter

On the basis of the physical state, matter is classified into three main types. They are solids, liquids and gases.

On the basis of chemical composition matter is classified into two main types. They are pure substances and mixtures.

Pure substances are of two types. They are elements and compounds. Mixtures are of two types. They are homogeneous mixtures and heterogeneous mixtures.

Pure substance	Mixture
Pure substance consists of a one type of substance.	Mixture consists of two or more pure substances.
Pure substance cannot be separated into other substances by physical methods.	Mixture can be separated into its components by physical methods.
Pure substance has its own definite properties.	Mixture shows the properties of all its components.

Homogeneous mixture

• It is a mixture which has a uniform composition.
• The particles of the mixture are not visible by the naked eye.
• The particles cannot be separated by filtration.
• The mixture is stable (the particles do not settle down). The path of a beam of light is not visible in the mixture.
• Example: Sugar in water. It has a uniform composition throughout its mass.

Heterogeneous mixture

• It is a mixture which has a non-uniform composition.
• The particles may or may not be visible by the naked eye.
• The particles can be separated by ordinary filtration/fine filtration.
• The mixture is unstable (the particles settle down).
• The path of a beam of light is visible in the mixture.
• Example: Mixture of sugar and sand. It does not have a uniform composition throughout its mass.

True solution

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

A solution has a solvent and solute as its components. The component in the larger amount is the solvent and the component in the lesser amount is the solute.

Example: Solution of salt in water, solution of sugar in water, iodine in water (tincture iodine), soda water etc.

Properties of true solutions

• True solution is a homogeneous mixture.
• The particles are cannot be seen by the naked eye.
• The solute particles cannot be separated by filtration.
• The solute particles do not settle down and the solution is stable.
• The particles do not scatter a beam of light passing through it and the path of light is not visible in the solution.

Colloidal solution

A colloidal solution is a heterogeneous mixture of two or more substances. For example, mixture of starch in water, mixture of egg albumin in water, milk, air containing dust and smoke etc.

Properties of colloidal solution

• Colloidal solution is a heterogeneous mixture.
• The particles cannot be seen by the naked eye.
• The solute particles cannot be separated by filtration.
• The solute particles do not settle down and the solution is stable.
• The particles scatter a beam of light passing through it and the path of light is visible in the solution.

Suspension

A suspension is a heterogeneous mixture of two or more substances. For example, solution of sand in water, solution of chalk powder in water etc.

Properties of suspension

• Suspension is a heterogeneous mixture.
• The particles can be seen by the naked eye.
• The solute particles can be separated by filtration.
• The solute particles settle down and the solution is unstable.
• The particles scatter a beam of light passing through it and the path of light is visible in the solution.

Tyndall effect

When a beam of light is passed through a colloidal solution, the colloid particles scatter the beam of light and the path of light becomes visible in the solution. This effect is called Tyndall effect.

Tyndall effect can be seen when light enters a room through a small hole due to scattering of light by the dust and smoke particles.

Tyndall effect can be seen in a dense forest due to scattering of light by water droplets in the mist.

Saturated solution

Saturated solution is a solution which cannot dissolve any more of a solute at a given temperature.

Solubility of a substance is the amount of solute present in a saturated solution of the substance.

Unsaturated solution is a solution which can dissolve some more of the solute at a given temperature.

Preparation of a saturated solution

• Take 50ml of water in two beakers. Add salt in one beaker with continuous stirring till no more salt dissolves in it. Similarly add sugar in the other beaker with continuous stirring till no more sugar dissolves in it. We get saturated solutions of salt and sugar.
• If the mixtures are heated it dissolves some more of the solute.
• The solubility of different substances are different.
• The solubility of substances varies with temperature.

Concentration of a solution

The concentration of a solution is the amount of solute present in a given amount of the solvent or solution.

Concentration of a solution = Amount of solute/Amount of solvent ✕ 100
Or, = Amount of solute/Amount of solution

The concentration of a solution can be expressed as mass by mass percentage or as mass by volume percentage.

Mass by mass percentage of a solution = Mass of solute/Mass of solution ✕ 100
Mass by volume percentage of a solution = Mass of solute/Volume of solution ✕ 100

Separating the components of a mixture

The components of a heterogeneous mixture can be separated by simple methods like hand picking, sieving, filtration etc.

Sometimes special techniques are used to separate the components of mixtures like

• Evaporation
• Centrifugation
• Decantation (Using separating funnel)
• Sublimation
• Centrifugation
• Chromatography
• Distillation and fractional distillation

Evaporation

Basic principal: Out of the two components of a mixture one can evaporate (i.e., has less boiling point) and other has higher boiling point. Example: Mixture of dye (higher boiling point) and water. Out of water and dye, water evaporates but dye is left behind in petri dish.

Centrifugation

Basic principle: Separation of Substances or particles on the basis of their density, when mixture is rotated very fast, then denser particles are forced at the bottom and lighter particles stay above.

By Separating Funnel

Basic principle: Two immiscible liquids (which do not dissolve in each other) can be easily separated by putting in a separating funnel.

Example: Water from oil can be separated by first opening the stop cock till water is removed in one beaker, then afterwards oil can be collected in a separate beaker.

Applications:

• Separation of oil from water.
• Extraction of iron from its ore. Lighter slag is removed from above the molten iron.

Sublimation

Basic principle: Out of the two components, one will sublime (directly converts to gas from solid) and other will not. Example: NH₄CI (ammonium chloride) and NaCl common salt mixture can be easily separated by heating so the NH₄Cl sublimes but common salt remains behind.

Applications:

Camphor, naphthalene, anthracene, NH₄Cl can sublime.

Chromatography

Basic principle: Coloured components of a mixture can be separated by using an Adsorbent on which they are adsorbed at different rates. Adsorption is the process of surface absorption.

Applications:

• To separate colours of a dye.
• To separate pigments from natural colours like chlorophyll.
• To separate drugs from blood.

Distillation

Basic principle: Based on Separating mixture of miscible liquids having different boiling points, followed by condensation. Out of the two components one has a lower boiling point and other has higher boiling point. This is used to separate two or more miscible liquids.

When water/any suitable solvent moves up the chromatography paper ink with two different colours separates because both colours are absorbed at different speeds.

Example: When mixture of acetone and water is heated, acetone having lesser boiling point, boils and moves to delivery tube, within which it condenses back to liquid with the help of a condenser clamped to it. Thus, acetone is separated out in a beaker and water is left in the distillation flask.

Note: If there are more than two components (liquids) mixed (with different boiling points) then we use a fractionating column to separate all the components from each other. This process is done for air, petroleum etc.

Petroleum is separated into paraffin wax, lubricating oil, diesel, kerosene, petrol and petrol gas by this method.

Fractional distillation

It is used for separating a mixture of two or more miscible liquids whose difference in boiling points is less than 25K.

The apparatus used for fractional distillation is similar to that used for simple distillation except that a fractionating column is fitted between the distillation flask and condenser. The fractionating column has glass beads which increases the surface for the vapours to cool and condense.

Fractional distillation is used for separating the components of petroleum, separating the different gases from air etc.

Separation of components of air

Air is a mixture of gases. The components of air can be separated by fractional distillation.

Air is compressed by increasing the pressure and cooled by decreasing the temperature to get liquid air. The liquid air is then allowed to warm up slowly in a fractional distillation column. Then the different components separate at different heights depending on their different boiling points.

Crystallisation

Basic principle: To remove impurities from a mixture by first dissolving in a suitable solvent and then crystallising out one component.

For example: Copper sulphate crystals (impure) are first dissolved in sulphuric acid and then heated to saturated solution. Now, this solution is left overnight. So, only pure copper sulphate crystals on filter paper.

Why is crystallisation better than evaporation?

• Some solids decompose or get charred upon heating to dryness during evaporation. example, sugar.
• Some impurities remain dissolved in solution after filtration. On evaporation, these impurities do not evaporate and remain with the mixture.

Applications:

• Purification of salt from sea water.
• Separation of crystals [example, alum (phitkari), copper sulphate] from their impure crystals.

Types of pure substances

Pure substances are of two types. They are elements and compounds.

Element is a basic form of matter which cannot be broken down into simpler substances by chemical reactions. Elements are of three types. They are metals, non metals and metalloids.

Properties of metals

They have lustre. They are malleable and ductile. They are good conductors of heat and electricity. They are sonorous.

Example, iron, aluminium, zinc, mercury, copper, silver, gold etc.

Properties of non metals

They do not have lustre. They are not malleable or ductile. They are poor conductors of heat and electricity. They are not sonorous. Example, hydrogen, oxygen, nitrogen, iodine, carbon, sulphur, phosphorus etc.

Properties of metalloids

Metalloids are elements which show some properties of metals and some properties of non metals.
Example, boron, silicon, germanium etc.

Compound

A compound is a substance composed of two or more elements chemically combined together in a fixed ratio.
Example, water, carbon dioxide, sugar, salt, iron sulphide etc.

Differences between mixtures and compounds

Mixture	Compound
Elements or compounds are simply mixed so no new substance is formed.	Substances are reacted together with each other to make a new substance.
Elements do not combine in a fixed ratio.	Composition of the components is fixed i.e., they combine together in a fixed radio according to their masses.
A mixture shows the properties of its components.	Compound doesn’t show the properties of component elements.
Components can be easily separated by any mechanical method which is suitable.	Components can’t be separated from each other by simple mechanical methods.
Example, Mixture of iron and sulphur.	Example, Iron and sulphur react to from iron sulphide.