Chemical Classification: Elements, Compounds and Mixtures

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Matter and its Different States:

  • The matter is any substances that have mass and that occupies space. There are three states of matter.
  • Solid: It has a definite shape and volume at given temperature and pressure.
  • Liquid: It will take the shape of the container, thus has indefinite shape but has a definite volume.
  • Gas: Gas has neither a definite shape nor a definite volume.

Note: Besides these three standard states, there are two more states called plasma state (Exists at very high temperature) and Bose-Einstein condensate (Exists at very cold condition).



Chemical Classification:

  • Depending upon the chemical composition matter is classified into two types a) Pure substances and b) Mixtures.
  • Pure substances are further classified into elements (e.g. iron, oxygen) and compounds (e.g. sodium hydroxide, ammonia).This classification was given by a French chemist Lavoisier. While the mixtures are further classified into homogeneous mixtures (e.g. a common salt solution in water)and heterogeneous mixtures (e.g. muddy water).

Chemical Classification of Matter elements compound and mixtures

 

Pure Substances:

  • A pure substance is the one which is made up of molecules containing the same kind of atoms. Let us consider the case of pure water. In pure water, all molecules are made up of two hydrogens and one oxygen.
  • Pure substances are further classified into elements and compounds.
  • All pure substances are homogeneous. But all homogeneous substances are not pure. For e.g. If we consider water its composition is same everywhere, thus it is homogeneous. If we take lime water its composition may be the same everywhere but it is not a pure substance. It is a mixture of water and lime juice.

Characteristics of the Pure Substances:

  • They have fixed composition.
  • They cannot be separated into simpler substances.
  • They can only be changed in identity and properties.
  • Their properties do not vary.

Elements:

  • An element may be defined as a pure substance which can neither be decomposed into nor built from simpler substances by any physical or chemical method.
  • This definition fails to explain modern techniques and processes like nuclear fission, nuclear fusion and artificial transmutations in nuclear chemistry.
  • Now atom is considered as a fundamental unit of matter. Hence an element may be defined as a pure substance which is made up of one kind of atoms.


Characteristics of Elements:

  • They cannot be divided into simpler substances.
  • They cannot be built from simpler substances.
  • They can only be changed in identity and properties.
  • Their properties do not vary.

Classification of Elements:

  • Based on the nature elements are classified into three types
    • Metals: These are generally solids (exceptions mercury and gallium) and have characteristics such as hardness malleability, high tensile strength, lustre and ability to conduct heat and electricity. Examples: Copper, iron, aluminium, zinc etc.
    • Non-metals: These are generally non-lustrousExamples: Sulphur, phosphorous, nitrogen, hydrogen
    • Metalloids: These elements have characteristics common to metals as well as non-metals.Examples: Arsenic, tin, bismuth, antimony
  • The number of atoms present in a molecule of an element is called its atomicity. Based on their atomicity they are classified as
    • Monoatomic elements: e.g. Silver (Ag), Gold (Au), Aluminium (Al), Helium (He), Neon(Ne), Boron (B), Carbon (C).
    • Diatomic elements: Hydrogen (H2), Chlorine (Cl2), Oxygen (O2).
    • Polyatomic elements: Ozone (O3), Phosphorous (P4), Sulphur (S8).

Compounds:

  • A compound may be defined as a pure substance which can be decomposed into simpler substances by some suitable chemical method.
  • A compound is formed by the combination of two or more elements in a definite proportion of a mass.
  • Examples:  Water (H2O) is a compound of hydrogen and oxygen in the ratio 1:8 by mass respectively. Carbon dioxide (CO2) is a compound of carbon and oxygen in the ratio 3:8 by mass respectively.

Characteristics of Compounds:

  • The constituents of a compound are always present in a fixed ratio by mass.
  • A compound is always homogeneous in nature.
  • The properties of the compound are different from those of its constituent elements.
  • The constituents of a compound cannot be separated by simple mechanical means. Energy in the form of light, heat or electricity is required to separate them.
  • Compounds are formed as a result of a chemical change.
  • Formation of a compound is always accompanied by absorption or evolution of heat, light or electrical energy.
  • Compounds in their purest form have sharp boiling and melting point.


Mixtures:

  • Mixtures can be defined as substances which are made up of two or more pure substances.
  • They can possess variable composition and can be separated into constituent components by some suitable physical means.
  • Example: Crude oil, air

Classification of Mixtures:

  • Mixtures are classified into two types
  • Homogeneous Mixtures: Homogeneous mixtures are the mixtures which have the same composition throughout. These mixtures are also known as solutions. The constituents of homogeneous mixtures are distributed uniformly throughout. The constituents of such mixtures cannot be seen even under the microscope.  Examples: Air, gasoline, sea water, stainless steel, brass, coloured glass etc.
  • Heterogeneous Mixtures: Heterogeneous mixtures are the mixtures which have different composition at different parts.  The constituents of heterogeneous mixtures are not distributed uniformly throughout. The constituents of such mixtures can be seen even by naked eyes or with the help of a microscope.  Examples: Mixture of iron filings and sulphur, muddy water, a mixture of sand and sugar etc.

Characteristics of Mixtures:

  • The constituents of the mixture may be present in any ratio.
  • Mixtures may or may not be homogeneous in nature.
  • The properties of constituents remain the same even in mixture form.
  • The constituents of a mixture can be separated by simple mechanical or physical means.
  • Mixtures are formed as a result of a physical change.6. Formation of a mixture is not accompanied by absorption or evolution of heat, light or electrical energy.
  • Mixtures don’t have sharp boiling and melting point.

Methods of Separation of Mixtures:

  • Most of the available substances in nature are in mixture form. The useful component of the mixture can be obtained by separating individual components of the mixture by a suitable method. The method of separation employed depends upon the nature of components of the mixture.

Separation of Solid-Solid Mixtures:

Mechanical Picking:

  • Property exploited: the physical appearance is different for different substances
  • In this method, the components are separated on the basis of physical characters like shape, size and appearance.
  • e.g. Removing stones or husk particles from grains.

Mixtures 03

Magnetic Separation:

  • Property exploited: the magnetic property of one component.
  • Ferromagnetic substances like iron, cobalt, nickel are separated from non-magnetic substances using magnetic separators or magnetic belts.
  • e.g. Separation of iron filings from sulphur using magnet.

Mixtures 02

Sublimation:

  • Property exploited: the ability of one component to sublime.
  • The solids which on heating gets converted into vapours directly (instead of into liquid) are called sublimate or volatile solid and the phenomenon is known as sublimation. This process is used to separate volatile solids from non-volatile solids. In this method, the mixture is heated. The sublimable solid gets converted into vapours living non-volatile substance behind. The vapours of sublimation can be condensed by cooling to get the volatile solid back.
  • e.g. Separation of iodine from sand or separation of ammonium chloride from sand.

Mixtures 01

Solvent Extraction:

  • Property exploited: Solubility of one component in the solvent.
  • In this process, one of the components is dissolved in a particular solvent in which it is soluble while other components do not dissolve.
  • e.g. Separation of sulphur and iron. Carbon disulphide is added to the mixture in which sulphur get dissolved and iron remains in solid state. Then the solution is filtered iron particles are filtered out. The filtrate contains carbon disulphide with sulphur dissolved in it. Carbon disulphite evaporates and sulphur remains.
  • All the compounds of ammonia, sodium, potassium and all the nitrates and nitrites of metal, All chlorides (except chlorides of mercury, silver and lead), all metal sulphates (except sulphates of calcium, lead and barium)n  are soluble in water. All metal oxides, hydroxides, carbonates and sulphides are insoluble in water.Organic substances are not soluble in water but are soluble in organic solvents like chloroform, benzene, alcohol etc. Some important substances and their solvents are as follows.
Substance Solvent used
Chlorophyll Methylated spirit
Grease Petrol
Iodine Ethyl alcohol
Nail polish Acetone
Naphthalene Benzene
Nitre Water
Oil Petrol
Paint Turpentine oil
Paraffin wax Turpentine oil
Phosphorous Carbon disulphide
Rust Oxalic acid
Rubber Benzene
Sulphur Carbon disulphide
Shellac Ethyl alcohol

Gravity Separation:

  • Property exploited: Difference in densities of components.
  • The mixture containing two components having different densities but both insoluble in particular solvent can be separated by this method.
  • A mixture of chalk powder and sand is added to water. Due to more density sand particles settle at the bottom of the vessel while chalk particles being lighter floats on the surface of the water.


Fractional Crystallization:

  • Property exploited: Difference in a solubility of the components in the same solvent.
  • A mixture of KNO3 and NaCl can be separated by this method. Both are soluble in water.  But solubility of KNO3 is more than NaCl. When the saturated solution containing KNO3 and NaCl is heated and allowed to cool down, NaCl being less soluble crystalizes first and it is filtered out the filtrate contains KNO3 which crystallizes on further cooling.

Separation of  Solid-Liquid Mixtures:

Filtration:

  • Property exploited: Size of the insoluble solid component is more than the size of the grid of the filter paper.
  • This method is used for separating the insoluble suspended particles from a liquid using filtering materials like filter paper, sand bed etc. The liquid that passes trough the filter is called the filrate and insoluble solid left on filter is called the residue. For colloidal particles parchment membrane is used for separating colloidal particles from the solution.
  • e.g. filtration of water.

Mixtures 05

Sedimentation and Decantation: 

  • Property exploited; The high density of insoluble solid component.
  • In this method, the liquid containing insoluble suspended particles is allowed to stand still for some time. The suspended particles settle down at the bottom under gravity. The solid substance that settles down is called sediment. The upper liquid can be separated by decantation. In decantation the upper liquid is carefully poured in another beaker without distrurbing sediments.
  • e.g. sedimentation of muddy water.

Mixtures 04

Mixtures 11

Evaporation:

  • Property exploited: A solid remains unaffected on heating the mixture up to the boiling point of the liquid. Liquid gets converted into vapours.
  • In this method, the solid soluble component of the mixture is separated from a liquid. The mixture is heated up to the boiling point of a liquid. The liquid evaporates living the solid behind. The vapours of liquid can be condensed to get liquid back.
  • e.g. Separation of sugar from sugar in water solution.

Mixtures 06

Distillation:

  • Property exploited: A solid remains unaffected on heating the mixture up to the boiling point of the liquid. Liquid gets converted into vapours.and vapours of liquid cooled down to obtain pure liquid.
  • This method is used to separate liquid from soluble solid.  In this method, the mixture is heated up to the boiling point of the liquid, at which the vapours of liquid are formed living the solid behind. The vapours of liquid on cooling condense to give the liquid. The condensed vapours are called distillates.
  • e.g. Separation of water from salt in water solution.

Mixtures 07

  • Note: The difference between the evaporation and distillation is that in case of evaporation, the component evaporated is lost permanently. While in distillation both the components one which is evaporated and one which has no effect are recovered.

Centrifuging:

  • Property exploited: Density of solid particles is different from that of liquid particles.
  • In this method, the mixture of solid in a liquid is rotated in a centrifuge. The centrifugal force causes more dense substances to separate out along the radial direction (the bottom of the tube). Whereas the lighter particles will tend to move to the top of the tube.
  • e.g. Blood plasma can be separated from its solid constituents (RBC, WBC, Platelets) by this method. When milk is stirred very fast the cream particles and milk particles are separated. When rotation or sturring stops cream particles float on the top.

Mixtures 08

Separation of  Liquid-Liquid Mixtures:

Separating Funnel:

  • Property exploited: Difference in densities of two immiscible liquid components
  • In this method, the mixture of two immiscible liquids having different densities is taken in separating funnel. The lighter (lower density) liquid forms the top layer while heavier(higher density)  liquid forms lower level. Now the bottom valve is open and the heavier liquid is separated.
  • e.g. Separation of kerosene from the water.

Mixtures 09

Fractional Distillation:

  • Property exploited: Difference in boiling points of liquids.
  • This method is used to separate two miscible liquids having close boiling points (Difference of about 10 K). In this method, the mixture is heated. The liquid having lower boiling point gets evaporated first living the liquid having a higher boiling point behind. The vapours of evaporated liquid can be condensed by cooling to get it back in the liquid state.
  • e.g. Separation of different components from petroleum.

Mixtures 10



Separation of  Gas-Gas Mixtures:

Diffusion:

  • Property exploited: Difference in densities of component gases.
  • The rate of diffusion is inversely proportional to its molar mass. It means the gas with lower molar mass diffuses faster than that with higher molar mass.
  • e.g. Separation of hydrogen (lower molar mass) from methane (higher molar mass)

Mixtures 13

Fractional Evaporation:

  • Property exploited: Difference in boiling points of component gases.
  • A mixture of liquefied gases is evaporated the respective gases separates out from the liquefied mixture at their boiling points.
  • e.g. When liquefied air is allowed to evaporate nitrogen having the lowest boiling point boils off the mixture leaving behind oxygen in the liquefied air. on further evaporation, oxygen boils off leaving behind remaining components of air.

Mixtures 14

Preferential Liquefication:

  • Property exploited: Difference in liquefication of gases under pressure.
  • a mixture of gases is liquefied using high pressure. The gas which is easily liquefiable gets liquefied first.
  • e.g. When a mixture of ammonia and hydrogen are subjected to high pressure, ammonia gets liquefied while hydrogen remains in gaseous state and hence can be separated very easily.

Dissolution in Suitable Solvent:

  • Property exploited: Difference in the solubility of components in the given solvent.
  • The mixture of gases is allowed to dissolve in the solvent such that one component is soluble in the solvent. Other component remains in the gaseous state and can be separated easily.
  • e.g. When a mixture of carbon dioxide and carbon monoxide is dissolved in KOH, carbon dioxide gets dissolved in it while carbon monoxide remains in the gaseous state and thus can be separated.

Separation of  Liquid-Gas Mixtures:

Heating:

  • Property exploited: As the temperature increases, the solubility of a gas in the solvent decreases.
  • When a solution of a gas in a liquid is heated slightly below the boiling point of the liquid The gas escapes out leaving behind liquid component. The escaping gas can be collected separately.
  • e.g. Separation of dissolved oxygen from water.

Lowering of Pressure:

  • Property exploited: The solubility of a gas in a liquid is directly proportional to the pressure on it.
  • In soda water bottle carbon dioxide gas is dissolved in water under pressure. When the cap of the bottle is open the solubility of the gas decreases and dissolved carbon dioxide gas escapes out leaving water behind.

Miscellaneous Method:

Chromatography:

  • Property exploited: The different rates of adsorption of different components.
  • This method was discovered by Michael Tswett in 1906. This technique is based on preferential adsorption of a substance at the surface of the adsorbent. In this method, the mixture is mixed with the liquid in which it is soluble. This liquid is called eluant. Then this solution is poured into a tube containing adsorbing column. Different components of the mixture would adsorb to different extent forming separate layers (Different colours) on the adsorbing column. Depending upon the fixed and moving phases, chromatography is further classified as a paper chromatography, column chromatography, thin layer chromatography and gas chromatography.
  • e.g. separation of coloured constituents in a mixture of ink.

Mixtures 15

  • In chromatography, a very small quantity present in micrograms can be separated. it is used for quantitative estimation. It is used for purification of many industrial products.

The Phlogiston Theory:

  • The theory stated that the substance undergoing combustion mainly contains a mysterious matter called phlogiston and some clax. If a substance is burnt, phlogiston evolves and goes into atmosphere and clax (meaning a non-useful thing)  is left behind as an ash.
  • When a candle burns in a closed container, the air in the container becomes saturated with phlogiston. Air cannot accommodate additional phlogiston and the candle is extinguished.

Wood (Phlogiston rich) → Burnt → Phlogiston in air + Clax (ash)

Challenges to the Phlogiston Theory:

  • In 1771 British chemist Joseph Priestley heating mercury oxide produced a gas (oxygen) and found that a candle burns vigorously in this gas than that in the air. He named this gas as dephlogisticated air.
  • In 1766, Sir Henry Cavendish produced another gas (Hydrogen) by the action of an acid with metal. He found that this gas burns in air and produce water. He called this gas as flammable air.  He further said that flammable air must be associated with the phlogiston.
  • French scientist Lavoisier ruled out the theory of phlogiston and proved that during burning a part of the air is used in the chemical reaction. He named this part of the gas as oxygen (acid forming). He also proved that the gas produced by Cavendish has nothing to do with phlogiston and called this gas as hydrogen (water forming).

Burning substance + Part of air (oxygen) → Ash



Science > Chemistry > Introduction to ChemistryYou are Here
Physics Chemistry  Biology  Mathematics

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