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COMPOUNDS

COMPOUNDS

  • A compound is a substance that can be broken down into two or more simpler substances by chemical means.
  • It is made by chemical combination of two or more elements in fixed proportions by mass.
  • Properties of a compound are different from those of its component elements. For example, water is a compound made up of two elements, hydrogen and oxygen.
  • Other examples of compounds: Common salt (sodium chloride), sand (silicon dioxide), marble (calcium carbonate), chalk (calcium carbonate), limestone (calcium carbonate), quick lime (calcium oxide), slaked lime (calcium hydroxide), baking soda (sodium bicarbonate), washing soda (sodium carbonate), methane, hydrochloric acid, sodium hydroxide, sugar, starch, etc.

Differences between Mixtures and Compounds: 

  Mixture Compound
  A mixture can be separated into its A compound can be separated into its
1. components by physical methods, like constituents only by chemical methods
  filtration, distillation, etc. and not by physical methods.
2. A mixture exhibits the properties of its A compound shows properties which are
  components. entirely    different   from   those    of    its
    components.
3. No energy changes are involved in the Energy is evolved or absorbed during
  formation of a mixture. formation of a compound.
4. Composition of a mixture is variable. Composition of a compound is fixed
5. A    mixture   does    not   have   a   fixed A compound has a fixed melting/ boiling
  melting/boiling point. point.

 

Solutions, Suspensions, and Colloids

  • The substance which dissolves in another substance to form a solution is called the solute, and the substance in which the solute dissolves is called the solvent.
  • Solute particles can also be referred to as the ‘dispersed particles’ and solvent as the ‘dispersion medium’.
  • The size of solute particles is minimum in solutions and maximum in suspensions.
  • Solutions in which the solvent is water are called aqueous solutions and those in which the solvent is an organic liquid are called non-aqueous solutions.

SOLUTIONS | COMPOUNDS

A solution is a homogeneous mixture. Examples: Salt solution, sugar solution, soft drinks, vinegar, sea water, air, and metal alloys like brass.

Important Characteristics of a Solution

  • A solution is a homogeneous mixture.
  • The solute particles are extremely small in size (less than 1 nm in diameter). They cannot be seen even with a microscope.
  • The solvate cannot be separated from the solvent by filtration.
  • The solute does not separate out on keeping.
  • A solution does not scatter light because the particles are extremely small.

Types of Solutions

  • Solid in solid : Metal alloys like brass (solution of zinc in copper), bronze (solution of tin in copper)
  • Solid in liquid : Solution of sugar in water, copper sulphate in water
  • Liquid in liquid: Vinegar (solution of acetic acid in water)
  • Gas in liquid: Carbonated drinks like Coca-Cola, soda-water
  • Gas in Gas: Air (solution of oxygen, carbon dioxide, argon, water vapour, etc., in nitrogen gas)

SUSPENSIONS | COMPOUNDS

A suspension is a heterogeneous mixture in which particles of a solid are dispersed in the liquid without dissolving in it.

Examples: Sand particles in water, mud in water, chalk particles in water, milk of magnesia (magnesium hydroxide in water).

Important Characteristics of a Suspension

  • A suspension is a heterogeneous mixture.
  • The solute particles in a suspension are quite large (more than 100 nm in diameter).
  • The particles in a suspension can be seen easily.
  • The particles can be separated from the dispersion medium by filtration.
  • The particles of a suspension settle down on keeping.
  • As the particles are large, a suspension scatters the beam of light passing through it.

COLLOIDS | COMPOUNDS

A colloid is a type of solution in which the particle size of the solute is bigger than that of a true solution but smaller than that of a suspension.

Colloidal solutions are heterogeneous.

Examples: Milk, blood, soap solution, starch solution, ink, jelly.

Important Characteristics of a Colloid

  • A colloid is heterogeneous even though it appears to be homogeneous.
  • The diameter of solute particles in a colloid is between 1 nm and 100 nm.
  • The particles of colloids cannot be seen even with a microscope.
  • A colloid can be separated by centrifugation but not by filtration.
  • The particles of a colloidal solution do not settle down on keeping.
  • A colloid scatters a beam of light passing through it. Scattering of light by colloidal particles is known as Tyndall Effect.

Types of Colloids | COMPOUNDS

Colloids can be classified into the following groups on the basis of the physical state of the dispersed phase and the dispersion medium —

  • Sol (tiny solid particles dispersed in a liquid). E.g. Soap solution, starch solution, ink, paint
  • Solid sol (solid particles dispersed in a solid). E.g. Coloured gemstones like ruby, sapphire, emerald
  • Aerosol (a solid or liquid dispersed in a gas) E.g. Smoke, automobile exhausts, fog, mist, clouds, hairspray
  • Emulsion (small drops of a liquid dispersed in another liquid). E.g. Body lotion, milk, butter
  • Foam (a gas dispersed in a liquid). E.g. Shaving cream soap bubbles, fire-extinguisher foam
  • Solid foam (a gas dispersed in a solid). E.g. Sponge, bread, foam rubber
  • Gel (a network of solid particles dispersed in a liquid). E.g. Gelatine, jelly, hair gel

Solubility

  • The concentration of a solution is defined as the amount of solute present in a given amount of the solution.
  • A solution in which some more solute can be dissolved without increasing its temperature is called an unsaturated solution, whereas a solution in which no more solute can be dissolved at that temperature is called a saturated solution.

The maximum quantity of a solute that can be dissolved in 100 grams of a solvent at a particular temperature is known as the solubility of the solute in that solvent at that temperature.

  • The solubility of a solid in a liquid generally increases on increasing the temperature, and decreases on decreasing the temperature.
  • It remains unaffected by changes ‘in pressure. The solubility of a gas in a liquid generally decreases on increasing the temperature, and increases on decreasing the temperature. In contrast, it increases on increasing the pressure, and decreases on decreasing the pressure.
  • For example, when water is heated, air dissolved in water comes out in the form of tiny bubbles. This shows that solubility of air (gas) in water (liquid) decreases with increase in temperature.
  • When a soda water bottle is opened, the pressure decreases and carbon dioxide gas dissolved in water escapes producing a fizz. This shows that solubility of a gas in a liquid decreases on decreasing the pressure.

SEPARATION OF MIXTURE | COMPOUNDS

Many procedures have been developed to separate mixtures into their components. The method which is used for this purpose depends upon the nature of the components present in the mixture.

  1. A mixture of two solids can be separated by one of the following methods:
  • Use of suitable solvent
  • A mixture of sugar and sand can be separated by adding water as the solvent which dissolves sugar but not sand.
  • Filtration of the solution leaves sand on the filter paper. Evaporation of water from the filtrate gives sugar.
  • Sublimation
  • The process of sublimation is used to separate the component which sublimes on heating from the one which does not.
  • Thus, naphthalene, which sublimes, can be easily separated from sodium chloride by this method.
  •   Use of a magnet
  • Iron is attracted by a magnet. Therefore, it can be separated from other components of a mixture with the help of a magnet.
  • In factories, scrap iron is separated from a heap of waste material with the help of electromagnets fitted to a crane.
  1. A mixture of a solid and a liquid can be separated by one of the following methods:
  • Filtration
  • Filtration is used to separate insoluble substances from a liquid, e.g., a mixture of sand and water can be separated by filtration.
  • Different kinds of filters can be used, e.g., filter paper, wire-mesh, cotton, muslin cloth or a layer of sand.
  • Used tea leaves are separated from prepared tea by filtration, using a tea strainer. Drinking water is filtered using water filters.
  • Centrifugation
  • The method of centrifugation is used to separate suspended particles from a liquid.
  • The mixture is separated by rotating it at high speed in a centrifuge. This process is used in dairies to separate cream from milk.
  • Evaporation
  • A solid substance dissolved in a solvent can be separated by the process of evaporation. The dissolved substance is left as a solid residue after the solvent has evaporated.
  • The solvent itself cannot be recovered by this method. Common salt is obtained from sea water by evaporation.
  • Sea water, trapped in shallow lakes called lagoons, is subjected to the heat of the sun.
  • Water evaporates leaving behind salt as a solid. If any impurities are present in the dissolved solid, they would still be present after its recovery by evaporation.
  • Crystallisation
  • When a hot, concentrated solution of a substance is allowed to cool slowly, crystals of pure solid are formed, while impurities remain dissolved in the solvent.
  • This process is called crystallisation. The crystals can be separated by filtration.
  • An impure sample of a compound, like copper sulphate or alum, can be purified by crystallisation.
  • Chromatography
  • Two or more dissolved solids present in a solution in very small amounts can be separated and identified by chromatography.
  • Though these substances are soluble in the same solvent, yet their solubilities may be different.
  • The components of the mixture dissolved in a solvent, move to different extents on an adsorbent material (filter paper, silica gel, etc.) and thus get separated.
  • There are many types of chromatography. In Paper Chromatography, a special kind of filter paper is used, whereas Thin Layer Chromatography (TLC) involves the use of a glass plate coated with silica gel.
  • In Column Chromatography a glass column packed with adsorbent material is used. Gas-Liquid Chromatography (GLC) is a very powerful technique for the rapid analysis of mixtures containing volatile components.
  • High Performance Liquid Chromatography (HPLC) is an important method for compounds which are non-volatile or thermally unstable. It is used to separate isomers.
  • Chromatography is used in forensic science for the detection and identification of minute amounts of substances, like poisons, in the contents of stomach or bladder.
  • It can be used to separate small amounts as well as large amounts of products of chemical reactions.

   

  • Distillation
  • In order to recover both the solute and the solvent from a solution, the process of distillation has to be used.
  • It is a process in which evaporation and condensation go on side by side.
  • When the solution is heated, the solvent evaporates, leaving behind the solid.
  • Vapours of the solvent are condensed to give the liquid, which is called the ‘distillate’.
  • Pure water is obtained from tap water by distillation. Distillation is used to obtain drinking water from sea water in hot and arid countries, which get little or no rain.
  1. A mixture of two or more liquids can be separated by one of the following methods:
  • Fractional Distillation
  • Liquids which mix together in all proportions to produce a single layer are called miscible liquids.
  • Two or more miscible liquids can be separated by fractional distillation using a fractionating column.
  • The distillate is collected in fractions, boiling at different temperatures. Mixtures of miscible liquids, like alcohol-water or acetone-water mixture, can be separated by this method.
  • Fractional distillation is used to separate crude petroleum into useful fractions, like kerosene, petrol, etc.
  • Use of Separating Funnel
  • Liquids which do not mix with each other and form separate layers are called immiscible liquids.
  • A separating funnel is used to separate two immiscible liquids.
  • The heavier liquid forms the lower layer. Mixture of oil and water, petrol and water, water and chloroform, ether and water can be separated by this method. COMPOUNDS

 

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