CLASS 9 Science
CHAPTER 2 Is Matte Around Us Pure? (NCERT Notes)
All the constituent particles of that substance are the same in their chemical nature. A pure substance consists of a single type of particles. In other words, a substance is a pure single form of matter. A pure substance always have the same characteristic properties. It can be seperated by physical process. For example sugar .
What is a Mixture?
A mixture contains more than one substance (element and/or compound) mixed in any proportion. Or mixtures are constituted by more than one kind of pure form of matter. A mixture has a variable composition.
TYPES OF MIXTURES
Depending upon the nature of the components that form a mixture .
Mixture which has a uniform composition throughout the solution or mixture . Such mixtures are called homogeneous mixtures or solutions. Sedimentation does not takes place. Examples of such mixtures are: salt dissolved in water , sugar dissolved in water.
Heterogeneous Mixture
Mixture which has does not have a uniform composition throughout the solution or mixture . Such mixtures are called heterogeneous mixtures or solutions. It gets sedimented (sedimentation ) when it is undisturbed. Examples of such mixtures are: sand and salt , sugar and salt , oil and water .
What is a Solution?
A solution has a solvent and a solute as its components. The component of the solution that dissolves the other component in it (usually the component present in larger amount) is called the solvent. The component of the solution that is dissolved in the solvent (usually present in lesser quantity) is called
the solute. Examples:
(i) A solution of sugar in water is a solid in liquid solution. In this solution, sugar is the solute and water is the solvent.
(ii) A solution of iodine in alcohol known as ‘tincture of iodine’, has iodine (solid) as the solute and alcohol (liquid) as the solvent.
(iii) Aerated drinks like soda water etc., are gas in liquid solutions. These contain carbon dioxide (gas) as solute and water (liquid) as solvent.
(iv) Air is a mixture of gas in gas. Air is a homogeneous mixture of a number of gases. Its two main constituents are: oxygen (21%) and nitrogen (78%). The other gases are present in very small
quantities.
Properties of a solution
• A solution is a homogeneous mixture.
• The particles of a solution are smaller than 1 nm (10-9 metre) in diameter. So, they cannot be seen by naked eyes.
• 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.
CONCENTRATION OF A SOLUTION
Depending upon the amount of solute present in a solution, it can be called a dilute, concentrated or a saturated solution.
At any particular temperature, a solution that has dissolved as much solute as it is capable of dissolving, is said to be a saturated solution. In other words, when no more solute can be dissolved in a solution at a given temperature, it is called a saturated solution.
The amount of the solute present in the saturated solution at this temperature is called its solubility.
If the amount of solute contained in a solution is less than the saturation level, it is called an unsaturated solution.
The concentration of a solution is the amount (mass or volume) of solute present in a given amount (mass or volume) of solution.
There are various ways of expressing the concentration of a solution, but here we will
learn only three methods.
(i) Mass by mass percentage of a solution = Mass of solute × 100 / Mass of solution
(ii) Mass by volume percentage of a solution = Mass of solute × 100 / Volume of solution
(iii) Volume by volume percentage of a solution = Volume of solute × 100 / Volume of solution
What is a suspension?
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 a suspension are visible to the naked eye.
Properties of a Suspension
• Suspension is a heterogeneous mixture.
• The particles of a suspension can be seen by the naked eye.
• The particles of a suspension scatter a beam of light passing through it and make its path visible.
• The solute particles settle down when a suspension is left undisturbed, that is, a suspension is unstable. They can be separated from the mixture by the process of filtration. When the particles
settle down, the suspension breaks and it does not scatter light any more.
WHAT IS A COLLOIDAL SOLUTION?
The particles of a colloid are uniformly spread throughout the solution. Due to the relatively smaller size of particles, as compared to that of a suspension, the mixture appears to be homogeneous. But actually, a colloidal solution is a heterogeneous mixture, for example, milk.
Because of the small size of colloidal particles, cannot be seen with naked eyes. But, these particles can easily scatter a beam of visible light . This scattering of a beam of light is called the Tyndall effect after the name of the scientist who discovered this effect.
Tyndall effect can also be observed when a fine beam of light enters a room through a small hole. This happens due to the scattering of light by the particles of dust and smoke in the air.
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.
Properties of a colloid
• A colloid is a heterogeneous mixture.
• The size of particles of a colloid is too small to be individually seen by naked eyes.
• Colloids are big enough to scatter a beam of light passing through it and make its path visible.
• They do not settle down when left undisturbed, that is, a colloid is quite stable.
• They cannot be separated from the mixture by the process of filtration. But, a special technique of separation known as centrifugation , can be used to separate the colloidal particles.
The components of a colloidal solution are the dispersed phase and the dispersion medium.
The solute-like component or the dispersed particles in a colloid form the dispersed phase, and the component in which the dispersed phase is suspended is known as the dispersing medium.
Colloids are classified according to the state (solid, liquid or gas) of the dispersing medium and the dispersed phase.
Alloys:
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. But still, an alloy is considered as a mixture because it shows the properties of its constituents and can have variable composition. For example, brass is a mixture of approximately 30% zinc and 70% copper.
Separating the Components of a Mixture
How we obtain coloured component (DYE) from blue /black ink ?
Ink is a mixture of a dye in water. Thus, we can separate the volatile component (solvent) from its non-volatile solute by the method of evaporation.
How can we separate cream from milk ?
We separate cream from milk by centrifugation. Centrifugation is process in which the denser particles are forced to the bottom and the lighter particles stay at the top when spun rapidly.
Applications
• Used in diagnostic laboratories for blood and urine tests.
• Used in dairies and home to separate butter from cream.
• Used in washing machines to squeeze out water from wet clothes.
How can we separate a mixture of two immiscible liquids ?
The principle is that immiscible liquids separate out in layers depending on their densities.
Applications
• To separate mixture of oil and water.
• In the extraction of iron from its ore, the lighter slag is removed from the top by this method to leave the molten iron at the bottom in the furnace.
How can we separate a mixture of salt and camphor ?
Camphor changes directly from solid to gaseous state on heating. So, to separate such mixtures that
contain a sublimable volatile component from a non-sublimable impurity, the sublimation process is used . Some examples of solids which sublime are ammonium chloride, naphthalene and anthracene.
Is the DYE in black ink a single colour ?
Separation of dyes in black ink using chromatography
Chromatography is a technique was first used for separation of colours. Chromatography is the
technique used for separation of those solutes that dissolve in the same solvent.
Applications
To separate
• colours in a dye
• pigments from natural colours
• drugs from blood.
How can we separate a mixture of two miscible liquids ?
Distillation 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.
To separate a mixture of two or more miscible liquids for which the difference in boiling points is less than 25 K, fractional distillation process is used, for example, for the separation of different gases from air, different factions from petroleum products etc.
How can we obtained different gases from air ?
Air is a homogeneous mixture and can be separated into its components by fractional distillation.
The air is compressed by increasing the pressure and is then cooled by decreasing the temperature to get liquid air. This liquid air is allowed to warm-up slowly in a fractional distillation column, where gases get separated at different heights depending upon their boiling points.
How can we obtained pure cupper sulphate from an impure sample?
The crystallisation method is used to purify solids. For example, the salt we get from sea water can have many impurities in it. To remove these impurities, the process of crystallisation is used. Crystallisation is a process that separates a pure solid in the form of its crystals from a solution. Crystallisation technique is better than simple evaporation technique as –
• some solids decompose or some, like sugar, may get charred on heating to dryness.
• some impurities may remain dissolved in the solution even after filtration. On evaporation these contaminate the solid.
Applications
• Purification of salt that we get from sea water.
• Separation of crystals of alum (phitkari) from impure samples.
Physical and Chemical Changes
The properties that can be observed and specified like colour, hardness, rigidity, fluidity, density, melting point, boiling point etc. are the physical properites.
The interconversion of states is a physical change because these changes occur without a change in composition and no change in the chemical nature of the substance.
Although ice, water and water vapour all look different and display different physical properties, they are chemically the same.
Both water and cooking oil are liquid but their chemical characteristics are different. They differ in odour and inflammability. We know that oil burns in air whereas water extinguishes fire. It is this chemical property of oil that makes it different from water .Burning is a chemical change. During this process one substance reacts with another to undergo a change in chemical composition.
Chemical change brings change in the chemical properties of matter and we get new substances.
A chemical change is also called a chemical reaction. During burning of a candle, both physical
and chemical changes take place.
What are the Types of Pure Substances?
On the basis of their chemical composition, substances can be classified either as elements or compounds.
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, chlorine etc.
Some elements have intermediate properties between those of metals and non-metals, they are called metalloids; examples are boron, silicon, germanium etc.
COMPOUND
A compound is a substance composed of two or more elements, chemically combined with one another in a fixed proportion.
( From NCERT Book )