Magnetism
Magnetism: The phenomenon of attracting magnetic substances like iron, cobalt, nickel etc. is called magnetism. A body possessing the property of magnetism is called magnet. Lodestone or magnetite is natural magnet. Earth is also a natural magnet. In magnetized substance all the atomic magnets are aligned in same direction and thus resultant magnetism is non-zero.
Bar Magnet
A bar magnet consists of two equal and opposite magnetic poles separated by a small distance. Poles are not exactly at the ends. The shortest distance between two poles is called effective length (\[{{L}_{e}}\]) and is less than its geometric length (\[{{L}_{g}}\]). For bar magnet \[{{L}_{g}}=2l\]and\[{{L}_{e}}=(5/6){{L}_{g}}\]
Properties of Magnet
(i) Attractive property: When a magnet is dipped into iron fillings it is found that the concentration of iron filings, i.e., attracting power of the magnet is maximum at two points near the ends and minimum at the centre. The places where its attracting power is maximum are called poles.
(ii) Directive property: When a magnet is suspended its length becomes parallel to N-S direction. The pole pointing north is called the north-pole while the other pointing in the geographical south is called the south pole of the magnet. The line joining the two poles of a magnet is called magnetic axis and the vertical plane passing through the axis of a freely suspended or pivoted magnet is called magnetic meridian.
(iii) Poles of a magnet always exist in pairs: In a magnet the two poles are found to be equal in strength and opposite in nature. If a magnet is broken into number of pieces, each piece becomes a magnet with two equal and opposite poles. This shows that monopole do not exist.
(iv) Repulsive property: A pole of a magnet attracts the opposite pole while repels similar pole.
Demagnetisation of Magnet
A magnet gets demagnetised, i.e., loses its power of attraction if it is heated, hammered or alternating current is passed through a wire wound over it.
Permanent and Temporary Magnets (Electromagnets)
The permanent artificial magnets are made of some metals and alloys like Carbon-steel, Alnico, Platinum-cobalt, Alcomax, Ticonal etc. The permanent magnets are made of ferromagnetic substances with large coercivity and retentivity. The temporary artificial magnets like electromagnets are prepared by passing current through coil wound on soft iron core. These cannot retain its strength for a long time. These are made from soft iron, non-metal and alloy. Electromagnets are stronger than permanent magnet.
Some Applications of Electromagnets
(i) Electric motors
(ii) Doorbells
(iii) In scrapyards to separate iron from other metals
Coulomb's Law in Magnetism
If two magnetic poles of strengths \[{{m}_{1}}\] and \[{{m}_{2}}\] are kept at a distance more...
On the basis of electrical conductivity\[(\sigma )\]or resistivity \[(\rho =1/\sigma )\] the solids are classified as
(i) Metals - have low resistivity
\[\rho \tilde{\ }{{10}^{-2}}\,to\,{{10}^{-8}}\Omega m\],
\[\sigma \tilde{\ }{{10}^{2}}\,to\,{{10}^{8}}S{{m}^{-1}}\]
(ii) Semiconductors - have intermediate resistivity
\[\rho \tilde{\ }{{10}^{5}}\,to\,{{10}^{0}}\Omega {{m}^{{}}}\]
\[\sigma \tilde{\ }{{10}^{-5}}\,to\,{{10}^{0}}S{{m}^{-1}}\]
(iii) Insulators - have high resistivity
\[\rho \tilde{\ }{{10}^{8}}\Omega {{m}^{{}}}\]
\[\sigma \tilde{\ }{{10}^{-8}}S{{m}^{-1}}\]
i.e. the Semiconductors are the materials whose conductivity is more than insulators but less than conductors.
Types of Semiconductors
Intrinsic semiconductors or pure semiconductors in semiconductors forbidden energy gap Eg is more than metals or conductors and less than insulators. Silicon (Si) and Germanium (Ge) are the examples of pure semi-conductors.
In pure or intrinsic semiconductor,
\[{{n}_{e}}={{n}_{h}}={{n}_{i}}\] Where\[{{n}_{e}}=no\].of electrons: \[{{n}_{h}}=no\]. Of holes and\[{{n}_{i}}=no\]. Of intrinsic carrier concentration. Impurity like pentavalent (As, Sb, P) or trivalent (In, B, Al) are added to increase conductivity.
Depending on doping type we have
(a) n- type semiconductor (b) p- type semiconductor
(a) n - type semiconductor: Si or Ge with pentavalent doping. An atom of valency +5 occupies the position of parent atom in crystal lattice. Four valence electrons form 4 covalent bonds but 5th electron is free and weakly bound to parent atom. The ionisation energy (~ 0.01 V for Ge and 0.05V for Si) is small and even at room temperature the electron jumps to conduction band. The dopant is called donor impurity (positively charged).
(b) p - type semiconductor: Si or Ge with trivalent doping means one less electron in the 4 covalent bonds, so the 4th neighbour has a vacancy or hole that can be occupied by an electron from another site. Thus a hole is available for conduction. The trivalent atom is negatively charged as it acquires an electron and is called acceptor atom or impurity.
Formation of p - n junction: Part of p-type can be converted into n - type by adding pentavalent impurity. There is concentration gradient between p and n sides, holes diffuse from p side to n side (\[(P\to n)\]and electrons move from \[(n\to p)\]creating a layer of positive and negative charges on n and p side respectively called depletion layer. External bias is applied to cause charges to flow.
Symbol of p-n junction diode
p-n junction under forward bias: When p-side is connected to positive terminal and n - side to negative terminal of external voltage, it is said to be forward biased.
The applied voltage V is opposite to build in Potential\[{{V}_{0}}\], hence depletion layer width decreases and barrier height is reduced to\[({{V}_{0}}-V)\]. There is minority carrier injection, hence charges begin to flow. Current is in the order of mA.
(c) p-n junction under reverse bias: When p-side of p-n junction is connected to -ve more...
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.
Pure substance:
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.
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 in case of a liquid substance.
Types of pure substances:
Two different types of pure substances are
(i) Element: 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.
(ii) Compound: A compound is a substance made up of two or more elements chemically combined in a fixed ratio by weight e.g. \[{{H}_{2}}O\] (water), \[NaCl\](sodium chloride) etc.
Mixture:
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.
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. e.g. a well-mixed sample of vinegar.
(ii) Heterogeneous mixture: It is one that is not uniform throughout. Different samples of a heterogeneous mixture may have different composition, e.g. a mixture of salt and pepper.
Solution:
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 mixture (i.e. whether it is a homogeneous mixture or heterogeneous mixture) different methods used are given below:
Mixture
Separation Method
1.
Insoluble solid in solvent
Sedimentation followed by filtration. In case of a fine solid centrifugation is used instead of filtration
Law of conservation of mass: This law was stated by Lavoisier in 1744. It states that "In all physical and chemical changes, the total mass of readouts is equal to total mass of products."
Law of constant proportions (or constant composition): This law was first stated by Proust in 1797. According to the law "a chemical compound is always found to be made up of the same elements combined together in the same proportions by weight" e.g. the ratio of hydrogen and oxygen in pure water is always 1: 8 by weight. This law is also called law of definite proportions.
Law of multiple proportions: This law was given by John Dalton (1803) and states that "when two elements combine to form two or more compounds, the different mass of one of the elements and the fixed mass of the one with which it combines always form a whole number ratio". This law explains the concept of formation of more than one compound by two elements.
Dalton's Atomic theory: Postulates of Dalton s Atomic Theory
(i) Matter is made up of extremely small indivisible particles called atoms.
(ii) Atoms of the same substance are identical in all respects i.e., they possess same size, shape, mass, chemical properties etc.
(iii) Atoms of different substances are different in all respects i.e., they possess different size, shape, mass etc.
(iv) Atom is the smallest particle that takes part in a chemical reaction.
(v) Atoms of different elements may combine with each other in a fixed simple, whole number ratio to form compound atoms.
(vi) Atoms can neither be created nor destroyed i.e., atoms are indestructible.
Atom: It is the smallest particle of an element which can take part in a chemical change. It may or may not be capable of independent existence.
Symbol: The abbreviation used for lengthy names of elements are termed as their symbols. The symbol of an element is the first letter or the first and another letter of English name or Latin name of the element. While writing a symbol, the first letter is always capital and the second is always small.
Molecule: It is the smallest particle of an element or compound that is capable of independent existence and shows all the properties of that substance. [The molecules of an element is made up of only one and same type of atoms, while the molecule of a compound is made up of dissimilar atoms]
Atomicity: The number of atoms present in a molecule of an element or a compound is known as its atomicity. e.g. the atomicity of oxygen is 2 while atomicity ozone is 3.
Ion: It is an electrically charged atom or group of atom. It is formed by the loss or gain of electrons by an atom. Ions are of two types :
Cation: It is positively charged ion and is formed by the loss of electron from an atom e.g. \[{{H}^{+}},\]\[N{{a}^{+}},C{{a}^{2+}},A{{l}^{3+}}\]\[,NH_{4}^{+}\], more...
Classification of Elements and Periodicity in Properties
Classification means identifying similar species and grouping them together.
Lavoisier divided elements into two main types known as metals and non-metals.
Doberiner's Law of triads:
According to this law, "in certain triads (group of three elements) the atomic mass of the central element was the arithmetic mean of the atomic masses of the other two elements.” But in some triads all the three elements possessed nearly the same atomic masses, therefore the law was rejected. e.g., atomic masses of Li, Na and K are respectively 7, 23 and 39, thus the mean of atomic masses of 1st and 3rd element is = 7 + 39 = 23
Limitations of Doberiner's Triads: He could identify only a few such triads and so the law could not gain importance. In the triad of Fe, Co, Ni, all the three elements have a nearly equal atomic mass and thus does not follow the above law.
Newland’s Law of octaves:
According to this law "the elements are arranged in such a way that the eighth element starting from a given one has properties which are a repetition of those of the first if arranged in order of increasing atomic weight like the eight note of musical scale.)
Drawback of Newland’s law of octaves:
(i) According to Newland only 56 elements exists in nature and no more elements would be discovered in the future. But later on several new element were discovered whose properties did not fit into law of octaves.
(ii) In order to fit new elements into his table Newland adjust two elements in the same column, but put some unlike elements under the same column. Thus, Newland's classification was not accepted.
Mendeleev’s periodic table:
Mendeleev arranged 63 elements known at that time in the periodic table. According to Mendeleev "the properties of the elements are a periodic function of their atomic masses." The table consists of eight vertical column called 'groups’ and horizontal rows called 'periods'.
Merits of Mendeleev's periodic table:
(i) At some places the order of atomic weight was changed in order to justify the chemical and physical nature.
(ii) Mendeleev left some gap for new elements which were not discovered at that time.
(iii) One of the strengths of Mendeleev's periodic table was that, when inert gases were discovered they could be placed in a new group without disturbing the existing order.
Characteristics of the periodic table: Its main characteristics are:
(i) In the periodic table, the elements are arranged in vertical rows called groups and horizontal rows called periods.
(ii) There are eight groups indicated by Roman Numerals I, II, III, IV, V, VI, VII, VIII. The elements belonging to first seven groups have been divided into sub-groups designated as A and B on the basis a/similarities. The elements that are present on the left hand more...
The term acid, in fact, comes from the Latin term acere, which means “Sour”. In everyday life we come across many compounds that chemists classify as acids. Bases are compounds which taste bitter eg. Milk of magnesia. Salts also have wide applications for example ammonium chloride is used as electrolyte in dry cells, sodium bicarbonote (baking powder) in the manufacture of glass etc.
Properties of acids and bases
A. Properties of acids
Chemical properties:
(i) Action of metals: Metals generally react with dilute acids to form their respective salt and hydrogen.
\[Metal+Acid\to Salt+Hydrogen\]
(ii) Action with metal oxides (Basic oxides) Metal oxides are generally basic oxides. These oxides get neutralised when they react with acids. These reactions are mostly carried upon heating e.g.
\[Basicoxide+Acid\to Salt+Water\]
\[\left( Neutralization\text{ }reaction \right)\]
(iii) Action with metal carbonates and metal hydrogen carbonates Acids react with carbonates and hydrogen carbonates to form their respective salts, water and carbon dioxide gas.
\[Carbonate/bicarbonate+Acid\to Salt+water+carbon\,\,dioxide.\]
B. Properties of bases
Chemical Properties:
(i) Reaction of metals with bases: Metals (e.g. Zn, Al, Sn) dissolve in NaOH (an alkali) to liberate hydrogen gas.
\[Zn+2NaOH\to N{{a}_{2}}Zn{{O}_{2}}+{{H}_{2}}\]
\[Sod.Zincate\]
(ii) Action with acids: Bases combine with acids to form salt and water only. It is a neutralisation reaction.
\[Base+Acid\to Salt+Water\] Non - metallic oxides react in the same way hence non- metallic oxides are acidic in nature.
Strength of Acids and Bases
The strength of an acid or a base can be easily estimated by making use of universal indicator which is a mixture of several indicators. The universal indicator show different colours at different concentrations of hydrogen ions in solution.
pH Scale
pH Scale: It is a scale that is used for measuring \[{{H}^{+}}\] ion (Hydrogen ion) concentration of a solution. The term pH stands for "potential" of "hydrogen". It is the amount of hydrogen ions in a particular solution.
For acids pH< 7
For bases pH > 7
For neutral substances pH = 7
Importance of pH in Daily Life
(i) Blood pH: For proper functioning our body needs to maintain blood pH between 7.35 and 7.45. Values of blood pH greater than 7.8 or less than 6.8 often results in death.
(ii) Acid rain: When pH of rain water is less than 5.6, it is called acid rain, when acid rain flows into rivers, it lowers the pH of river water.
(iii) pH in our digestive system: We know that hydrochloric acid (HC1) produced in our stomach helps in digestion of food without harming stomach. However excess of acid causes indigestion and leads to pain as well as irritation. To get rid of this people use bases called "antacids".
(iv) pH of the soil: For their healthy growth plants require a specific pH. Soils with high peat content more...
Metals and Non-metals: There are more than 114 elements present in the periodic table. These elements can be broadly classified into two categories i.e., metals and non-metals. Out of 114 elements, 22 are non-metals.
Physical properties of metals:
(i) They are usually shiny i.e. have a metallic luster.
(ii) Metals have a high density
(iii) Metals are ductile i.e. they can be drawn into wires.
(iv) Metals are malleable i.e. they can be founded into thin sheets.
(v) Metals are good conductors of electricity.
(vi) Metals have high melting point and are generally in solid state at room temperature.
(vii) Metals are good conductors of heat and sound.
Uses of metals:
(i) Metals are very important for modern humans it is not possible to imagine our life without them.
(ii) Metals are used in manufacturing of bridges, railways, aeroplanes, diesel mobile units (DMU), electric mobile units (EMU), motor cars, electric motors, telephones, televisions, interplanetary space vehicles, or even common articles like cooking utensils and coins.
(iii) Metals are very important for the economy of a country. Some metals, such as titanium, chromium, manganese and zirconium are strategic metals. These metals and their alloys find wide applications in atomic energy, space science projects, jet engines and high grade steels.
(iv) Gold and silver ornaments are obtained from small pieces of metals by hammering.
Noble metal: Noble metals are metals that are resistant to corrosion or oxidation, unlike most base metals. Examples include tantalum, gold, platinum, and rhodium.
Precious metal: A precious metal is a rare metallic chemical element of high economic value precious metals include the platinum group metals: ruthenium, rhodium, palladium, osmium, indium, and platinum, of which platinum is the most widely traded.
Alloy: An alloy is a mixture of two or more elements in solid solution in which the major component is a metal. Most pure metals are either too soft, brittle or chemically reactive for practical use. Combining different ratios of metals as alloys modify the properties of pure metals to produce desirable characteristics. The aim of making alloys is generally to make them less brittle, harder, resistant to corrosion, or have a more desirable color and luster. Examples of alloys are steel (iron and carbon), brass (copper and zinc), bronze (copper and tin), and duralumin (aluminium and copper).
Alloy
Composition
Uses
1. Brass
Cu = 80%, Zn = 20%
For making utensils and cartridges.
2. Bronze
Cu = 90%, Sn = 10%
For making statues, medals, ships, coins and machines
3. Solder
Sn = 50%, Pb = 50%
For joining metals, solding wire and electronic components etc.
The pollutants may be inorganic, biological or radiological in nature.
(i) Bio-degradable pollutants are domestic wastes which are rapidly decomposed by micro-organisms.
(ii) Non-biodegradable pollutants include chemicals, mercuric salts, lead compounds, pesticides, etc.
(iii) Natural pollution is caused by radioactive substances, volcanic eruptions, forests and mines fires floods, etc.
(iv) Artificial pollution is caused by industries, thermal plants, automobile, exhausts, sewage, etc.
Environment
Environment: The conditions existing around animal or human life.
Atmosphere: The gaseous envelop surrounding the earth. It has been classified into following regions:-
(i) Stratosphere: The layer of the earth's atmosphere above the troposphere and below the mesosphere.
(ii) Troposphere: The lowest region of the atmosphere extending from earth's surface to the lower boundary of the stratosphere. In this region, human beings along with other organisms live. It contains water vapour and is greatly affected by air pollution.
Note: The other two layers are Thermosphere and Mesosphere.
Air pollution
Air pollution: The major air pollutants are
(i) Carbon monoxide (CO): It is produced by incomplete combustion of gasoline in motor vehicles, wood, coal, inceneration and forest fires. It is treacherous and deadly poisonous gas. It induces headache, visual difficulty coma and death. It blocks the normal transport of oxygen from the lungs to other parts of the body.
(ii) Sulphur dioxide\[(S{{O}_{2}})\]: It is produced by petrol combustion, coal combustion, and petrol refining and smelting operations. It hinders the movement of air in and out of lungs. It is particularly poisonous to trees causing chlorosis and dwarfing. In presence of air it is oxidised to \[S{{O}_{3}}\] which is also irritant.
\[2S{{O}_{2}}+{{O}_{2}}(air)\to 2S{{O}_{3}}\]
In presence of moisture \[S{{O}_{3}}\] is converted into highly corrosive sulphuric acid.
\[S{{O}_{3}}+{{H}_{2}}O(moisture)\to {{H}_{2}}S{{O}_{4}}\]
It attacks marble, limestone, vegetation, paper and textiles and injurious to human beings.
(iii) Oxides of nitrogen: \[N{{O}_{2}}\] and NO, Source - combustion of coal, gasoline, natural gas, petroleum refining, chemical plants, manufacturing explosives and fertilizers, tobacco smoke. Breathing \[N{{O}_{2}}\] causes chlorosis to plants and chronic lung conditions leading to death. \[N{{O}_{2}}\] reacts with moisture to form acids.
\[2N{{O}_{2}}+{{H}_{2}}O\to HN{{O}_{2}}+HN{{O}_{3}}\],
\[3HN{{O}_{2}}\to 2NO+HN{{O}_{3}}+{{H}_{2}}O\]
(iv) Smoke, dust:
Sources: cement works, iron and steel works, gas works, power generating stations.
Smog: It is a mixture of smoke and fog in suspended droplet form. It is of two types:
London smog or classical smog: It is coal smoke plus fog. The fog part is mainly \[S{{O}_{2}}\] and \[S{{O}_{3}}\].It has sulphuric acid aerosol. It causes bronchial irritation and acid rain. It is reducing in nature.
Photochemical smog or Los Angeles smog: The oxidised hydrocarbons and ozone in presence of humidity cause photochemical smog. Hydrocarbons \[+{{O}_{2}}\], \[N{{O}_{2}}\] NO, O, \[{{O}_{3}}\] \[\to \] Peroxides, formaldehyde, peroxyacetylnitrate (PAN), acrolein etc. It is oxidising in nature and causes irritation to eyes, lungs, nose, and asthamatic attack and damage plants.
Acid rain: The oxides of C, N and S present in more...
Chemistry is a branch of science which deals with study of matter and various changes it undergoes. It deals with the preparation, properties, reactions and structures of chemical elements and compounds. For convenience the study of chemistry is sub-divided into various branches such as:
(i) Inorganic chemistry (ii) Organic chemistry (iii) Physical chemistry
(iv) Analytical chemistry (v) Industrial chemistry
Valency
During the formation of molecules of compounds, atoms combine in certain fixed proportions. This is because of the fact that different atoms have different combining capacities. The combining capacity of an atom or radical is known as its valency.
Ions or radicals
In addition to atoms and molecules, a third type of particles occurs in substances. These particles, called ions, or atoms or group of atoms that carry an electrical charge. An ion is formed when electrons are removed from or added to an atoms or group of atoms. When electrons is/are removed the resulting ion is called a cation or basic radical. A cation is positively charged ion. (e.g\[N{{a}^{+}}\]. When electron is/are added the resulting ion is called an anion or acidic radicals. An anion is negatively charged ion e.g.,\[(C{{l}^{-}},O_{2}^{2-})\]An ion or radical is classified as monovalent, divalent, trivalent or tetravalent when the number of charges over it is 1,2,3 or 4 respectively.
Formula of Elements and Compounds
Formula of dements: The molecule of an element is denoted by writing the symbol of the element and, to the right and below it, a number expressing the number of atoms in the molecule.
Formula of compound: A molecule of a compound maybe made up of atom of different elements linked up together chemically and in definite proportion by weight.
Chemical formula: It is of two types:
(i) Molecular formulae: Chemical formulae that indicate the actual number and type of atoms in a molecule is called molecular formulae.
(ii) Empirical formulae: Chemical formulae that indicate only the relative number of atoms of each type in a molecule is called empirical formulae.
Equivalent weight
(i) Equivalent weight of element \[=\frac{Atomic\,\,weight\,\,of\,\,element}{Valency\,\,of\,\,element}\]
(ii) Eq. wt of an acid/base\[=\frac{Molecular\,\,mass}{Basicity\,\,of\,\,acid/Acidity\,\,of\,\,base}\]
(iii) Eq. wt of salts\[=\frac{Formula\,\,mass}{(Valency\,\,of\,\,cation)(No.of\,\,cations)}\]
Expression of strength /concentration of Solution
(i) Mass percent = \[\frac{Weight\,\,of\,\,solute(gm)}{Weight\,\,of\,\,solution(gm)}\times 100\]
(ii) Normality \[=\frac{numer\,\,of\,\,gram\,\,equivalents\,\,of\,solute}{Volume\,\,of\,solution\,(lit.)}\]
(iii) Molarity =\[\frac{Number\,\,of\,\,gram\,\,moles\,\,of\,\,solute}{Volume\,\,of\,\,solution(lit.)}\]
(iv) Molality \[=\frac{Gram\,\,moes\,\,of\,\,solute}{weight\,\,of\,\,solent\,(kg)}\]
(v) Mole fraction: Mole fraction of solute \[={{X}_{A}}=\frac{{{n}_{A}}}{{{n}_{A}}+{{n}_{B}}}\]
Mole fraction of solvent \[={{X}_{B}}=\frac{{{n}_{B}}}{{{n}_{A}}+{{n}_{B}}}\]
\[{{X}_{A}}+{{X}_{B}}=1\]
Chemical reaction: The process in which a substance undergoes change to produce new substances with new properties are known as chemical reaction. For example magnesium carbonate when heated produces magnesium oxide and carbon dioxide (i.e. new substances with new properties). A chemical change is generally accompanied by a change of state, change of colour, evolution of a gas or change of temperature etc.
Chemical equation: The qualitative representation of a chemical reaction in a more...
Chemistry has helped significantly in meeting human needs by providing chemical fertilizers, improved varieties of insecticides and pesticides to increase the yield of crops and fruits. It has given us a large number of lifesaving drugs. Also chemical industries manufacturing polymers, soaps, detergents, glass, ceramics etc.
Industrially Important Compounds Glass
It consists of a mixture of two or more silicates.
Preparation of glass:
Common glass (or soft glass): It is used to make bottles, glass wares etc. and is obtained by heating together silica (in the form of sand), sodium carbonate or sodium sulphate and chalk or lime stone (calcium carbonate). Some broken glass and a little coke are usually added. The glass so prepared consists of silicates of sodium and calcium.
Hard glass: For preparation of hard glass \[{{K}_{2}}C{{O}_{3}}\] is used in place of\[N{{a}_{2}}C{{O}_{3}}\]. It consists of a mixture of calcium and potassium silicates.
Physical properties of glass: Hard, rigid, high viscosity, bad conductor of heat and electricity, brittle, etc.
Blowing: It is a method to cast the molten glass into different moulds. There are two different methods of glass blowing (i) Free blowing and (ii) Mould blowing
Free blowing: It involves the blowing of air to inflate the molten glass which is gathered at one end of the blow pipe to give the desired shape.
Mould blowing: This method was developed after' the technique of free blowing. In this method, molten glass is inflated into a wooden or metal carved mould with the help of blow pipe which gives the molten glass the shape and design of the interior of the mould.
Chemical properties of glass
(i) It is resistant to action of air and acids except hydroflouric acid.
(ii) It is alkaline in nature.
(iii) It slowly reacts with water to form alkaline solution.
Types of Glass
(i) Silica glass: For this type of glass the raw material used is 100% pure form of quartz. It is quite expensive. It is used in the manufacture of laboratory apparatus. It has low thermal expansion. Its softening point is very high and it is resistant to a wide variety of chemicals.
(ii) Alkali silicate glass: For it the raw materials used are sand and soda. It is also called water glass because it is soluble in water and used only as a solution. It is generally used to make gums and adhesives.
(iii) Lead glass: For this type of glass lead oxide is added to ordinary glass. The addition of lead oxide increases the density and also the refractive index. This type of glass is used for the manufacture of ornamental glass ware, decorative articles etc.
(iv) Optical glass: This type of glass is used in the manufacture of optical instruments like binoculars, spectacles, lenses, prisons, telescopes, microscopes etc. It is transparent and can be grounded into the required shape. It generally more...
Symbol of p-n junction diode
p-n junction under forward bias: When p-side is connected to positive terminal and n - side to negative terminal of external voltage, it is said to be forward biased.
The applied voltage V is opposite to build in Potential\[{{V}_{0}}\], hence depletion layer width decreases and barrier height is reduced to\[({{V}_{0}}-V)\]. There is minority carrier injection, hence charges begin to flow. Current is in the order of mA.
(c) p-n junction under reverse bias: When p-side of p-n junction is connected to -ve more... 
