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Chemistry · NDA

CN01 — Basic Concepts of Chemistry

📖 Chapter CN01 · NDA Class 11–12 Level 🎯 NDA Level : High Priority

Basic Concepts is the foundation of all chemistry. Questions on matter classification, laws of chemical combination, and mole concept appear consistently in NDA. Average students who can recognise elements vs compounds, apply the mole formula confidently, and recall the laws of conservation can solve most of these questions with straightforward calculation.

📌 What to expect in NDA (based on 2022–2025 pattern):
(1) Physical vs chemical changes; properties of states of matter;
(2) Elements, compounds, mixtures — classification and separation methods;
(3) Chemical symbols, formulae, and balancing equations;
(4) Laws of Chemical Combination — conservation of mass, constant proportions, multiple proportions;
(5) Mole concept — molar mass, Avogadro's number (6.022 × 10²³), stoichiometry calculations.

Topics at a Glance

① Matter & Its Changes

States, physical & chemical changes, properties

② Elements, Compounds & Mixtures

Classification, separation techniques

③ Symbols, Formulae & Equations

Valency, balancing, symbols of elements

④ Laws of Chemical Combination

Conservation, constant proportions, multiple proportions

⑤ Mole Concept

Atomic/molecular weight, molar mass, Avogadro, stoichiometry

1. Matter and Its Changes

1.1

States of Matter — Solid, Liquid, Gas

Everything around us is matter — it has mass and occupies space

Matter exists in three physical states. The difference lies in the arrangement and energy of particles. Temperature and pressure determine the state of a given substance.

Property	Solid	Liquid	Gas
Shape	Definite	No definite (takes container shape)	No definite
Volume	Definite	Definite	No definite (fills container)
Particle arrangement	Closely packed, ordered	Close but disordered	Far apart, random
Intermolecular forces	Very strong	Moderate	Very weak
Compressibility	Negligible	Very low	High
Examples	Iron, ice, NaCl	Water, mercury, ethanol	O₂, CO₂, steam

Fig. 1 — State changes and the names for each transition (heating = green arrows; cooling = orange arrows)

✅ Physical Changes

No new substance formed; original can be recovered
Change of state: melting, boiling, freezing
Dissolving sugar in water (physical, reversible)
Cutting, shaping, stretching a material
Key test: composition remains same

⚡ Chemical Changes

New substance(s) formed with different properties
Generally irreversible (unless by another reaction)
Burning wood, rusting of iron, cooking food
Signs: colour change, gas evolved, precipitate, heat/light
Key test: chemical composition changes

2. Elements, Compounds, and Mixtures

2.1

Classification of Matter

Pure substances vs mixtures — the most-tested distinction in NDA Chemistry

Fig. 2 — Classification of Matter: Pure substances vs Mixtures

⚛ Elements

Made of only one type of atom
Cannot be broken down by chemical means
118 known elements (92 natural)
Examples: H, O, Fe, Au, Na, Cu, Cl
Metals, non-metals, metalloids

🔬 Compounds

Two or more elements chemically combined
Fixed ratio by mass; can be broken by chemical methods
Properties differ from constituent elements
H₂O (H:O = 1:8 by mass); NaCl; CO₂; H₂SO₄
Na is soft metal + Cl₂ is toxic gas → NaCl is edible salt

🔄 Mixtures

Components not chemically combined; variable ratio
Each component retains its own properties
Separated by physical methods
Homogeneous: uniform throughout (alloys, air, sea water)
Heterogeneous: non-uniform (soil, smoke, blood)

📋 Separation Methods (NDA-tested):
Filtration — separates insoluble solid from liquid (sand from water)
Distillation — separates liquids of different boiling points (alcohol-water)
Crystallisation — purifies soluble solids (salt, sugar from impure solution)
Chromatography — separates dyes/pigments based on differential adsorption
Magnetic separation — iron filings from non-magnetic mixture
Centrifugation — cream from milk (different densities)

📝 TOPIC-WISE PYQ

Matter Classification & Changes — NDA Pattern Questions

Q1. Which of the following is a pure substance?

(a) Air (b) Sea water (c) Carbon dioxide (d) Steel

Answer: (c) Carbon dioxide (CO₂)
CO₂ is a compound — a pure substance with fixed composition (C:O = 3:8 by mass). Air is a homogeneous mixture of N₂, O₂, Ar etc. Sea water is a heterogeneous mixture. Steel is an alloy (mixture of iron, carbon, and other metals).

Q2. Rusting of iron is a —

(a) Physical change (b) Chemical change (c) Both physical and chemical (d) None of the above

Answer: (b) Chemical change
Rusting: 4Fe + 3O₂ + 6H₂O → 4Fe(OH)₃. A new substance (iron oxide/hydroxide) is formed. The composition changes permanently — this is irreversible without chemical treatment. Signals: change in colour and physical properties.

Q3. Which separation technique is used to separate cream from milk?

(a) Filtration (b) Distillation (c) Centrifugation (d) Sublimation

Answer: (c) Centrifugation
Centrifugation separates components of different densities by spinning at high speed. Cream (fat) is less dense than the aqueous part of milk; spinning makes the denser component settle outward. Used in dairies, blood banks, and laboratories.

3. Symbols, Chemical Formulae & Balancing Equations

3.1

Chemical Symbols and Common Elements

IUPAC symbols — many are from Latin names (must memorise)

Element	Symbol	Origin	Atomic No.	Valency
Sodium	Na	Natrium (Latin)	11	+1
Potassium	K	Kalium (Latin)	19	+1
Iron	Fe	Ferrum (Latin)	26	+2, +3
Copper	Cu	Cuprum (Latin)	29	+1, +2
Gold	Au	Aurum (Latin)	79	+1, +3
Silver	Ag	Argentum (Latin)	47	+1
Lead	Pb	Plumbum (Latin)	82	+2, +4
Mercury	Hg	Hydrargyrum	80	+1, +2
Tungsten	W	Wolfram (German)	74	+6
Tin	Sn	Stannum (Latin)	50	+2, +4

📌 NDA Trap — Symbols from Latin: Na, K, Fe, Cu, Au, Ag, Pb, Hg, W, Sn — these 10 non-obvious symbols appear repeatedly in NDA. The rest (C, H, O, N, Cl, Ca, Mg, Al, Si, P, S, Zn, Br, I) match the English name and are straightforward.

3.2

Valency, Chemical Formulae & Balancing Equations

Writing and balancing equations — a tested skill in NDA

Valency is the combining capacity of an element — the number of electrons it donates, accepts, or shares to form bonds. Chemical formulae are written using the cross-valency method.

📈 Common Valencies

+1: Na, K, H, Li, Ag, NH₄⁺
+2: Ca, Mg, Fe²⁺, Cu²⁺, Zn, Pb²⁺, Ba
+3: Al, Fe³⁺, Cr³⁺
−1: Cl, F, Br, I, OH⁻, NO₃⁻
−2: O, S²⁻, SO₄²⁻, CO₃²⁻
−3: N³⁻, PO₄³⁻

📋 Writing Formulae (Cross Method)

Aluminium sulfate: Al³⁺ + SO₄²⁻ → Al₂(SO₄)₃
Calcium chloride: Ca²⁺ + Cl⁻ → CaCl₂
Iron(III) oxide: Fe³⁺ + O²⁻ → Fe₂O₃
Magnesium nitrate: Mg²⁺ + NO₃⁻ → Mg(NO₃)₂
Sodium carbonate: Na⁺ + CO₃²⁻ → Na₂CO₃

🔧 Balancing Equations — Hit & Trial Method (NDA approach):
Step 1: Write unbalanced equation with correct formulae.
Step 2: Count atoms of each element on both sides.
Step 3: Add coefficients (whole numbers) to balance — never change subscripts.
Step 4: Verify: atoms equal on both sides; coefficients in lowest ratio.

Example: Burning of magnesium: Mg + O₂ → MgO (unbalanced)
→ 2Mg + O₂ → 2MgO (balanced: 2 Mg, 2 O on each side) ✓

📝 NDA-style balancing examples

1. Electrolysis of water: 2H₂O → 2H₂ + O₂

2. Burning of methane: CH₄ + 2O₂ → CO₂ + 2H₂O

3. Formation of ammonia (Haber): N₂ + 3H₂ ⇌ 2NH₃

4. Iron + steam: 3Fe + 4H₂O → Fe₃O₄ + 4H₂

5. Photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

🧠 TRICKY QUESTIONS

Symbols, Formulae & Equations — Watch Out!

Q. The chemical formula of rust is often stated as Fe₂O₃. What is its correct hydrated form?

Answer: Fe₂O₃·xH₂O (hydrated iron(III) oxide)
Rust is not simply Fe₂O₃. The actual product is hydrated iron(III) oxide, Fe₂O₃·xH₂O, where x varies. This is why iron rusts faster in humid environments — water is a reactant, not just a medium. NDA sometimes tests whether students know rust is hydrated. Plain Fe₂O₃ is iron(III) oxide but anhydrous.

Q. While balancing 2H₂ + O₂ → 2H₂O, a student changed O₂ to O to balance oxygen. Is this correct?

Answer: No — Never change subscripts while balancing.
O₂ means two oxygen atoms bonded together — its molecular formula is fixed. Changing O₂ to O would describe a different substance (monatomic oxygen, which exists only in plasma). Balancing is only done by changing coefficients (the numbers in front of formulae). The correct balanced equation stays as 2H₂ + O₂ → 2H₂O.

4. Laws of Chemical Combination

4.1

The Three Fundamental Laws

Basis of quantitative chemistry — frequently tested in NDA statement-based MCQs

⚛ Laws of Chemical Combination — Summary

LAW 1 — Conservation of Mass (Lavoisier, 1789): "In any chemical reaction, total mass of reactants = total mass of products" Example: 2H₂ + O₂ → 2H₂O 4g H₂ + 32g O₂ = 36g H₂O ✓ (mass conserved) KEY: This is the basis of stoichiometry — "matter is neither created nor destroyed" LAW 2 — Constant (Definite) Proportions (Proust, 1799): "A compound always contains same elements in same ratio by mass, regardless of source" Example: H₂O always has H:O = 1:8 by mass — whether from river, ocean or lab KEY: If ratio changes → different compound (H₂O₂ has H:O = 1:16) LAW 3 — Multiple Proportions (Dalton, 1803): "When two elements form more than one compound, masses of one element that combine with fixed mass of the other are in simple whole number ratios" Example: CO and CO₂ both contain C and O Fix Carbon = 12g → O in CO = 16g; O in CO₂ = 32g → Ratio = 16:32 = 1:2 (simple!)

Gay-Lussac's Law (bonus): Gases combine in simple volume ratios at same T and P. H₂:O₂:H₂O = 2:1:2 by volume. Avogadro's Hypothesis extends this to explain molecular formulae.

📌 Law 1 — Conservation of Mass

Proposed by Lavoisier (1789)
Verified in open and closed systems
Holds for all ordinary chemical reactions
Does NOT hold for nuclear reactions (mass↔energy)
NDA: "burning wood loses mass" — FALSE (gases escape)

📌 Law 2 — Constant Proportions

Proposed by Proust (1799)
Also called Law of Definite Proportions
H₂O: H:O = 1:8 by mass — always
CO₂: C:O = 3:8 by mass — always
Source of sample does not matter

📌 Law 3 — Multiple Proportions

Proposed by Dalton (1803)
Only for pairs of compounds from same elements
CO vs CO₂: O ratio = 1:2
NO vs NO₂: O ratio = 1:2
SO₂ vs SO₃: O ratio = 2:3

📝 TOPIC-WISE PYQ

Laws of Chemical Combination — NDA Pattern Questions

Q1. 5.6 g of iron reacts with sulphur to give 8.8 g of iron sulphide. What mass of sulphur is consumed?

(a) 2.2 g (b) 3.2 g (c) 4.4 g (d) 14.4 g

Answer: (b) 3.2 g
By Law of Conservation of Mass: mass of reactants = mass of products.
Mass of S = mass of product − mass of Fe = 8.8 − 5.6 = 3.2 g

Q2. Carbon combines with oxygen to form CO and CO₂. If 12 g of carbon combines with 16 g oxygen in CO, how much oxygen combines with the same carbon in CO₂?

(a) 16 g (b) 24 g (c) 32 g (d) 48 g

Answer: (c) 32 g
Law of Multiple Proportions: the ratio of oxygen combining with fixed carbon must be a simple whole number ratio (1:2). If CO has 16 g O per 12 g C, then CO₂ must have 2 × 16 = 32 g of O per 12 g C. Ratio = 16:32 = 1:2. ✓

Q3. Water obtained from a river and from electrolysis of acid always has the same ratio of H:O by mass. This illustrates:

(a) Law of Conservation of Mass (b) Law of Constant Proportions (c) Law of Multiple Proportions (d) Gay-Lussac's Law

Answer: (b) Law of Constant Proportions
Proust's law states that a compound always has the same fixed composition by mass regardless of its source or method of preparation. H₂O: H:O = 1:8 by mass — always, everywhere. This is distinct from Law 3 (multiple proportions), which deals with different compounds of the same elements.

🧠 TRICKY QUESTIONS

Laws of Chemical Combination — Conceptual Traps

Q. When magnesium burns in air, the ash (MgO) appears lighter than the original magnesium strip. Does this violate the Law of Conservation of Mass?

Answer: No — the law is NOT violated.
The apparent decrease in mass is because students forget that oxygen from the air is a reactant: 2Mg + O₂ → 2MgO. The MgO formed is actually heavier than the original Mg. The ash may appear to scatter or some may rise as smoke — but if all products (including MgO smoke particles) are collected, total mass equals Mg + O₂ consumed. This is a classic NDA conceptual trap.

5. Mole Concept & Stoichiometry

5.1

Atomic Weight, Molecular Weight & Molar Mass

The bridge between atoms/molecules and measurable grams

Atoms are too tiny to count individually, so chemists use the mole as a counting unit — like a "dozen" for atoms. One mole = 6.022 × 10²³ particles (Avogadro's number, N_A). The molar mass (in g/mol) numerically equals the atomic or molecular weight in amu.

🔮 Mole Concept — Core Formulae

Avogadro's Number: Nₐ = 6.022 × 10²³ mol⁻¹ Number of moles: n = Given mass (g) / Molar mass (g/mol) n = Number of particles / Nₐ n = Volume at STP (L) / 22.4 L (for gases only, STP = 0°C, 1 atm) Number of particles: N = n × Nₐ Mass from moles: Mass = n × Molar mass Molecular mass: Sum of atomic masses of all atoms in the formula Example: H₂SO₄ = 2(1) + 32 + 4(16) = 2 + 32 + 64 = 98 g/mol Percentage composition: % of element X = (mass of X in 1 mol of compound / molar mass) × 100 Example: % O in H₂O = (16/18) × 100 = 88.9%

At STP (Standard Temperature and Pressure): 1 mole of any gas occupies 22.4 litres. This is called the molar volume. At SATP (25°C, 1 bar) it is 24.8 L — NDA uses 22.4 L unless stated otherwise.

🔮 Common Molar Masses (memorise)

H = 1, C = 12, N = 14, O = 16, Na = 23
Mg = 24, Al = 27, S = 32, Cl = 35.5, K = 39
Ca = 40, Fe = 56, Cu = 63.5, Zn = 65
H₂O = 18; CO₂ = 44; NaCl = 58.5
NH₃ = 17; H₂SO₄ = 98; CaCO₃ = 100

⚛ Stoichiometry Approach

Write balanced equation first
Mole ratio from coefficients is the key
Convert given quantity to moles → use ratio → convert to required unit
N₂ + 3H₂ → 2NH₃: 1 mol N₂ gives 2 mol NH₃
Check: use limiting reagent if two reactants given

📝 TOPIC-WISE PYQ

Mole Concept — NDA Pattern Questions

Q1. How many molecules are present in 18 g of water?

(a) 6.022 × 10²³ (b) 3.011 × 10²³ (c) 12.044 × 10²³ (d) 1.8 × 10²³

Answer: (a) 6.022 × 10²³
Molar mass of H₂O = 18 g/mol. 18 g of H₂O = 18/18 = 1 mole.
Number of molecules = 1 × 6.022 × 10²³ = 6.022 × 10²³.

Q2. What volume (at STP) is occupied by 44 g of carbon dioxide?

(a) 11.2 L (b) 22.4 L (c) 44.8 L (d) 4.4 L

Answer: (b) 22.4 L
Molar mass of CO₂ = 12 + 16×2 = 44 g/mol. Moles = 44/44 = 1 mol.
At STP, 1 mol of any gas = 22.4 L.

Q3. How many moles of H₂ are required to completely react with 1 mole of N₂ to form NH₃? (N₂ + 3H₂ → 2NH₃)

(a) 1 mol (b) 2 mol (c) 3 mol (d) 6 mol

Answer: (c) 3 mol
The balanced equation N₂ + 3H₂ → 2NH₃ shows the mole ratio directly: 1 mol N₂ reacts with 3 mol H₂ to give 2 mol NH₃. Mole ratios from balanced equations are the foundation of all stoichiometry — always write the balanced equation first.

Q4. The percentage by mass of oxygen in H₂SO₄ (molar mass = 98 g/mol) is approximately:

(a) 32.7% (b) 49.1% (c) 65.3% (d) 16%

Answer: (c) 65.3%
H₂SO₄: mass of O = 4 × 16 = 64 g/mol.
% O = (64/98) × 100 = 65.3%.
For reference: % S = (32/98)×100 = 32.7%; % H = (2/98)×100 = 2.04%.

🧠 TRICKY QUESTIONS

Mole Concept — Classic NDA Calculation Traps

Q. 1 mole of N₂ and 1 mole of O₂ are mixed. Which statement is correct?
(a) Both have same number of molecules (b) Both have same mass (c) Both occupy same volume at STP (d) Both (a) and (c)

Answer: (d) Both (a) and (c)
1 mole of ANY substance contains the same number of particles = 6.022 × 10²³. ✓
1 mole of ANY gas at STP occupies 22.4 L. ✓
But masses differ: N₂ = 28 g/mol; O₂ = 32 g/mol. So (b) is FALSE.
This question tests whether students confuse moles (fixed count) with mass (element-specific).

Q. How many atoms are in 1 mole of CO₂?

Answer: 3 × 6.022 × 10²³ = 18.066 × 10²³ atoms
Common trap: students answer 6.022 × 10²³ — that's the number of molecules. But each CO₂ molecule has 3 atoms (1 C + 2 O). So total atoms = 3 × Nₐ. Always distinguish between molecules and atoms when moles are given.

📄 CN01 Formula & Fact Sheet — Quick Reference

⚛ Matter & Changes

Physical change: no new substance, reversible
Chemical change: new substance, mostly irreversible
Solid → Liquid: Melting (↑T)
Liquid → Gas: Vaporisation (↑T)
Solid → Gas: Sublimation (e.g. dry ice, iodine, camphor)

🔄 Pure Substances vs Mixtures

Element: one type of atom (e.g. H, O, Fe)
Compound: fixed ratio, chemical combination (e.g. H₂O)
Homogeneous mixture: uniform (air, alloys, sea water)
Heterogeneous mixture: non-uniform (soil, blood, smoke)
Separation: filtration, distillation, centrifugation, chromatography

⚡ Laws of Chemical Combination

Conservation of Mass (Lavoisier): mass reactants = mass products
Constant Proportions (Proust): fixed ratio by mass in compound
Multiple Proportions (Dalton): simple whole number ratio of masses
CO:CO₂ — oxygen ratio = 1:2 (for fixed mass of carbon)
Exception: nuclear reactions violate mass conservation

🔮 Mole Concept Formulae

Nₐ = 6.022 × 10²³ mol⁻¹
n = mass / molar mass
n = particles / Nₐ
n = volume at STP / 22.4
% composition = (mass of element / molar mass) × 100

🔋 Key Molar Masses (g/mol)

H₂O = 18; CO₂ = 44; O₂ = 32; N₂ = 28
NaCl = 58.5; NH₃ = 17; HCl = 36.5
H₂SO₄ = 98; NaOH = 40; CaCO₃ = 100
CH₄ = 16; C₆H₁₂O₆ (glucose) = 180
Ca(OH)₂ = 74; Na₂CO₃ = 106

📌 Latin Symbol Traps

Na (Natrium) = Sodium; K (Kalium) = Potassium
Fe (Ferrum) = Iron; Cu (Cuprum) = Copper
Au (Aurum) = Gold; Ag (Argentum) = Silver
Pb (Plumbum) = Lead; Hg (Hydrargyrum) = Mercury
W (Wolfram) = Tungsten; Sn (Stannum) = Tin

⚡ Quick Revision Booster — CN01 Basic Concepts

🔋 Matter Classification

Pure substance = element OR compound (fixed composition)
Mixture = variable composition, physical separation
Air = homogeneous mixture (NOT a compound)
Alloys = mixtures (not compounds)
Diamond & graphite = both pure carbon (allotropes)

🔬 Physical vs Chemical

Dissolving sugar = physical (solute recoverable)
Burning = chemical (CO₂ + H₂O formed)
Electrolysis of water = chemical (H₂O → H₂ + O₂)
Melting ice = physical (same H₂O molecules)
Fermentation = chemical (new products formed)

📝 Law Shortcuts

Lavoisier → Conservation (same total mass)
Proust → Constant Proportions (same compound, same ratio)
Dalton → Multiple Proportions (different compounds, whole ratios)
Gay-Lussac → Gas volumes (simple ratios at same T, P)
Avogadro → Equal volumes of gases = equal molecules at same T, P

🔮 Mole Shortcuts

18 g H₂O = 1 mol = 6.022×10²³ molecules
22.4 L of any gas at STP = 1 mol
1 mol CO₂ contains 3 × Nₐ atoms (not 1 × Nₐ)
Moles × molar mass = grams (always)
Same moles → same number of molecules (different masses)

📈 Balancing Tips

Never change subscripts — only coefficients
Balance metals first, then non-metals, H and O last
Reduce coefficients to lowest whole number ratio
Fractions allowed temporarily — then multiply through
Check both sides atom by atom before finalising

🚨 Common Traps

Air is a mixture — not a compound (cannot be expressed as formula)
Rust is Fe₂O₃·xH₂O — not just Fe₂O₃
Burning magnesium: MgO is heavier than Mg (O added from air)
1 mole of N₂ has 2Nₐ atoms (diatomic!)
CaCO₃ = 100 g/mol → simplest stoichiometry standard

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