In sigma bond linear overlapping occurs while in pi bond parallel overlapping occurs.
Explanation:
In sigma bond the bonding occurs by the linear overlap of the orbitals whereas in the pi bond parallel overlapping occurs.
Electron density is maximum between the nuclei of bonded atoms in sigma bond as compared to the pi bond.

It is the temperature at which Non-ideal gas exhibits almost ideal behaviour for a considerable range of pressure. Boyle's temperature (TB) is related to Vander Waal’s constant a, b as follows:
T_B = a/R_b

The combination of smoke and fog is called smog. It is of two types:
(a) London smog or sulphurous smog or classical smog: London smog formation is initiated by a mixture of SO₂ particulate and high humidity in the atmosphere. The chemicals present in the particulates catalyse the conversion of SO₂ to SO₃ which then combines with H₂O of the humidity forming a fog of sulphuric acid droplets. These then condense on the surface of the particulate.
2SO₂ + O₂  → 2SO₃
SO₃ + H₂O  → H₂SO₄
This type of smog is seen in the early morning hours of winters months.
(b) Photochemical smog or Los Angeles smog: It is formed when the air contains NO₂ and hydrocarbons and the mixture is exposed to sunlight. As the reaction occurs in the presence of sunlight to form the smog, it is called photochemical smog) This type of smog is formed in the months of summer (strong sunlight) during the daytime when NO₂  and hydrocarbons are present in very large amounts due to heavy vehicular traffic.

Reaction of ethanol with excess concentrated sulphuric acid acting as dehydrating agent at 443 K results in the dehydration of Ethanol leading to the formation of Ethene.
2CH₃CH₂OH {tex}\;\xrightarrow[{H_2SO_4}]{Hot\;Conc}{/tex} 2CH₂=CH₂  + H₂O
Ethanol                      Ethene

 Electrophiles (Electrophilic reagents): Electrophiles are electron loving chemical species having an atom which is deficient in electrons. There are two types of electrophiles:

(a) Positively charged ions or positive electrophiles: These are deficient in electrons and carry a positive charge. For example H⁺ (hydrogen ion) H₃O⁺(hydronium ion),  (Nitronium ion),


(b) Neutral electrophiles. These are electrophilic reagents in which the electron deficient atom does not carry any charge. For example AlCl₃, FeCl₃, BF₃, SnCl₄. Carbenes also act as electrophiles because the carbon in them has only six electrons. 

Since both positively charged and neutral electrophiles are short by a pair of electrons, they have strong tendency to attract electrons from other sources and hence behave a Lewis acids.
 
Electrophiles always attack the substrate molecule at the point of high electron density.
 

The word smog is derived from smoke and fog and is the most common example of air pollution. It reduces visibility and causes breathing issues. Smog falls under particulate pollutants, as it is composed of very minute particles.

CrO₅ is cromium with four peroxy oxygen and one cordinated oxygen. it is butterfly type structure .central cromium surround with 4 peroxy cyclic oxygen and 1 cordinate oxygen. In this oxidation number of cromium is +6

Unit Cell is the smallest part (portion) of a crystal lattice. It is the simplest repeating unit in a crystal. The entire lattice is generated by the repetition of the unit cell in different directions.
Primitive Unit Cells: When the constituent particles occupy only the corner positions, it is known as Primitive Unit Cells.
Centered Unit Cells: When the constituent particles occupy other positions in addition to those at corners, it is known as Centred Unit Cell.

BH₃  molecule is electron deficient in nature as the B atom has only 6 electrons around it so it exist in the form of dimer i.e. B₂H₆. BF₃ is also electron deficient but because of larger size of F atom it can not undergo dimerization.

c) Even though Fluorine has the highest electronegativity among all the  elements and it should have the highest electron gain enthalpy among all  the halogens but  this is an exception and chlorine has higher electron gain enthalpy than Fluorine.   The reason for this is that the size of Fluorine atom is very small and  hence there is very high inter-electronic repulsion among the electrons  of fluorine.  This makes incoming of another electron not very favourable.  Even though fluorine has large negative electron gain enthalpy but for chlorine its even more negative.

Photoelectric effect: J.J. Thomson observed that when light of a certain frequency strikes the surface of a metal, electrons are ejected from the cathode metal. They completed the circuit through the gas in the tube which was otherwise a non-conductor. This phenomenon is called photoelectric effect. Only a few metals show this effect under the action of more energetic ultraviolet light is a discharge tube at low voltage.

Water gas shift reaction involves water vapor reaction with carbon monoxide to give carbon dioxide and hydrogen gas. The reaction is

CO + H₂O  → CO₂ + H₂

Felice Fontana discovered this water gas shift reaction in 1780.  It was made in England in the year 1828.  The water gas was produced by blowing steam through red hot coke, the reaction is   

C + H₂O → CO + H_2.

A substance's solubility product, Ksp, is the mathematical product of its dissolved ion concentrations  with each concentration raised to the power of their stoichiometric coefficients. 
For example the solubility product of ionic compound Ag2S is
Ksp =[Ag⁺]²[S^-2]