Elements, Compounds and Mixtures

Size: px

Start display at page:

Download "Elements, Compounds and Mixtures"

Thomas Phelps
6 years ago
Views:

1 L. R. & S. M. VISSANJI ACADEMY SECONDARY SECTION CHEMISTRY - GRADE: VIII Elements, Compounds and Mixtures ELEMENTS An element is a pure substance which cannot be converted into anything simpler than itself by any physical or chemical process. Thus, elements are the basic substance from which all other substances are made. An element is a pure substance composed of only one kind of atom. CLASSIFICATION OF ELEMENTS Elements are classified on the basis of their properties. METALS 1. Metals are monoatomic elements. 2. They are hard solids. 3. They have lustre. 4. They are good conductors of heat and electricity. 5. They are ductile i.e. they can be drawn into wires. 6. They are malleable i.e. they can be beaten into sheets. 7. Most of them have high melting point and high boiling point. 8. They produce a sound when they are struck i.e. they are sonorous substances. Examples: Magnesium, Mercury, Copper, Silver, Gold, Aluminium, Lead, Tin, Iron etc. Exceptions: i. Mercury, Gallium and Caesium are liquids ( at 30 o C ) ii. Zinc is non-ductile and non-malleable. It is brittle in nature. iii. Sodium and Potassium are soft solids. iv. Tungsten is a poor conductor of electricity. v. Lead has low melting point.

2 NON-METALS 1. Non-metals are either monoatomic elements or polyatomic elements. 2. They exist in all the three physical states i.e. solid, liquid and gas. Example Solid Carbon, Sulphur, Phosphorus and Iodine are solids. Liquid Bromine is a liquid. Gases hydrogen, oxygen, Nitrogen, Chlorine etc are gases. 3. They do not have lustre. 4. They are bad conductors of heat and electricity. 5. They are neither malleable nor ductile, rather they are brittle. 6. They have low melting point and low boiling point. 7. They do not produce a sound when they are struck i.e. they are not sonorous substances. Exceptions i. Graphite and Iodine have lustre. ii. Graphite is a good conductor of electricity. It also had high melting point. iii. Diamond has high melting point and high boiling point. METALLOIDS Elements which show the properties of metals as well as non-metals are called as Metalloids. Examples: Boron, Silicon, Germanium, Arsenic, Antimony and Tellurium. INERT GASES OR NOBLE GASES Elements that are chemically inactive or inert are known as Noble Gases or Inert Gases. They occur in traces in the atmosphere. Examples: Helium, Neon, Argon, Krypton, Xenon and Radon. COMPOUNDS A compound is a pure substance made up to two or more elements combined chemically in a fixed proportion. CHARACTERISTICS OF A COMPOUND 1. A compound contains atoms of two or more elements combined by chemical forces. 2. It has a homogenous composition. 3. The elements in a compound are present in a definite proportion. 4. The properties of compounds are different from those of the elements of which they are made. 5. Compound can be broken down into their constituent elements only by chemical means, not by physical means. 6. During the formation of a compound by a combination of elements, energy is either liberated or absorbed. MIXTURES A mixture is made up of two or more elements or compounds or both mechanically mixed together in any proportion.

TYPES OF MIXTURES 1. HOMOGENOUS MIXTURE They have same composition and properties throughout their mass. Example: sugar solution. 2.

METHODS OF SEPARATION I. SOLID SOLID MIXTURES 1.

For this method, one of the components must be capable of subliming on heating.

3 TYPES OF MIXTURES 1. HOMOGENOUS MIXTURE They have same composition and properties throughout their mass. Example: sugar solution. 2. HETEROGENOUS MIXTURE They have different composition and properties in different parts of their mass. Example: sand mixed with salt. METHODS OF SEPARATION I. SOLID SOLID MIXTURES 1. SUBLIMATION Sublimation is the process of conversion of a solid into vapour and back to the solid state, without passing through the liquid state. For this method, one of the components must be capable of subliming on heating. Examples: A mixture of sand and iodine (sublimes ) or of common salt and ammonium chloride (sublimes) can be completely separated by this method. 2. MAGNETIC SEPARATION This is possible only when one of the components is magnetic. Examples: a mixture of iron filings and powdered sulphur can easily be separated by using this method.

3. SOLVENT EXTRACTION In this process one of the components of mixture dissolves in a particular liquid, either water or any other solvent, and the other component, which does not dissolve, is

Carbon disulphide or carbon tetrachloride is used as a solvent in which sulphur dissolves and carbon gets separated as a residue.

4 3. SOLVENT EXTRACTION In this process one of the components of mixture dissolves in a particular liquid, either water or any other solvent, and the other component, which does not dissolve, is separated as residue by filtration. Examples: A mixture of charcoal and sulphur is separated by filtration. Carbon disulphide or carbon tetrachloride is used as a solvent in which sulphur dissolves and carbon gets separated as a residue. Note: dissolved substance can be separated from solution by evaporation where solvent evaporates leaving behind solute. 4. CHROMATOGRAPHY It is a method in which the separation of a mixture of substances is done by the flow of solvents on a special type of paper known as Whatman chromatographic paper. A good quality filter paper may be used for separating dyes present in ordinary black ink. A thin strip of rectangular shaped filter paper is used as absorbent. A pencil line is drawn about 3 cm from one edge. This is called base line. A small drop of water soluble black ink is applied with the help of a capillary tube at the centre of the line and is allowed to dry. The filter paper strip is now suspended into a large size gas jar containing water as the solvent in such a way that the drop of ink on the paper is just above the water level. The gas jar is covered with a lid and is kept undisturbed for about one hour. Water is drawn by capillary action and moves slowly upwards. As the water rises up on the filter paper, it takes along with it the dye particles. Different dyes present in the ink spot move with different velocities and so different dyes settle at different heights on the paper. The strip of filter paper is called chromatogram.

Thus, components of black ink are separated by this method. II. SOLID - LIQUID MIXTURES 1.

Example: A mixture of sand and water can be separated by this method. 2.

The insoluble solid substance settles down, and a clear liquid is left standing. It is called supernatant liquid.

5 Thus, components of black ink are separated by this method. II. SOLID - LIQUID MIXTURES 1. FILTRATION This method is used when the solid is insoluble in the liquid and thus forms a heterogenous mixture. Example: A mixture of sand and water can be separated by this method. 2. SEDIMENTATION This method is used to separate a heterogenous mixture containing an insoluble solid in a liquid. The mixture is allowed to stand. The insoluble solid substance settles down, and a clear liquid is left standing. It is called supernatant liquid. The solid substance that settles down is called sediment, and the whole process is known as sedimentation. The clear liquid is then carefully poured out into another beaker, leaving the sediment undisturbed. This process is known as decantation. 3. EVAPORATION This method is suitable if one of the components is a soluble solid and the other liquid that forms a homogenous solution with the solid component. The liquid can easily be evaporated, and its loss can be ignored.

4. DISTILLATION Distillation is the process of converting a liquid into vapour (by heating) and the subsequent condensation of the vapour back into the liquid.

III. LIQUID - LIQUID MIXTURES 1. SEPARATING FUNNEL If the liquids are immiscible, put the mixture in a separating funnel and leave it undisturbed for a while.

6 4. DISTILLATION Distillation is the process of converting a liquid into vapour (by heating) and the subsequent condensation of the vapour back into the liquid. This method is used to separate a solid from a liquid in which it is dissolved. Unlike evaporation, distillation permits separation and recovery of both the dissolved solid and the solvent liquid. III. LIQUID - LIQUID MIXTURES 1. SEPARATING FUNNEL If the liquids are immiscible, put the mixture in a separating funnel and leave it undisturbed for a while. It will be separated the mixture of the two liquids into two distinct layers; the heavier liquid will form the lower layer, while the lighter one will form the upper layer. Open the stop-cock and collect the heavier liquid in a beaker. Now place another beaker under the funnel, open the stop cock again, and let the lighter liquid run into it. Example: A mixture of carbon tetrachloride and water can be separated with the help of separating funnel. A mixture of oil and water can be separated with the help of separating funnel. 2. FRACTIONAL DISTILLATION If the liquid in a mixture are miscible and have different boiling points, they can be separated by fractional distillation. Here, an advantage is taken of the fact that there is a difference in the boiling points of the two liquids. On heating the mixture in a distilling flask, the liquid with the lower boiling point changes into vapour first, and this vapour, on being cooled by the condenser, collects in the receiver. The temperature remains stationary till all the liquid with the lower boiling point distils over. The receiver is then changed, and heating is continued so as to allow the second liquid to distil.

Chapter-2: IS MATTER AROUND US PURE

1 Chapter-2: IS MATTER AROUND US PURE Revision Notes: # A pure substance consists of a single type of particles. # A mixture contains more than one substance (element and/or compound) mixed in any proportion.