CBSE 9th Science | Matter around us: Concept review
QUICK CONCEPT REVIEW
SUBSTANCE (OR CHEMICAL SUBSTANCE):
A “substance” is a kind of matter that can not be separated into other kinds of matter by any physical process, e.g. gold, silver, iron, sodium chloride, calcium carbonate etc.
[NOTE: Most of the naturally occurring materials on the earth are mixtures, substances (i.e. elements and compounds in pure state) are rare.]
is one that is a single substance and has a uniform composition. Such a substance always have the same texture and taste, e.g. water, salt, sugar etc.
Thus, those substances, which always have the same colour, taste or texture at a given temperature and pressure are pure substances.
TESTING THE PURITY OF A SUBSTANCE:
The purity of substance can easily be checked by checking its melting points in case of a solid substance or by checking its boiling points is case of a liquid substance.
A pure substance has a fixed melting point or boiling point at constant pressure, e.g. the boiling point of pure water is 373K at atmospheric pressure.
[NOTE: The boiling point increases if impurities are present.]
TYPES OF PURE SUBSTANCES:
Two different types of pure substances are
An element is a substance which can not be split up into two or more simpler substances by usual chemical methods of applying heat, light or electric energy, e. g. hydrogen, oxygen, sodium, chlorine etc.
A compound is a substance made up of two or more elements chemically combined in a fixed ratio by weight e.g. H20 (water), NaCl (sodium chloride) etc.
A mixture is a substance which consists of two or more elements or compounds not chemically combined together, e.g. Air is a mixture of nitrogen, oxygen, inert gases, water vapour, carbon dioxide etc.
A mixture possesses a variable composition and the components of a mixture can be separated by suitable physical method, e.g. mixture of sand and common salt, crude oil, sea water, brass, coloured glass etc.
TYPES OF MIXTURES:
Mixtures are impure substances. They are of two types :
(i) Homogeneous mixture:
It has a uniform composition throughout and its components can not be distinguished visually.
Further more, any microscopically small portion of the sample has the same composition as any^ other portion, e.g. a well mixed sample of vinegar.
(ii) Heterogeneous mixture:
It is one mat is not uniform throughout. Different samples of a heterogeneous mixture may have different composition, e.g. a mixture of salt and pepper.
It is a homogeneous mixture of two or more substances whose composition can be varied, e. g. solution of common salt in water, solution of ammonia in water. Some other examples are lemonade, coke, pepsi etc.
SEPARATING THE COMPONENTS OF A MIXTURE:
Various methods are used for separating the constituents of a mixture. Depending upon the type of scare (i.e. whether it is a homogeneous mixture or heterogeneous mixture) different methods used are given
|1. Insoluble solid in solvent||Sedimentation followed by filtration. In case of afine solid centrifugation is used instead of filtration|
|2. Solution of solid in liquid||Evaporation, crystallization, distillation|
|3. Miscible mixture of liquids.||Fractional distillation|
|4. Immiscible mixture of liquids.||Separating funnel|
|5. Mixture of two solids one of which is sublime.||Sublimation|
|6. Mixture of substances in solution.||Chromatography|
The component of solution that is dissolved and present in smaller quantities in a solution is known as solute e.g. common salt in case of solution of common salt in water and ammonia in case of solution of ammonia in water.
The component of solution in which solute is dissolved is known as solvent. It is always present in larger amount in a solution, e.g. water in case of the solution of common salt or ammonia in water.
A solution in which no more solute can be dissolved at the same temperature is called Saturated solution.
It is a solution in which more solute can be dissolved at the same temperature.
It is a solution which contains more mass of the dissolved solute than the saturated solution at the same temperature and pressure.
TYES OF SOLUTION:
Solution are of the following types depending upon the state of solute and solvent.
|(i) gas in gas
(i) Liquid in gas
(iii) solid in gas
(iv) gas in liquid
(v) liquid in liquid
(vi) solid in liquid
(vii) gas in solid
(viii) solid in solid
(ix) liquid in solid
Chloroform vapours mixed with nitrogen
camphor vapours in nitrogen
aerated drinks, oxygen dissolved in water
vinegar ethanol dissolved in water
solution of sugar in water
solution of common salt in water
solution of hydrogen in palladium
Brass (70% Cu, 30% Zn) In it Zn is solute.
Amalgam of mercury with sodium
NOT: Air is a gaseous solution.
Alloys are solid solution.
Alloys are homogeneous mixtures of metal and can not separated into their components by physical methods.
However, Alloy is considered as a mixture because it shows the properties of its constituents and have a variable composition, e.g. Brass is a mixture of copper (Cu) and zinc (Zn).
PROPERTIES OF SOLUTIONS:
(i) It is a homogeneous mixture.
(ii) The particles of solution are smaller than 1 nm (10 _9m) in diameter and can not be seen by naked eye.
(iii) Particles of solution do not scatter a beam of light passing through solution i.e. they do not show Tyndall effect,
(iv) The components of a solution can not be separated by filtration,
(v) The particles of a solution do not settle when left undisturbed.
CONCENTRATION OF A SOLUTION:
There are various ways to express the concentration of a solution.
Concentration of a solution is the amount of solute presents in a given amount (mass or volume) of a solution or the amount of solute dissolved in a given mass or volume of a solvent.
The various ways of expressing the concentration of a solution, are :
(i) Mass percentage and
(ii) Volume percentage
(i) Mass percentage:
It is the amount of solute in grams dissolved per l00g of solution, e.g. 20% solution of sodium, chloride means 20g of solid sodium chloride dissolved per l00g of solution. It is the number of parts by mass of solute per 100 parts by mass of solution, e. g. 10% sugar solution in water.
We can find mass % by using the relation
(ii) Volume percentage: It is the mass of solute in grams present per 100 ml of solution.
It is defined as the amount of solute dissolved in l00g of solvent to form a saturated solution.
(i) solubility of a substance may be different in different solvents.
(ii) solubility of a substance may be different at different temperature for the same solvent].
It is a non-homogeneous mix true in which solids are dispersed in liquids. In it the solute particles do not dissolve but remains suspended through out the bulk of the medium. The particles in a suspension are of a size larger than 10-5cm (or 10-7 m) in diameter and can be seen with naked eye, e.g. muddy water.
PROPERTIES OF A SUSPENSION:
(i) It is a heterogeneous mixture.
(ii) The particle size is larger than 10_5cm (100 nm) in diameter.
(iii) Particles in a suspension can be seen by naked eye.
(iv) Particles of a suspension scatter a beam of light passing through it and it makes the path visible, i.e. they show Tyndall effect.
(v) Particles of a suspension settle down when suspension is left undisturbed i.e. suspension is unstable.
(vi) Components of a suspension can be separated by process of filtration.
COLLOIDS OR COLLOIDAL SOLUTION :
Colloid is a heterogeneous mixture. The size of particles of a colloid is intermediate between true solutions and suspensions (i. e between l nm and 100 nm). The particles of a colloid can not be seen with naked eye.
PROPERTIES OF COLLOIDS:
(i) Colloid is a heterogeneous mixture.
(ii) The size of colloidal particles lies between 1 nm and 100 nm.
(iii) Colloidal particles can not be seen with naked eye.
(iv) Colloidal particles scatter a beam of light passing through a colloidal solution and make its path visible i.e. they show Tyndall effect.
(iii) Colloidal particles can not be separated from the mixture by the process of filtration. However they can be separated by centrifugation process,
(vi) Colloidal particles carry charge and move in zigzag motion. (Brownian movement)
TYPES OF COLLOIDAL SOLUTION:
Since colloidal solution is heterogeneous mixture it consists of two phases. These are
(i) dispersed phase (colloidal particles)
(ii) dispersion medium (The medium in which colloidal particles are dispersed.)
DIFFERENT TYPES OF COLLOIDAL SOLUTION:
Eight different types of colloidal solution on the basis of state of dispersed phase and dispersion medium are;
|S.No.||Dispersed||Dispersant||Name of Colloidal solution||Example|
|1.||Gas||Liquid||Foam||Soap, lather, whipped cream, soda water|
|2.||Gas||Solid||Solid foam||Pumice stone, foam, rubber, bread.|
|3.||Liquid||Gas||Aerosol||Mist, fog, cloud, insecticide spray|
|4.||Liquid||Liquid||Emulsion||Milk, emulsified oil, medicines, rubber|
|5.||Liquid||Solid||gels||Latex, jelly, butter, cheese|
|Boot polish, aerosol smoke, bust,
Storm, volcanic dust, haze.
|7.||Solid||Liquid||Sols||Paints, starch dispersed in water, gold
sol, muddy water.
|8.||Solid||Solid||Solid||Alloy, coloured glass, gem, stone, ruby, glass, minerals.|
Emulsions are liquid-liquid colloids.
TYPES OF EMULSION:
Emulsions are of two types-
(i) water in oil
(ii) oil in water
Emulsifiers are those substances that help in forming stable emulsions of oil and water, e.g. milk, cod-liver oil, cold creams, vanishing creams, moisturizing cream, paints, etc.
Application of colloidal solution:
Some important applications are:
(i) To stop bleeding from a cut: For this purpose we apply alum or ferric chloride solutions on the cuts.
(ii) Medicines in colloidal form can be easily absorbed by body.
(iii) Cleansing action of soap.
(iv) Formations of delta when river comes in contact with sea water.
(v) Blue colour of sky: It is due to the scattering of sunlight by fine dust particles present in atmosphere.
During such a change no new substances is formed and there is no change in the chemical properties of the substances. There is only a change in physical state of the substance and such a change can be easily reversed by a slight change of conditions. Thus it is a temporary change, e.g. change of ice in water, melting of wax etc.
Such a change is accompanied by change in chemical properties and formation of new substances. Such a change can not be easily reversed and so it is a permanent change, e.g. burning of paper, burning of wood, burning of candle etc.
Elements are a type of pure substances. An element is a substance that can not be split into two or more simpler substances by usual chemical methods of applying heat, light or electric energy. An elements is made up of atoms, e.g. hydrogen, oxygen, nickel, gold etc. At present 115 elements are known. 92 elements are natural others are man-made.
TYPES OF ELEMENTS:
Elements have been divided into metals and non-metals. All metals (except mercury) are solids, mercury is a liquid, e.g. sodium, potassuim, gold, silver etc.
All non-metals are solids or gases (Bromine is an exception as it is a liquid non-metal) e.g. hydrogen, oxygen, carbon, bromine, chlorine, iodine etc.
A compound is a substance made up of two or more elements chemically combined in a fixed ratio by weight, e.g. water (H20) is a compound made up of two elements Hydrogen and Oxygen chemically combined in a fixed proportion of 1: 8 by weight.
PROPERTIES OF METALS:
(i) They have a luster (shine).
(ii) They conduct heat and electricity.
(iii) They are ductile (e.g. they can be drawn, into wire).
(iv) They are malleable (i.e. they can be hammered into thin sheets).
(v) They are sonorous (i.e. they produce a tinkling sound when hit).
PROPERTIES OF NON-METALS:
(i) They have different colours.
(ii) They are poor conductors of heat and electricity.
(iii) They are non-lustrous.
(iv) They are not sonorous.
(v) They are not malleable.