Nitrogen and Phosphorus in Agriculture, Environment & Industry – NDA Chemistry Notes

This chapter covers nitrogen and phosphorus: two elements that are essential for life, essential for agriculture, and regularly tested in NDA. Questions from this chapter are a mix of direct identification, match lists, and assertion-reason pairs.

The chapter is practical and factual. Memorise the key compounds, their properties, and the specific facts that examiners test repeatedly.

1. Nitrogen

Nitrogen is the most abundant gas in the atmosphere, making up 78% of air by volume. It is a colourless, odourless, tasteless gas. It does not support combustion and does not burn itself.

Nitrogen molecules have a very strong triple bond (N≡N). This bond requires enormous energy to break. This is why nitrogen is chemically inert under normal conditions. It does not react easily with other substances at room temperature.

Despite being so abundant in air, plants and animals cannot use nitrogen gas directly. Nitrogen must first be converted into usable compounds, nitrates (NO₃⁻) or ammonium compounds (NH₄⁺), before living organisms can use it.

The Nitrogen Cycle

Nitrogen moves between the atmosphere, soil, and living organisms in a continuous cycle.

Nitrogen from air cannot be used directly by most plants. Nitrogen-fixing bacteria, such as Rhizobium, live in the root nodules of leguminous plants (peas, beans, lentils). These bacteria convert atmospheric nitrogen (N₂) into ammonia (NH₃) and then into nitrates. Plants absorb these nitrates from the soil. Animals get nitrogen by eating plants or other animals.

When organisms die, decomposer bacteria break down their nitrogen compounds and release nitrogen back into the soil and eventually into the atmosphere.

This process of converting atmospheric nitrogen into compounds that plants can use is called nitrogen fixation.

Oxides of Nitrogen

Nitrogen forms several oxides. Each has distinct properties and each is asked in NDA.

Oxide	Formula	Common Name	Nature	Key Fact
Nitrous oxide	N₂O	Laughing gas	Colourless, sweet smell	Anaesthetic in dentistry [NDA 2010-I]
Nitric oxide	NO	—	Colourless, toxic	Formed in diesel engines at high temperature [NDA 2010-II]
Nitrogen dioxide	NO₂	—	Brown/reddish-brown gas	Causes acid rain
Dinitrogen pentoxide	N₂O₅	—	White solid	Anhydride of nitric acid [NDA 2016-I]

N₂O₅ is the anhydride of nitric acid. This means when N₂O₅ dissolves in water, it forms nitric acid: N₂O₅ + H₂O → 2HNO₃. [NDA 2016-I]

Nitrogen dioxide (NO₂) dimerises: two NO₂ molecules combine to form N₂O₄ (dinitrogen tetroxide). [NDA 2025-I]

Nitric oxide (NO) is produced when nitrogen and oxygen from air react at very high temperatures, for example in diesel engines. [NDA 2010-II] This explains why diesel engines produce more NO pollutants than petrol engines: diesel burns at a higher temperature at which N₂ and O₂ from air combine.

Acid Rain

Oxides of nitrogen (NO and NO₂) dissolve in rain water to form nitric acid.

Oxides of sulphur (SO₂, SO₃) similarly dissolve to form sulphuric acid.

Both these oxyacids make rain water acidic. This is acid rain. 

Normal rain water is slightly acidic (pH ~5.6) because of dissolved CO₂. Acid rain has pH below 5.6, sometimes as low as 4 or even lower.

Oxides of nitrogen are responsible for acid rain, not oxides of carbon. CO₂ makes rain slightly acidic naturally, but does not cause acid rain in the harmful sense.

Ammonia (NH₃)

Think of the strong smell inside a public toilet. That is ammonia. It is one of the most important industrial chemicals in the world.

Ammonia is a colourless gas with a sharp, pungent smell. It is lighter than air. It is highly soluble in water. When dissolved in water it forms ammonium hydroxide (NH₄OH), a weak base.

Industrial Manufacture (Haber Process): [NDA 2009-I]

N₂ + 3H₂ ⇌ 2NH₃

Conditions: Iron catalyst | Temperature: 400–500°C | Pressure: 200 atm.

Nitrogen comes from the air. Hydrogen comes from natural gas (methane). The reaction is reversible, moderate temperature and high pressure are used to get a good yield.

Uses of Ammonia:

• Manufacturing fertilisers: urea, ammonium nitrate, ammonium sulphate
• Manufacturing nitric acid (via Ostwald process)
• Refrigerant (liquid ammonia)
• Cleaning agent (dilute ammonium hydroxide solution)

Nitric Acid (HNO₃)

Nitric acid is made industrially from ammonia by the Ostwald process.

Step 1: 4NH₃ + 5O₂ → 4NO + 6H₂O (ammonia oxidised to nitric oxide, over platinum catalyst at 850°C). Step 2: 2NO + O₂ → 2NO₂ (nitric oxide oxidised to nitrogen dioxide). Step 3: 4NO₂ + O₂ + 2H₂O → 4HNO₃ (nitrogen dioxide absorbed in water to give nitric acid).

Uses of Nitric Acid:

• Making fertilisers (ammonium nitrate)
• Making explosives (TNT, dynamite, nitroglycerin)
• Making dyes and plastics
• Component of aqua regia (with HCl, dissolves gold and platinum)

Nitric acid is used in the production of fertilisers.

Nitrogen Fertilisers

This is the most asked topic in this chapter for NDA. Examiners ask which compound has the MOST or LEAST percentage of nitrogen by mass.

Fertiliser	Formula	% Nitrogen by Mass
Urea	CO(NH₂)₂	46.7%, HIGHEST
Ammonium nitrate	NH₄NO₃	35%
Ammonium sulphate	(NH₄)₂SO₄	21.2%
Calcium ammonium nitrate (CAN)	NH₄NO₃ + CaCO₃	~26%
Ammonium phosphate	(NH₄)₃PO₄	~28% (least among phosphorus-containing)

Urea has the highest percentage of nitrogen among common fertilisers. [NDA 2009-I]

Which has the LEAST percentage of nitrogen? Among the options NDA gives, (NH₄)₃PO₄ (ammonium phosphate) has the least because phosphorus and oxygen make up most of its mass. [NDA 2012-I]

Macro Nutrients from Inorganic Fertilisers

The three primary macro nutrients provided by inorganic fertilisers are: Nitrogen (N), Phosphorus (P), and Potassium (K), commonly called NPK.

These are called macronutrients because plants need them in large quantities. Other nutrients like iron, manganese, and zinc are micronutrients, needed in small quantities.

Calcium Ammonium Nitrate (CAN) is a popular nitrogen fertiliser because it is a slow supplier of nitrogen. [NDA 2013-II] It releases nitrogen gradually, which prevents runoff and provides sustained nutrition to crops.

Excessive use of nitrogen fertilisers can cause nitrates to leach into groundwater, making it toxic. 

Gunpowder (Black Powder)

Gunpowder is one of the oldest known chemical explosives. It contains three components.

Composition of gunpowder: Potassium nitrate (KNO₃): 75% + Charcoal (Carbon): 15% + Sulphur: 10%.

Potassium nitrate (KNO₃) is also called saltpetre. It is the oxidising agent in gunpowder. It provides oxygen to support combustion of carbon and sulphur at a rate fast enough to cause an explosion.

The presence of sulphur in gunpowder decreases the ignition temperature, making it easier to ignite. [NDA 2013-II]

Gases evolved during gunpowder explosion: When gunpowder explodes, nitrogen and carbon dioxide are evolved in the proportion of 1:3 (N₂:CO₂). [NDA 2011-II]

2. Phosphorus

Phosphorus is a non-metal that exists in several allotropic forms. The most important allotropes for the exam are white phosphorus and red phosphorus.

Property	White Phosphorus	Red Phosphorus
Colour	White/yellow-white waxy solid	Dark red powder
Reactivity	Highly reactive	Much less reactive
Ignition	Catches fire spontaneously in air	Does not catch fire spontaneously
Toxicity	Very toxic	Non-toxic
Smell	Garlic-like smell	Odourless
Storage	Stored under water (to prevent ignition)	Can be stored in open air
Phosphorescence	Shows phosphorescence in dark	Does not show phosphorescence
Use	Not used in safety matches	Used in safety matches

White phosphorus is highly reactive and catches fire spontaneously when exposed to air. This is why it is stored under water.

Red phosphorus is used in the manufacture of safety matches.

Red phosphorus is used in safety matches because at ordinary temperature it is less reactive than other varieties of phosphorus. It does not ignite spontaneously. When the match head is struck against the rough surface (which contains red phosphorus), friction generates enough heat to ignite the match.

White phosphorus, despite being more reactive, is NOT used in safety matches because it would ignite too easily, even at normal temperatures, making it dangerous.

Phosphorus Compounds

Phosphoric Acid (H₃PO₄):

• Triprotic acid, can donate three H⁺ ions
• Used in making fertilisers and food additives (acidity regulator in soft drinks)

Superphosphate of Lime: Superphosphate is a phosphorus fertiliser made by treating calcium phosphate (rock phosphate) with sulphuric acid.

Ca₃(PO₄)₂ + 2H₂SO₄ → Ca(H₂PO₄)₂ + 2CaSO₄

The product Ca(H₂PO₄)₂, calcium dihydrogen phosphate, is the active fertiliser component.

The metal present in superphosphate is calcium (Ca). [NDA 2010-I]

Superphosphate of lime can be assimilated (absorbed) by plants because it is soluble in water. Ordinary calcium phosphate (rock phosphate) is insoluble and cannot be absorbed by plants.

NPK Fertilisers: Modern fertilisers often combine all three macro nutrients, nitrogen, phosphorus, and potassium, in one product. These are called NPK fertilisers or compound fertilisers.

Dolomite powder (CaMg(CO₃)₂) is applied to agricultural land to increase the pH of the soil. It neutralises acidic soil.

Phosphorus in Matches

Safety matches use red phosphorus on the striking surface (rough strip on the side of the matchbox). The match head contains antimony sulphide and an oxidising agent. When struck, friction between the match head and the red phosphorus strip generates enough heat to ignite the match.

Old-style (non-safety) matches used white phosphorus, which was highly toxic and caused a disease called “phossy jaw” (phosphorus necrosis of the jaw) in match factory workers. White phosphorus was banned in matchmaking in the early 20th century.

3. Key Nitrogen Compounds

Compound	Formula	Key Property / Use
Ammonia	NH₃	Pungent gas, Haber process, fertilisers
Nitric acid	HNO₃	Strong acid, fertilisers, explosives, Ostwald process
Urea	CO(NH₂)₂	Highest % nitrogen fertiliser (46.7%)
Ammonium nitrate	NH₄NO₃	Fertiliser + explosive
Ammonium sulphate	(NH₄)₂SO₄	Common nitrogen fertiliser
Potassium nitrate	KNO₃	Saltpetre, oxidising agent in gunpowder
Nitrous oxide	N₂O	Laughing gas, anaesthetic
Nitrogen dioxide	NO₂	Brown gas, acid rain
Dinitrogen pentoxide	N₂O₅	Anhydride of nitric acid

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