Nitrogen is used because it doesn't react with the hot filament. Then, it will carry heat away form the filament to cool it.

Nitrogen is an inactive element as it has a high bond dissociation energy due to the presence of triple bond $N\equiv N$. Hence, viewed as a non-reactive element.

We use Claude's apparatus for the fractional evaporation for the fractional evaporation of liquid air.

$N _2$ does not directly react with chlorine because it is noble unreactive gas at room temperature as its bond dissociation energy is high.

$ CaC _2 + N _2 \rightarrow CaNCN + C$

 $CaNCN$ or $CaCN _2$ is the molecular formula of calcium cyanamide. 

Hydride of nitrogen is called as ammonia. The molecular formula of ammonia is $NH _3$.

Liquid nitrogen is a cryogenic liquid. Like dry ice, the main use of liquid nitrogen is as a refrigerant. 

Nitrogen trichloride $ { NCl } _{ 3 } $ is a dense, volatile, and explosive liquid.

The triple bond is pretty strong. Therefore, it needs a lot of energy to break those bonds and participate in a reaction. So nitrogen is commonly referred to as and used as an inert gas (high abundance & low cost.) Dissociation energy of its molecule is fairly high.

Nitrogen is commonly referred to as and used as an inert gas (high abundance & low cost.) Due to its inert nature it does not react with other compounds. Hence, Nitrogen gas is very difficult to identify because of its nonreactive nature. 

You can't fill a bulb with air or combustion of hot tungsten wire at high temperature will take place in presence of oxygen. You can't maintain vacuum either or external atmospheric pressure will break the glass. So, you have to fill it with non-reactive gas like nitrogen.

The fixation of nitrogen can be brought artificially also by usage of Nitrogen Fertilizers.

Nitrogen is a neutral gas hence, it does not show any change with Litmus and does not change the color of litmus paper.

Nitrogen gas is used in food packaging to prolong shelf life and preserve the freshness, integrity, and quality of the delivered product. The presence of oxygen introduces moisture which can deteriorate the food quality.

Nitrogen exhibits several oxidation states, ranging between -3 to +5, which includes +1.

$NCl _3$ is explosive. 

Kjeldahl's method is a method for the quantitative determination of nitrogen contained in organic substances as well as nitrogen contained in unorganic compounds ammonia and ammonium.

to prepare pure nitrogen in laboratory a mixture of ammonium chloride and sodium nitrite is heated.  The product formed is ammonium nitrite.  This compound is very unstable and breaks into nitrogen and water.

Reaction:
$CaC _2+N _2\overset{\Delta}{\rightarrow}  Ca(N=C=N)$
$BaC _2+N _2\overset{\Delta}{\rightarrow}  Ba ( CN) _2$

Nitrogen fixation is a process in which nitrogen ($N _2$) in the atmosphere is converted into ammonium ($NH _4^+$). Nitrogen fixation, natural and synthetic, is essential for all forms of life because nitrogen is required to biosynthesize basic building blocks of plants, animals and other life forms, e.g., nucleotides for DNA and RNA and amino acids for proteins. 

Nitrogen is not combustible under ordinary circumstances. In fact, the National Fire Protection Association has given nitrogen a flammability rating of zero so it has no effect on burning.

$Ca+C _2\rightarrow CaC _2+N _2\rightarrow  Ca(CN) _2$

Percentage of $N _2$ in following compounds is calculated by formula 
$(atomic weight of N _2 in compound / Atomic weight  the compound)\times100$
So, percentage of $N _2$ in compounds is given as
In ${(NH _4)} _{2}SO _4$ it is 21%
In $Ca{(NO _3)} _{2}$ it is 17%
In $NH _2CONH _2$ it is 47%
In $NH _4NO _3$ it is 35%.

d-orbitals are not available for bonding in N. 
there are not any 2d orbitals 
Electronic configuration: $1s^{2} 2s^{2} 2p^{3}$

Catalyst increases the rate of reaction both in forward and reverse direction by decreasing the activation energy.
In cyanamide process, calcium chloride is used to increase the rate.

Animals need oxygen for breathing. If they don't get oxygen they die.

The bond dissociation energy is inversely proportional to reactivity.
Nitrogen is inactive in normal chemical reactions due to its high bond dissociation energy.

Nitrogen burns with a blue flame and is not very soluble in water.

Usually, compounds comprising of only non-metals tend to have covalent bonds. Therefore, among the given options, only A and C are covalent compounds. However, between the two, $N _2$ exists as a gas. $CCl _4$ is a covalent compound that exists as a liquid at standard temperature and pressure.

Silver nitrate $\displaystyle AgNO _3$ forms a white precipitate of $AgCl$ when added to a solution of $NaCl$. 

Nitrogen is the key element in soil fertilizer. It is essential for plant growth but plants cannot utilize atmospheric nitrogen as it is inert due to presence of triple bond. Hence, it is supplied through fertilizer.

$\displaystyle CaCO _3 + 2HCl \rightarrow CaCl _2 + H _2O + CO _2 \uparrow$
Thus, calcium carbonate reacts with $HCl$ to form carbon dioxide gas.

At STP, one mole of any gas occupies 22.4litres of volume. Nitrogen exists in molecular form as $N _2$. 

Nitrogen occurs in the free state as $N _2$ as it has less size it has more tendency to from pi bonds with same elements and get stability.
Hence option  A is correct.

Nitrogen and oxygen react to form nitrous oxide.

$O _2$ is paramagnetic due to the presence of $2$ unpaired $e^-$ in the $\Pi$-antibonding orbital.

$(NH _4) _2Cr _2O _7\longrightarrow N _2+Cr _2O _3+H _2O$

$NO _2$ is called as a mixed anhydride of nitrogen because it gives a mixture of $ HNO _2$ and $HNO _3$. Hence option (B) is correct .

$(NH _4) _2Cr _2O _7  \rightarrow N _2 + Cr _2O _3 + 4H _2O$
$3Mg + N _2 \rightarrow Mg _3N _2$
Orange solid is $(NH _4) _2Cr _2O _7$
Colourless gas is $N _2$
Green residue is $Cr _2O _3$

Except from Cs, all these metals form metal nitride $Na _3N, Li _3N, Mg _3N _2$.

At high temperatures, nitrogen directly combines with $Mg$ and$ Al$; forms nitride $AlN$ and $Mg _3N _2$.

${ Mg } _{ 3 }{ N } _{ 2 }+{ 6H } _{ 2 }O\rightarrow { 2NH } _{ 3 }+3Mg\left( OH \right) _{ 2 }$

$a−3$
The oxidation state of $N$ in $NaN _3$ is $-1/3$.
The oxidation state of $Na$ is $+1$. So total oxidation state of $3 N$ atoms will be $-1$.

$b−4$
The oxidation state of $N$ in $N _2H _2$ is $-1$.
The oxidation state of $H$ is $+1$. So oxidation state of $N$ atom will be $-1$.

$c−2$
The oxidation state of $N$ in $NO$ is $+2$.
The oxidation state of $O$ is $-2$. So oxidation state of N atom will be $+2$.

$d−1$
The oxidation state of $N$ in $N _2O _5$ is $+5$.
The oxidation state of $O$ is $-2$.
Total oxidation state of $5 O$ atoms will be $5 \times (-2) = -10$.
Total oxidation state of $2 N$ atoms will be $+10$.
So oxidation state of $N$ atom will be $+5$.

Fixation  - conversion of free atmospheric nitrogen into nitrogen compounds.

Mg and Al give nitrides at higher temperature.
$3Mg + N _{2} \rightarrow Mg _{3}N _{2}$
$Al + \frac{1} {2}N _{2} \rightarrow AlN$

Soil fertilizer contains Nitrogen as a key element.
Nitrogenous fertilizers react with moisture in the soil and produce ammonia which is converted to nitrates by soil bacteria and stored as plant food.

Nitrogen gas is absorbed by calcium hydroxide at low temperatures. 

Nitrogen molecule is a non-polar covalent molecule. It has zero dipole moment. Two $N$ atoms in nitrogen molecule have zero electronegativity difference. The bond pairs of electrons are equally distributed between two $N$ atoms.

Nitrogen is a neutral gas hence, it does not show any change with litmus. Nitrogen is inert at atmospheric conditions. Hence, it is noncombustible nor it does not support combustion.  Liquid nitrogen is nitrogen in a liquid state at an extremely low temperature. Hence, at low temperature, nitrogen can be liquefied on compression. Nitrogen is non-poisonous gas and constitutes around 79% of the air.

Kjeldahl method is a method that is used the quantitatuin estimation of nitrogen conlaining in inorganic substances like $NH _3/NH _4^+$, and in organic substances.In kjeldah's method, notrogen is estimated in following steps : 

Solid $N _2O _5$ is a salt, consisting of separated anions and cations. The cation is the linear nitronium ion $NO _2^+$ and the anion is the planar nitrate $NO _3^−$ ion. Thus, the solid could be called nitronium nitrate. Both nitrogen centers have oxidation state +5.

Calcium carbide reacts with nitrogen at high temperature to form calcium cyanamide which is used as fertilizer.
$\displaystyle Ca+C _{2} \rightarrow CaC _{2}\xrightarrow{N _{2}}CaCN _2 + C$

Water gas is a synthesis gas, containing carbon monoxide and hydrogen. It is a useful product but requires careful handling due to its flammability and the risk of carbon monoxide poisoning. 
Producer gas: a gas composed of carbon monoxide, hydrogen, and nitrogen, obtained by passing air and steam through incandescent coke.
Syngas/synthesis gas, is a fuel gas mixture consisting primarily of hydrogen, carbon monoxide, and very often some carbon dioxide.
Marsh gas is methane $CH _4$.