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

Everything around you is made of matter. The air you breathe, the water you drink, the food you eat, the chair you sit on — all of it is matter. Matter is anything that has mass and takes up space.

But not all matter is the same. Some matter is a pure substance. Some matter is a mixture of two or more things. Some pure substances are elements — the simplest building blocks. Others are compounds — elements joined together by chemical bonds.

This chapter teaches you how to tell the difference between elements, compounds, and mixtures. It also teaches you how mixtures can be separated. NDA asks questions from this chapter every year. The most tested topics are: physical vs chemical change, methods of separation, and whether a given substance is an element, compound, or mixture.

1. Matter

Matter is anything that has mass and occupies space. Matter exists in three main states — solid, liquid, and gas. The state of matter depends on temperature and pressure.

All matter is made of tiny particles called atoms. Atoms are the smallest units of an element that can take part in a chemical reaction.

2. Pure Substances and Mixtures

Matter is first divided into two broad categories — pure substances and mixtures.

A pure substance has a fixed composition. It contains only one type of particle throughout. Every sample of a pure substance is identical.

A mixture contains two or more substances mixed together. The substances in a mixture are not chemically joined. They can be separated by physical methods. The composition of a mixture is not fixed — it can vary.

[Image Suggestion: Classification of matter flowchart — Matter branching into Pure Substances (→ Elements, Compounds) and Mixtures (→ Homogeneous, Heterogeneous), showing at a glance how the whole chapter fits together]

3. Elements

An element is the simplest form of a pure substance. It cannot be broken down into simpler substances by any chemical method.

Each element is made of only one type of atom. There are 118 known elements. Some are metals like iron and copper. Some are non-metals like oxygen and sulphur. Some are metalloids like silicon.

Elements are represented by symbols. Fe = iron, Cu = copper, O = oxygen, N = nitrogen, Au = gold, Ag = silver.

4. Compounds

A compound is a pure substance made of two or more elements chemically joined together in a fixed ratio.

The elements in a compound are combined by chemical bonds. They cannot be separated by simple physical methods. You need a chemical reaction to break them apart.

The properties of a compound are completely different from the properties of its elements.

For example, water (H₂O) is a compound of hydrogen and oxygen. Hydrogen is a flammable gas. Oxygen supports burning. But water is a liquid that puts out fire. The compound has entirely new properties.

Property	Element	Compound
Made of	One type of atom	Two or more elements chemically joined
Composition	Fixed	Fixed
Can be broken down?	No — by any method	Yes — by chemical methods only
Properties	Properties of that element	Entirely new properties
Examples	Fe, Cu, O₂, N₂	H₂O, NaCl, CO₂, CaCO₃

Iodised salt is a mixture — not a compound. It is common salt (NaCl) mixed with a small amount of potassium iodide (KI). The two are physically mixed, not chemically joined. [NDA 2010-II]

Glass is not a compound. It is an amorphous solid — a supercooled liquid with no fixed composition. Its composition can vary depending on how it is made. [NDA 2013-II]

5. Mixtures

A mixture contains two or more substances that are physically combined. The substances keep their own properties. They can be separated by physical methods.

Types of Mixtures

A homogeneous mixture looks the same throughout. You cannot see the individual components. Salt dissolved in water is a homogeneous mixture. The salt is spread evenly throughout the water. It is also called a solution.

A heterogeneous mixture does not look the same throughout. You can see or detect the individual components. Soil is a heterogeneous mixture. Sand and iron filings mixed together is a heterogeneous mixture.

Property	Element	Compound	Mixture
Composition	Fixed	Fixed	Variable
Components	One type of atom	Two or more elements (chemically combined)	Two or more substances (physically combined)
Separation	Cannot be separated	Only by chemical methods	By physical methods
Properties	Own properties	New properties	Components keep their own properties
Example	Copper, Oxygen	Water, Salt	Air, Soil, Seawater

6. Physical Change vs Chemical Change

This is one of the most tested topics in this chapter for NDA.

A physical change is a change in the physical appearance or state of a substance. No new substance is formed. The chemical composition does not change. The change can usually be reversed.

Examples of physical change: melting of ice, boiling of water, dissolving sugar in water, cutting paper, bending iron rod, breaking glass.

A chemical change is a change in which one or more new substances are formed. The chemical composition changes. The change is usually not easily reversed.

Examples of chemical change: burning of wood, rusting of iron, cooking food, souring of milk, digestion of food, explosion of crackers.

[Image Suggestion: Side-by-side visual — melting ice (physical change) versus rusting iron (chemical change) — illustrating “no new substance” against “new substance formed”]

How to Identify a Chemical Change

• A new substance is formed
• A gas is produced (bubbles)
• A precipitate forms (solid appears in liquid)
• Heat or light is given out or absorbed
• A permanent colour change happens
• The change cannot be easily reversed

Property	Physical Change	Chemical Change
New substance formed?	No	Yes
Composition changed?	No	Yes
Reversible?	Usually yes	Usually no
Examples	Melting ice, dissolving sugar	Burning wood, rusting iron

When copper sulphate (CuSO₄) is heated, it loses its water of crystallisation and turns white. This appears to be a physical change. But when it reacts with iron (Fe + CuSO₄ → FeSO₄ + Cu), that is a chemical change — a new substance is formed. [NDA 2012-I]

Browning of an apple after cutting is a chemical change. It is caused by oxidation of polyphenolic compounds in the presence of oxygen. [NDA 2012-I]

7. Atomicity

Atomicity is the number of atoms present in one molecule of an element.

Some elements exist as single atoms. These are called monoatomic. Noble gases like helium (He), neon (Ne), and argon (Ar) are monoatomic.

Some elements exist as molecules with two atoms. These are called diatomic. Hydrogen (H₂), oxygen (O₂), nitrogen (N₂), fluorine (F₂), chlorine (Cl₂), bromine (Br₂), and iodine (I₂) are diatomic.

Some elements exist as molecules with four atoms. Phosphorus (P₄) is tetraatomic.

Sulphur is the most interesting. It exists as S₈ — eight atoms per molecule. It is polyatomic. [NDA 2024-II]

Atomicity	Meaning	Examples
Monoatomic	1 atom per molecule	He, Ne, Ar, Kr
Diatomic	2 atoms per molecule	H₂, O₂, N₂, Cl₂, Br₂, I₂
Tetraatomic	4 atoms per molecule	P₄
Polyatomic	More than 4 atoms	S₈

★ IMPORTANT Atomicity at a glance — Monoatomic: He, Ne, Ar (noble gases). Diatomic: H₂, O₂, N₂, F₂, Cl₂, Br₂, I₂. Tetraatomic: P₄ (phosphorus). Polyatomic: S₈ (sulphur). [NDA 2024-II]

8. Methods of Separation of Mixtures

The components of a mixture can be separated using physical methods. The method chosen depends on the properties of the components.

1. Hand Picking — Used when the components are large enough to be picked by hand. Example: separating stones from rice or dal.

2. Threshing — Used to separate grain from stalks after harvesting. The stalks are beaten to loosen the grains.

3. Winnowing — Used to separate lighter components from heavier ones using wind or blowing air. Example: separating husk from grain. The husk is lighter and blown away. The grain falls down.

4. Sieving — Used to separate components of different sizes using a sieve. Example: separating flour from bran, sand from gravel.

5. Sedimentation and Decantation — Sedimentation: heavy, insoluble particles settle at the bottom when a mixture is left undisturbed. Decantation: the clear liquid on top is carefully poured off without disturbing the settled particles.

6. Filtration — Used to separate insoluble solids from liquids. The mixture is passed through a filter paper. Solid particles are trapped on the filter paper. The liquid (filtrate) passes through.

7. Evaporation — Used to separate a dissolved solid from a liquid. The liquid is heated until it evaporates. The solid is left behind. Example: getting salt from sea water.

8. Distillation — Used to separate a liquid from a solution, or to separate two liquids with different boiling points. The mixture is heated. The component with a lower boiling point evaporates first. The vapour is then cooled and collected as a pure liquid (distillate).

9. Fractional Distillation — Used when two or more liquids have close but different boiling points. Example: separating crude oil into petrol, diesel, and kerosene. Separating liquid air into nitrogen and oxygen.

10. Sublimation — Used to separate a substance that directly converts from solid to gas (and back to solid) without becoming liquid. Example: separating ammonium chloride (NH₄Cl) or iodine from a mixture with sand. Camphor, naphthalene, and dry ice (CO₂) also sublime.

11. Chromatography — Used to separate components of a mixture based on how fast they move through a medium. Example: separating colours in ink, separating pigments in leaves. Commonly used in forensic science and food testing.

12. Magnetic Separation — Used when one component is magnetic and the other is not. Example: separating iron filings from sulphur powder using a magnet.

13. Centrifugation — Used to separate components of different densities by spinning at high speed. Example: separating cream from milk, separating blood components.

[Image Suggestion: Labelled diagram of a simple distillation apparatus — round-bottom flask, condenser, and receiving flask — showing vapour rising, cooling, and collecting as distillate]

NDA asked a multi-step separation question: A mixture of sand, common salt, and ammonium chloride can be separated by first using sublimation (to remove ammonium chloride), then filtration (to remove sand), then evaporation (to get salt). [NDA 2010-II]

Substance / Situation	Best Method
Iron filings from sulphur	Magnetic separation
Salt from sea water	Evaporation
Cream from milk	Centrifugation
Ammonium chloride from salt	Sublimation
Petrol from crude oil	Fractional distillation
Husk from grain	Winnowing
Mud from water	Sedimentation + Decantation + Filtration
Colours in ink	Chromatography
Anthracene from salt	Sublimation

9. Important Distinctions

Ore vs Mineral

A mineral is a naturally occurring substance containing a metal. An ore is a mineral from which a metal can be profitably extracted. All ores are minerals, but not all minerals are ores.

Atom vs Molecule

An atom is the smallest unit of an element. A molecule is made of two or more atoms joined together. A molecule can be made of atoms of the same element (O₂) or different elements (H₂O).

Element vs Alloy

An element is a pure substance with only one type of atom. An alloy is a mixture of two or more metals (or a metal and a non-metal). Alloys are mixtures — not compounds. Their composition can vary.

Brass is a mixture of copper and zinc. It is not a compound.

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