Nitrogen is the first member of group 15 or VA of the periodic table. It consists of five elements Nitrogen (N), Phosphorus (P), Arsenic (As), Antimony (Sb) and Bismuth (Bi). The elements of this group are collectively called pnicogens and their compounds as pniconides. The name is derived from Greek word "Pniomigs" meaning suffocation. Pniconide contain M3− species.
(1) Electronic configuration
(1) Physical state : Nitrogen– (gas), phosphorus – (solid) (vaporises easily), As, Sb, Bi–solids.
Nitrogen is the most abundant gas in the atmosphere. It constitutes about 78% by volume of the atmosphere. Phosphorus is the most reactive element in this group and its yellow form is always kept under water.
(2) Atomic radii : Atomic radii increases with atomic number down the group i.e., from N to Bi due to addition of extra principal shell in each succeding elements.
(3) Ionisation energy : The ionisation values of the elements of this group decreases down the group due to gradual increases in atomic size.
(4) Electronegativity : Generally the elements of nitrogen family have high value of electronegativity.
This value shows a decreasing trend in moving down the group from nitrogen to bismuth.
(5) Non - metallic and metallic character : Nitrogen and phosphorus are non - metals, Arsenic and Antimony are metalloids (semi‐metal) and Bismuth a typical metal.
(6) Molecular state : Nitrogen readily forms triple bond (two pπ –pπ bonds) and exists as discrete diatomic gaseous molecule (N - N) at room temperature. Phosphorus, arsenic and antimony exist in the form of discrete tetra atomic molecules such as P4 , As4 , Sb4 in which the atoms are linked to each other by single bonds.
(7) Melting and boiling points : The melting points and boiling points of group 15 elements do not show a regular trend.
Melting point first increases from N to As and then decreases from As to Bi. Boiling point increases from N to Sb. Boiling point of Bi is less than Sb.
(8) Allotropy : All the members of group 15 except Bi exhibit the phenomenon of allotropy.
(i) Nitrogen exists in two solid and one gaseous allotropic forms.
(ii) Phosphorus exists in several allotropic forms such as white, red, scarlet, violet and black form.
(a) White or yellow phosphorus : White phosphorus is prepared from rock phosphate
Ca3 (PO4)2 , SiO2 and coke which are electrically heated in a furnace.
2Ca3 (PO4)2 + 6SiO2 6CaSiO3 + P4O10
P4O10 + 10C P4 + 10CO
When exposed to light, it acquires a yellow colour.
(b) Red phosphorus : It is obtained by heating yellow phosphorus, between 240 - 250°C in the presence of an inert gas. Yellow phosphorus can be separated from red phosphorus by reaction with NaOH (aq) or KOH (aq) when the former reacts and the latter remains unreacted.
(iii) Arsenic exists in three allotropic forms namely grey, yellow and black. Antimony also exists in three forms, viz., metallic, yellow and explosive.
(9) Oxidation state : The members of the group 15 exhibit a number of positive and negative oxidation states.
(i) Positive oxidation states : The electronic configuration (ns2np3) for the valence shell of these elements shows that these elements can have +3 and +5 oxidation states. In moving down this group, the stability of +3 oxidation state increases. It may be pointed out here that nitrogen does not exhibit an oxidation state of +5, because it fails to expand its octet due to nonavailability of vacant d‐orbitals.
(ii) Negative oxidation states : For example oxidation state of nitrogen is –3. The tendency of the elements to show –3 oxidation state decreases on moving down the group from N to Bi.
(10) Catenation (self linkage) : Elements of group 15 also show some tendency to exhibit catenation. This tendency goes on decreasing in moving down the group due to gradual decrease in their bond (M–M) energies.
(1) Hydrides : All the members form volatile hydrides of the type AH3. All hydrides are pyramidal in shape. The bond angle decreases on moving down the group due to decrease in bond pair–bond pair repulsion.
NH3 PH3 AsH3 SbH3 BiH3
107º 94º 92º 91º 90º
The decreasing order of basic strength of hydrides is as follows : NH3 > PH3 > AsH3 > SbH3 > BiH3 .
The increasing order of boiling points is as follows :
PH3< AsH3< NH3< SbH3.
NH3 is thermally most stable and BiH3 is least stable. This is because in NH3, N – H covalent bond is the strongest due to small size of N atom. Hence, the decomposition temperature of NH3 will be the highest.
The increasing order of reducing character is as follows, NH3< PH3< AsH3< SbH3< BiH3.
(2) Halides : The members of the family form trihalides (MX3) and pentahalides (MX5) . The trihalides are sp3 -hybridized with distorted tetrahedral geometry and pyramidal shape while pentahalides are sp3d - hybridized and are trigonal bipyramidal in shape. The trihalides are hydrolysed by water and ease of hydrolysis decreases when we move down the group. Hence, NCl3 is easily hydrolysed but SbCl3 and BiCl3 are partly and reversibly hydrolysed. NF3 is not hydrolysed due to lack of vacant d‐orbital with nitrogen. PF3 and PF5 are also not hydrolysed because the P–F bond is stronger than P - O covalent bond. The hydrolysis products of the halides are as follows :
NCl3 + 3H2O → NH3 + 3HOCl
PCl3 + 3H2O → H3PO3 + 3HCl
2AsCl3 + 3H2O → As2O3 + 6HCl
SbCl3 + H2O → SbOCl + 2HCl
BiCl3 + H2O → BiOCl + 2HCl
Their basic character follows this decreasing order as NI3 > NBr3 > NCl3 > NF3 . Except NF3 , the trihalides of nitrogen are unstable and decompose with explosive violence. NF3 is stable and inert. NCl3 is highly explosive. Trifluorides and trichlorides of phosphorus and antimony act as Lewis acid. The acid strength decreases down the group. For example, acid strength of tri‐chlorides is in the order ; PCl3 > AsCl3 > SbCl3 .
Nitrogen does not form pentahalides due to non‐availability of vacant d‐orbitals. The pentachloride of phosphorus is not very stable because axial bonds are longer (and hence weaker) than equitorial bond.
Hence, PCl5 decomposes to give PCl3 and Cl2 ;
PCl5 ⇋ PCl3 + Cl2 .
The unstability of PCl5 makes it a very good chlorinating agent. All pentahalides act as lewis acids since they can accept a lone pair of electron from halide ion.
Solid PCl5 is an ionic compound consisting of [PCl4]+, [PCl6]−. [PCl4]+ has a tetrahedral structure, while [PCl6]− has an octahedral structure.
Since, PCl5 reacts readily with moisture it is kept in well stoppered bottles.
PI5 does not exist due to large size of I atoms and lesser electronegativity difference between phosphorus and iodine.
Down the group, the tendency to form pentahalides decreases due to inert pair effect. e.g., BiF5 does not exist.
(3) Oxides : These elements form oxides of the type X2O3 , X2O4 and X2O5.
The acidic strength of oxides :
N2O < NO < N2O3< N2O4< N2O5.
The decreasing order of stability of oxides of group 15 follows as,
P2O5 > As2O5 > Sb2O5 > Bi2O5
The nature of oxides of group 15 elements is as follows,
N2O3 and P2O3 (acidic) ; As2O3 and Sb 2O3 (amphoteric);
(4) Oxyacids : N2 and P4 of this group forms oxyacids which are discussed further. In this chapter.
Anamalous behaviour of Nitrogen
Nitrogen is known to differ form other members of the family because of the following facts,
(1) Its small size (2) Its high electronegativity (3) Its high ionisation energy (4) non - availability of d - orbital in the valence shell. (5) Its capacity to form pπ - pπ multiple bonds.
The main points of difference are,
(i) Nitrogen is a gas while other members are solids.
(ii) Nitrogen is diatomic while other elements like phosphorus and arsenic form tetra - atomic molecules (P4 , As4) .
(iii) Nitrogen form five oxides (N2O, NO, N2O3 , N2O4 and N2O5) while other members of the family form two oxides (tri and pentaoxides).
(iv) Hydrides of nitrogen show H‐bonding while those of other elements do not.
(v) Nitrogen does not show pentacovalency because of absence of d‐orbitals while all other elements show pentacovalency.
(vi) Nitrogen does not form complexes because of absence of d ‐ orbitals while other elements show complex formation e.g., [PCl6]− , [AsCl6]− etc.
(vii) The hydride of nitrogen (NH3) is highly basic in nature while the hydrides of other elements are slightly basic.
(viii) Except for NF3, other halides of nitrogen e.g., NCl3 , NBr3 and NI3 are unstable.
Nitrogen and its compounds
N2 was discovered by Daniel Rutherford. It is the first member of group 15 in the periodic table.
Occurrence : N2, occurs both in the free state as well as in the combined state. N2 occurs in atmosphere to the extent of 78% by volume in free state. N2 is present in many compounds such as potassium nitrate (nitre). Sodium nitrate (Chile salt peter) and many ammonium compounds. N2 is an important constituent of proteins in plants and animals in combined state.
Preparation : It is prepared by the following methods,
(1) Laboratory method : In the laboratory N2 is prepared by heating an aqueous solution containing an equivalent amounts of NH4Cl and NaNO2.
NH4Cl (aq.) + NaNO2 (aq.) N2(g) + 2H2O(l) + NaCl
(2) Commercial preparation : Commercially N2 is prepared by the fractional distillation of liquid air.
Physical properties : N2 is a colourless, odourless and tasteless gas. It is a non - toxic gas. It’s vapour density is 14. It has very low solubility in water.
(1) N2 is neutral towards litmus. It is chemically unreactive at ordinary temp. It is neither combustible nor it supports combustion.
(2) The N – N bond in N2 molecule is a triple bond (N N) with a bond distance of 109.8 pm and bond dissociation energy of 946 kJ mol-1
(3) Combination with compounds : N2 combines with certain compounds on strong heating . eg
CaC2 + N2 CaCN2 + C
Calcium carbide Calcium cyanamide
Al2O3 + N2 + 3C 2AlN + 3CO
Aluminium oxide Al.nitride
Both these compounds are hydrolysed on boiling with water to give ammonia.
CaCN2 + 3H2O CaCO3 + 2NH3
AlN + 3H2O Al (OH)3 + NH3
Therefore, calcium cyanamide is used as a fertilizer under the name nitrolim (CaCN2 + C)
Uses of nitrogen : N2 is mainly used in the manufacture of compounds like NH3, HNO3, CaCN2 etc.
Compounds of nitrogen
(1) Hydrides of nitrogen – Ammonia
Preparation of ammonia : Ammonia is prepared in the laboratory by heating a mixture of NH4Cl and slaked lime, Ca(OH)2
2NH4Cl + Ca(OH)2 CaCl2 + 2NH3 + 2H2O
Moist NH3 gas is dried over quick lime, CaO . However, it cannot be dried over conc. H2SO4, P2O5 because being basic it forms salts with them. Anhydrous CaCl2 also cannot be used because it forms a complex CaCl2 .8 NH3 with it.
Manufacture : (i) Ammonia is manufacture by Haber’s process. A mixture of pure N2 and H2 (in the ratio 1 : 3 by volume) is compressed to 200 - 1000 atmospheres and passed over finely divided Fe (as catalyst) and Mo (as promoter) at 750 K
N2 + 3H2 2NH3 + 93.6 KJ mol−1
Favourable conditions for maximum yield of NH3 are :
(a) excess of reactants ( N2 and H2 ) (b) high pressure (c) low temperature and (d) use of catalyst and a promoter.
(ii) By the hydrolysis of calcium cyanamide (CaCN2) with super-heated steam at 450 K . CaCN2 itself is obtained by heating CaC2 and N2 at 1270 K .
CaC2 + N2 CaCN2 + C
CaCN2 + 3H2O
Properties of NH3: It is a colourless gas with pungent smell, highly soluble in H2O and basic in nature. It liquefies on cooling under pressure to give liquid ammonia (bp. 240K). On heating, it causes intense cooling and hence is used as a refrigerant in ice, factories and cold storages.
It burns in excess of air to give N2 and H2O and is oxidised to NO when passed over heat Pt at 1075 K .
4 NH3 + 3O2 2N2 + 6H2O
4NH3 + 5O2 4NO + 6H2O (ostwald process)
It reduces heated CuO to Cu and Cl2 to HCl (which combines with NH3 to give NH4Cl ).
2NH3 + 3CuO 3Cu + 3H2O + N2
8NH3 + 3Cl2
With excess of Cl2 , it gives NCl3 . With Br2 it gives NH4Br and N2 is set free.
NH3 + 3Cl2 NCl3 + 3HCl
8NH3 + 3Br2 6NH4Br + N2
With I2, it gives nitrogen triiodide ammonia (brown ppt) which is explosive in dry state and decomposes when struck
2NH3 + 3I2 NH3 .NI3 + 3HI
8NH3.NI3 5N2 + 9I2 + 6NH4I
It forms amides with active metals like Na, K etc.
2Na + 2NH3 2NaNH2 + H2
It forms complexes with many substances, e.g.,
[Ca(NH3)6]Cl2 , [Co(NH3)6]Cl2 , [Cu(NH3)4]SO4 ,
[Ag(NH3)2]Cl , [Cd(NH3)4]Cl2 etc.
Its aqueous solution is weakly basic due to the formation of OH− ions,
NH3 + H2O NH4+ + OH−
With sodium hypochlorite in presence of glue or gelatine, excess of ammonia gives hydrazine
2NH3 + NaOCl NH2 .NH2 + NaCl + H2O
It undergoes self ionization in liquid state and acts as a solvent.
Many polar compounds are soluble in liquid ammonia.
With Nessler’s reagent (an alkaline solution of K2Hgl4) , ammonia and ammonium salts give a brown precipitate due to the formation of Millon’s base.
K2HgI4 ⇋ 2KI + HgI2
HgI2 + 2NH3 I − Hg − NH2 + NH4I
2NH2 − Hg − I + H2O NH2 − Hg − O − Hg − I + NH4I
or 2K2HgI4 + NH3 + 3KOH H2N − Hg − O − Hg − I + 7KI + 2H2O
It is used as a refrigerant and in the manufacture of fertilizers.
Strcture of NH3 : The N atom in NH3 is sp3 -hybridized containing a lone pair of electrons due to which the H − N − H bond angle is 107 .5o . As a result NH3 molecule is pyramidal.
(2) Oxides of nitrogen : Nitrogen combines with O2 under different conditions to form a number of binary oxides which differ with respect to the oxidation state of the nitrogen atom. The important oxides are N2O, NO, N2O3 , NO2 , N2O4 and N2O5 . N2O and NO both are neutral. Nitrous oxide ( N2O ) has a sweet taste and its main use is as anaesthetic. When inhaled in mild quantities it causes hysterical laughter so it is also called Laughing gas. Nitric oxide (NO) can be obtained by treating a mixture of sodium nitrite and ferrous sulphate with
dil. H2SO4 . N2O5 is the strongest oxidising agent.
(3) Oxyacids of nitrogen : Oxyacids of nitrogen are HNO2, HNO3, H4N2O4 (Nitroxylic acid) and HNO4 (Pernitric acid), which are explosive.
(i) Nitrous acid (HNO2) : It is prepared by adding ice cold dil.HCl or dil.H2SO4 to a well cooled solution of any nitrite (NaNO2 , Ba(NO2)2 etc.).
NaNO2 + HCl NaCl + HNO2
2KNO2 + H2SO4 K2SO4 + 2HNO2
It oxidises H2S to S, Kl to I2 and acts as a reducing agent in presence of strong oxidising agent, i.e., it reduces acidified KMnO4, K2Cr2O7, H2O2 etc. to Mn2+, Cr3+ and H2O respectively.
(ii) Nitric acid (HNO3) : HNO3 is called aqua fortis. It is prepared in the laboratory by distillation of nitre with conc. H2SO4 .
2NaNO3 + H2SO4
Commercially, it is obtained by Ostwald’s process. In this process, NH3 is first catalytically oxidised to NO which is cooled to about 300K and then oxidised by air to NO2. Absorption of NO2 in water in presence of oxygen gives HNO3
4NH3 + 5O2 4NO + 6H2O
2NO + O2 ⇌ 2NO2
4 NO2 + 2H2O + O2 4HNO3
From air (Birkeland Eyde electric arc process)
N2 + O2⇌ 2NO ; ΔH of = + 135 mol kJ -1
2NO + O2 2NO2
2NO2 + H2O
3HNO2 HNO3 + H2O + 2NO
Properties: It is a very strong acid and decomposes on boiling or in presence of sunlight. It acts as a strong oxidising agent. It oxidises nonmetals and metalloids to their respective oxyacids, i.e., C to H2CO3, S to H2SO4, P to H3PO4, Cl2 to HClO3, As to H3AsO4 (arsenic acid) and Sb to H3SbO4 (antimonic acid), while nitric acid itself is reduced to NO2.
I2 + 10HNO3 2HIO3 +10NO2 + 4H2O
Nitric acid reacts with metals to form nitrates and is itself reduced to NO, N2O, NO2 or NH3 (which further reacts with HNO3 to give NH4NO3) depending upon the concentration of the acid, activity of the metal and the temperature of the reaction.
(i) Very active metals such as Mn, Mg, Ca, etc. give H2 on treatment with very dilute HNO3 (2%).
(ii) Less active metals like Cu, Hg, Ag, Pb etc. give NO with dil. HNO3. Zinc, however, gives N2O with dil HNO3 and NH4NO3 with very dilute HNO3.
Zn + 10HNO3 (dilute) 4 Zn(NO3)2 + N2O + 5H2O
Zn + 10HNO3 (very dilute)
Similarly, Fe and Sn react with dilute nitric acid to give NH4NO3.
(iii) Conc. HNO3 gives NO2 both with active metals (Zn, Pb etc.) and less active metals (Cu, Hg, Ag etc.)
Cu + 4HNO3 (Conc.) Cu(NO3)2 + 2NO2 + 2H2O
Tin is, however, oxidized by conc. HNO3 to metastannic acid (H2SnO3) .
Sn + 4HNO3 H2SnO3 + 4NO2 + H2O
Passivity: Fe, Cr, Ni and Al become passive in conc. HNO3 (i.e., lose their normal reactivity) due to the formation of a thin protective layer of the oxide on the surface of the metal which prevents further action. Nitric acid has no action on noble metals (Au, Pt) but these metals dissolve in aqua regia (3 vol. HCl + 1 vol. HNO3) forming their respective chlorides.
HNO3 + 3HCl 2H2O + NOCl + 2[Cl]
Au + 3[Cl] AuCl3
Pt + 4[Cl]
These chlorides subsequently dissolve in excess of HCl forming their corresponding soluble complexes. Thus,
AuCl3 + HCl HAuCl4
Auric chloride Aurochloric acid
PtCl4 + 2HCl H2PtCl6
Platinic chloride Chloro platinic acid
Sugar on oxidation with nitric acid gives oxalic acid. Nitric acid reacts with glycerine to give glycerol trinitrate or nitro glycerine, with toluene it gives 2, 4, 6-trinitrotoluene (T.N.T.) and with cellulose (cotton) it gives cellulose trinitrate (gun cotton). All these are used as explosives.
Oxyacids of nitrogen
Phosphorus and its compounds
It is the second member of group 15 (VA) of the Periodic table. Due to larger size of P, it can not form stable Pπ - Pπ bonds with other phosphorous atoms where as nitrogen can form Pπ – Pπ bonds.
(1) Occurrence : Phosphorous occurs mainly in the form of phosphate minerals in the crust of earth. Some of these are :
(i) Phosphorite Ca3(PO4)2 (ii) Fluorapatite Ca5(PO4)3 F (iii) Chlorapatite 3Ca3(PO4)2 .CaCl2 (iv) Hydroxyapatite
Ca5(PO4)3OH . Phosphates are essential constituents of plants and animals. It is mainly present in bones, which contains about 58% calcium phosphate.
(2) Isolation : Elemental phosphorus is isolated by heating the phosphorite rock with coke and sand in an electric furnace at about 1770K,
2Ca3 (PO4)2 + 6SiO2 6CaSiO3 + P4O10
P4O10 + 10C
(3) Allotropic forms of phosphorus :
Phosphorus exists in three main allotropic forms,
(i) White phosphorus, (ii) Red phosphorus, (iii) Black phosphorus
(i) White or yellow phosphorus : It is obtained from phosphate rock or phosphorite as explained above. It exists as P4 units where four P atoms lie at the corners of a regular tetrahedron with PPP = 60o. Each P atom is linked to three other P atoms by covalent bonds. there are total six bonds and four lone pairs of electrons present in a P4 molecule of white phosphorus.
Properties : White phosphorus is extremely reactive due to strain in the P4 molecule, poisonous, soft, low melting (317K) solid, soluble in CS2, alcohols and ether. It has a garlic odour. Persons working with white P develop a disease known as Phossy jaw in which jaw bones decay. It turns yellow on exposure to light. Hence, it is also called yellow phosphorus.
It spontaneously catches fire in air with a greenish glow which is visible in the dark
(P4 + 3O2 P4O6) . This phenomenon is called phosphorescence. Because of its very low ignition temperature (303K) , it is always kept under water.
With sulphur it gives tetraphosphorus trisulphide with explosive violence which is used in "strike anywhere matches".
8P4 + 3S8 8P4S3
With metals phosphorus forms phosphides. For example,
P4 + 6Mg 2Mg3P2
With aqueous alkalies, on heating, white phosphorus gives phosphine
It is an example of a disproportionation reaction where the oxidation state of P decreases from 0 to – 3 (in PH3) and increases to +1 (in NaH2PO2)
White phosphorus acts as a strong reducing agent. It reduces HNO3 to NO2 and H2SO4 to SO2 . It also reduces solutions of Cu, Ag and Au salts to their corresponding metals. For examples,
P4 + 8CuSO4 + 14 H2O 8Cu + 8H2SO4 + 4H3PO4
P4 + 20AgNO3 + 16H2O 20 Ag + 4H3PO4 + 20HNO3
(ii) Red phosphorus : It is obtained by heating white phosphorus at 540 − 570 K out of contact with air in an inert atmosphere (CO2 or coal gas) for several hours.
Red phosphorus exists as chains of P4 tetrahedra linked together through covalent bonds to give a polymeric structure as shown.
Due to its polymeric structure, red phosphorus is much less reactive and has m.p. much higher than that of white phosphorus.
Properties : Red phosphorus is a hard, odourless, non poisonous solid, insoluble in organic solvents such as CS2, alcohol and ether. Its ignition temperature is much higher than that of white phosphorus and thus does not catch fire easily. It does not show phosphorescence.
It sublimes on heating giving vapours which condense to give white phosphorus. It is denser than white phosphorus and is a bad conductor of electricity.
It burns in oxygen at 565 K to give phosphorus pentoxide, reacts with halogens, sulphur and alkali metals only when heated forming their corresponding salts.
It does not react with caustic alkalies and this property is made use in separating red phosphorus from white phosphorus.
(iii) Black phosphorus : It is obtained by heating white phosphorus at 470 K under high pressure (4000 – 12000 atm) in an inert atmosphere.
It has a double layered structure. Each layer is made up of zig-zag chains with P − P − P bond angle of 99º . Since it is highly polymeric, it has high density. It is the most stable (inactive) form of phosphorus and has a black metallic luster. It is a good conductor of heat and electricity.
(4) Compounds of phosphorus
(i) Oxides and oxyacids of phosphorus :
Phosphorus is quite reactive and forms number of compounds in oxidation states of –3 , +3 and +5. Phosphorus forms two common oxides namely, (a) phosphorus trioxide (P4O6) and (b) phosphorus penta oxide (P4O10) .
(a) Phosphorus (III) oxide (P4O6) :
It is formed when P is burnt in a limited supply of air, P4 + 3O2 P4O6.
It is a crystalline solid with garlic odour. It dissolves in cold water to give phosphorous acid, P4O6 + 6H2O(cold) 4H3PO3,
It is therefore, considered as anhydride of phosphorus acid.
With hot water, it gives phosphoric acid and inflammable phosphine,
P4O6 + 6H2O (hot)
It reacts vigorously with Cl2 to form a mixture of phosphoryl chloride and meta phosphoryl chloride.
P4O6 + 4Cl2 2POCl3 + 2PO2Cl
Metaphosphoryl chloride Phosphoryl chloride
(b) Phosphorus (V) oxide (P4O10):
It is prepared by heating white phosphorus in excess of air, P4 + 5O2 (excess )
P4O10 is a very strong dehydrating agent. It extracts water from many compounds including H2SO4 and HNO3.
(ii) Oxyacids of phosphorus : Phosphorus forms a number of oxyacids which differs in their structure and oxidation state of phosphorus. These are H3PO2 , H3PO3, H4P2O6 , H3PO4 , (HPO3)n , H4P2O5 , H4P2O7 . From these H3PO2 , H3PO3 are reducing agents. H4P2O5 (pyrophosphoric acid) is dibasic acid.
(HPO3)n is formed by dehydration of H3PO4 at 316ºC .
Oxyacids of phosphorus
(5) Chemical Fertilizers : The chemical substances which are added to the soil to keep up the fertility of soil are called fertilizers.
Types of fertilizers : Chemical fertilizers are mainly of four types,
(i) Nitrogenous fertilizers : e.g. Ammonium sulphate (NH4)2SO4 , Calcium cyanamide CaCN2 , Urea NH2CONH2 etc.
(ii) Phosphatic fertilizers :
e.g. Ca(H2PO4)2 .H2O (Triple super phosphate), Phosphatic slag etc.
(iii) Potash fertilizers : e.g. Potassium nitrate (KNO3), Potassium sulphate (K2SO4)etc.
(iv) Mixed fertilizers : These are made by mixing two or more fertilizers in suitable proportion.
e.g. NPK (contains nitrogen, phosphorus and potassium).
NPK is formed by mixing ammonium phosphate, super phosphate and some potassium salts.