Chemistry — NDA General Knowledge
GK Chemistry is not NDA's version of an entrance-exam organic chemistry paper — there are no reaction mechanisms, no IUPAC nomenclature trees, no mole-concept numericals. It is CBSE Class 9–10 general science: a closed set of definitions, formulas and reaction types that either you recall correctly under a four-mark, four-option, negative-marking clock, or you don't. Learn the compound formulas precisely, keep the reaction-type classification automatic, and treat every "sounds right but isn't quite" option — a swapped formula, a misclassified reaction, a reversed pH direction — as the trap it's designed to be.
1. What NDA actually asks
Weightage: 15% of General Knowledge — behind Physics (25%), Geography (20%) and History & Freedom Movement (20%), but ahead of General Science (10%) and Current Events (10%). GK itself is Part B of the GAT paper: 100 of the GAT's 150 questions, worth 400 of its 600 marks. A 15% share of GK Chemistry therefore works out to roughly 15 of the 100 GK questions — 60 of the 400 GK marks. Every GAT question, English or GK, carries +4 for a correct answer and −1.3333 for a wrong one (1/3 of 4 marks); an unattempted question costs nothing.
Chemistry questions in NDA GK come in three recurring shapes:
- Factual recall — name the chemical formula of a common compound, the discoverer of a subatomic particle, the correct definition of valency or an isotope.
- Correct-statement / classification identification — four short statements or four labelled reactions, only one of which correctly names a reaction type, an indicator's colour change, or a bonding mechanism.
- Simple equation and formula checks — a chemical equation with one option correctly balanced, or a compound formula built from stated valencies; grade 9–10 reasoning, no calculator needed, no multi-step derivation.
Nothing here goes beyond a CBSE Class 9–10 general science syllabus — this is the chemistry a generalist officer is expected to know, not the physical or organic chemistry a science-stream Class 12 student uses in engineering or medical entrance exams.
2. Matter and its changes
States of matter. Matter exists as solid (fixed shape and volume, tightly packed particles), liquid (fixed volume, takes the shape of its container, particles close but able to move past one another) and gas (no fixed shape or volume, particles far apart and moving freely). Changing state changes only the arrangement and energy of particles, not their chemical identity:
| Change | Direction |
|---|---|
| Melting (fusion) | Solid → liquid |
| Freezing | Liquid → solid |
| Boiling/vaporisation | Liquid → gas |
| Condensation | Gas → liquid |
| Sublimation | Solid → gas directly (e.g. camphor, dry ice) |
| Deposition | Gas → solid directly |
Physical vs. chemical change. A physical change alters only the form or state of a substance — no new substance forms, and it is usually reversible (melting ice, dissolving sugar, cutting paper). A chemical change produces one or more new substances with different properties and is usually irreversible (rusting of iron, burning of paper, souring of milk) — it is always accompanied by a chemical reaction.
Elements, compounds and mixtures. An element is a pure substance made of only one kind of atom (e.g. iron, oxygen). A compound is two or more elements chemically combined in a fixed ratio, with properties different from its constituent elements (e.g. water, H₂O). A mixture is a physical combination of two or more substances in any ratio, which retain their individual properties and can be separated by physical means (filtration, evaporation, distillation, sublimation, chromatography, decantation, centrifugation, magnetic separation) — homogeneous mixtures (solutions) have uniform composition throughout; heterogeneous mixtures (suspensions, most colloids) do not.
3. Atomic structure basics
Subatomic particles:
| Particle | Charge | Location | Discovered by |
|---|---|---|---|
| Electron | Negative | Orbits nucleus, in shells | J.J. Thomson |
| Proton | Positive | Nucleus | E. Goldstein |
| Neutron | Neutral | Nucleus | James Chadwick |
Atomic number (Z) = number of protons in an atom = number of electrons in a neutral atom (this uniquely identifies the element). Mass number (A) = number of protons + number of neutrons. So number of neutrons = A − Z. Isotopes are atoms of the same element (same atomic number) with different mass numbers — they differ only in neutron count and have identical chemical properties (e.g. carbon-12, carbon-13, carbon-14).
Valency is the combining capacity of an element, decided by the number of electrons in its outermost (valence) shell. To write a compound's formula, cross the numerical valencies of the two combining elements/radicals as subscripts, then reduce to the simplest ratio:
| Element | Symbol | Common valency |
|---|---|---|
| Hydrogen | H | 1 |
| Oxygen | O | 2 |
| Nitrogen | N | 3 |
| Carbon | C | 4 |
| Sodium | Na | 1 |
| Calcium | Ca | 2 |
| Chlorine | Cl | 1 |
| Aluminium | Al | 3 |
A molecule is two or more atoms chemically bonded together — a molecule of an element (O₂, N₂) contains only one kind of atom; a molecule of a compound (H₂O, CO₂) contains atoms of different elements. An ion is a charged atom or group of atoms — a cation (positive) has lost electrons, an anion (negative) has gained electrons.
4. The periodic table
The modern periodic law (Moseley) states that the properties of elements are a periodic function of their atomic number — refining Mendeleev's original table, which was arranged by atomic mass. The modern periodic table has 18 groups (vertical columns) and 7 periods (horizontal rows).
- Elements in the same group have the same number of valence electrons and therefore similar chemical properties.
- Elements in the same period have the same number of electron shells; properties change progressively across a period.
| Group | Name | Valence electrons | Examples |
|---|---|---|---|
| 1 | Alkali metals | 1 | Li, Na, K |
| 2 | Alkaline earth metals | 2 | Mg, Ca, Ba |
| 17 | Halogens | 7 | F, Cl, Br, I |
| 18 | Noble gases | 8 (2 for He) | He, Ne, Ar |
Metals, non-metals and metalloids:
| Property | Metals | Non-metals |
|---|---|---|
| Physical state | Mostly solid (mercury is liquid) | Solid, liquid or gas |
| Malleability/ductility | Malleable and ductile | Brittle if solid |
| Conductivity | Good conductors of heat and electricity | Poor conductors (graphite is an exception) |
| Lustre | Shiny | Dull (iodine is an exception) |
| Electron behaviour | Lose electrons to form cations | Gain electrons to form anions |
Metalloids — boron, silicon, germanium, arsenic, antimony and tellurium — sit along the "staircase" border between metals and non-metals and show intermediate properties (silicon, for instance, is a semiconductor). General periodic trends: atomic size decreases across a period and increases down a group; metallic character decreases across a period and increases down a group.
5. Chemical bonding basics
Atoms bond to attain a stable, noble-gas-like outer electron configuration.
| Feature | Ionic bond | Covalent bond |
|---|---|---|
| Formed between | A metal and a non-metal | Two non-metals |
| Mechanism | Complete transfer of electrons | Sharing of electron pairs |
| Example | NaCl, MgO | H₂O, CO₂, CH₄ |
| Melting/boiling point | High | Generally low |
| Conducts electricity | Yes, when molten or in solution | Generally no |
In sodium chloride, sodium (2,8,1) loses its single outer electron to become Na⁺ (2,8), and chlorine (2,8,7) gains that electron to become Cl⁻ (2,8,8); the oppositely charged ions then attract each other electrostatically. In water, oxygen shares one electron pair with each of two hydrogen atoms — nothing is transferred, and the resulting molecule is held together by shared, not exchanged, electrons.
6. Chemical reactions and equations
A chemical equation represents a reaction symbolically; it must be balanced — the same number of atoms of each element on both sides — because matter can neither be created nor destroyed (law of conservation of mass).
| Type | General form | Example |
|---|---|---|
| Combination | A + B → AB | CaO + H₂O → Ca(OH)₂ |
| Decomposition | AB → A + B | CaCO₃ →(heat) CaO + CO₂ |
| Displacement | A + BC → AC + B | Fe + CuSO₄ → FeSO₄ + Cu |
| Double displacement | AB + CD → AD + CB | AgNO₃ + NaCl → AgCl↓ + NaNO₃ |
A displacement reaction happens only when the free element is more reactive than the one it replaces, per the reactivity series: K > Na > Ca > Mg > Al > Zn > Fe > Pb > (H) > Cu > Ag > Au (most to least reactive). A double displacement reaction exchanges ions between two compounds, often producing an insoluble precipitate.
Oxidation is the gain of oxygen or loss of electrons; reduction is the loss of oxygen or gain of electrons — the two always occur together (a redox reaction). Rusting and rancidity (fats/oils turning stale) are everyday oxidation reactions. Reactions that release heat are exothermic (combustion, respiration); those that absorb heat are endothermic (photosynthesis, the thermal decomposition of CaCO₃ above).
7. Acids, bases and salts
Acids taste sour, turn blue litmus red, and release H⁺ ions in water; they react with reactive metals to release hydrogen gas and with carbonates to release CO₂. Common examples: hydrochloric acid (gastric juice), citric acid (citrus fruits), acetic acid (vinegar), tartaric acid (tamarind), ascorbic acid (Vitamin C). Bases taste bitter, feel soapy, turn red litmus blue, and release OH⁻ ions in water; a base soluble in water is called an alkali. Common examples: sodium hydroxide (caustic soda), potassium hydroxide (caustic potash), calcium hydroxide (lime water), magnesium hydroxide (milk of magnesia). Neutralisation is Acid + Base → Salt + Water.
The pH scale runs from 0 to 14 and measures H⁺ ion concentration: pH < 7 is acidic (the lower the number, the more strongly acidic), pH = 7 is neutral, and pH > 7 is basic (the higher the number, the more strongly basic). Pure water has pH 7; human blood is about pH 7.4; gastric juice is about pH 1.5–2.
| Indicator | In acid | In base |
|---|---|---|
| Litmus | Red | Blue |
| Phenolphthalein | Colourless | Pink |
| Methyl orange | Red | Yellow |
Everyday salts and compounds:
| Compound | Formula | Everyday use |
|---|---|---|
| Baking soda (sodium bicarbonate) | NaHCO₃ | Antacid, baking, fire extinguishers |
| Washing soda (sodium carbonate) | Na₂CO₃ | Cleaning agent, glass/soap industry |
| Bleaching powder (calcium oxychloride) | CaOCl₂ | Bleaching, disinfecting drinking water |
| Plaster of Paris | CaSO₄·½H₂O | Casts, moulds, sculpture |
| Caustic soda | NaOH | Soap-making, industrial cleaning |
| Quicklime / slaked lime | CaO / Ca(OH)₂ | Cement, whitewashing |
| Common salt | NaCl | Food, raw material for the above |
8. Carbon compounds and fuels
Carbon is tetravalent (valency 4) and can link into long chains, branches and rings — the basis of the vast diversity of carbon compounds. Hydrocarbons contain only carbon and hydrogen: saturated hydrocarbons (alkanes, e.g. methane CH₄) have only single bonds; unsaturated hydrocarbons (alkenes, alkynes) have at least one double or triple bond.
Common fuels: LPG (Liquefied Petroleum Gas) is mainly butane and propane; CNG (Compressed Natural Gas) is mainly methane; petrol, diesel and kerosene are liquid hydrocarbon fractions distilled from crude petroleum; coal is a solid fossil fuel used chiefly in thermal power plants. Complete combustion (adequate oxygen) burns a fuel to CO₂ and H₂O with a clean blue flame; incomplete combustion (limited oxygen) produces poisonous carbon monoxide and soot with a sooty yellow flame.
Other everyday carbon compounds: ethanol (C₂H₅OH — fuel additive, antiseptic, beverages), acetic acid (CH₃COOH — vinegar), and soaps/detergents (sodium or potassium salts of long-chain fatty acids). Carbon also exists as pure allotropes: diamond (each carbon tetrahedrally bonded to four others — extremely hard, poor conductor) and graphite (layered structure, soft, conducts electricity — used in pencils, electrodes and as a lubricant).
9. Everyday and applied chemistry
Metal extraction depends on reactivity: highly reactive metals (Na, Al) are extracted by electrolysis; moderately reactive metals (Fe, Zn) are extracted by reduction with a reducing agent such as carbon (iron ore is reduced in a blast furnace); the least reactive metals (Au, Pt) occur free/native in nature. Rusting is the corrosion of iron into hydrated iron(III) oxide (Fe₂O₃·xH₂O) in the presence of both oxygen and moisture; it is prevented by painting, oiling, galvanising (a protective zinc coating) or alloying.
Common alloys:
| Alloy | Composition | Use |
|---|---|---|
| Brass | Copper + Zinc | Utensils, fittings |
| Bronze | Copper + Tin | Statues, medals |
| Steel | Iron + Carbon | Construction, tools |
| Stainless steel | Iron + Chromium + Nickel + Carbon | Cutlery, surgical instruments |
| Solder | Lead + Tin | Soldering electrical joints |
| Duralumin | Aluminium + Copper + Magnesium + Manganese | Aircraft bodies |
Alloying makes a metal harder, more corrosion-resistant, or otherwise better suited to a purpose than the pure metal alone — exactly why aircraft-grade aluminium and surgical steel are never used in their pure elemental form.
Worked examples
Q1 (Physical vs. chemical change). Which of the following is a chemical change?
Show explanation
Solution. Rusting of iron — it forms a new substance, hydrated iron oxide, and is not simply reversed the way melting ice or dissolving sugar can be.
Q2 (Atomic structure). An atom has atomic number 11 and mass number 23. How many neutrons does it have?
Show explanation
Solution. Neutrons = mass number − atomic number = 23 − 11 = 12.
Q3 (Reaction type). CaCO₃ →(heat) CaO + CO₂ — what type of reaction is this?
Show explanation
Solution. One reactant breaks down into two simpler products under heat — a decomposition reaction.
Q4 (Balancing). Which is the correctly balanced equation for magnesium reacting with hydrochloric acid?
Show explanation
Solution. Mg + 2HCl → MgCl₂ + H₂ — Mg: 1=1, Cl: 2=2, H: 2=2, all balanced.
Q5 (pH scale). Which pH value represents the most strongly acidic solution: 9, 7, 4, or 1?
Show explanation
Solution. pH 1 — the lower the pH, the more strongly acidic; pH 7 is neutral and pH 9 is basic.
Q6 (Everyday compound). What is the chemical formula of baking soda?
Show explanation
Solution. NaHCO₃ (sodium bicarbonate) — not to be confused with washing soda, Na₂CO₃.
11. Common traps
- Confusing atomic number with mass number, or miscalculating neutron count — atomic number is always protons (= electrons in a neutral atom); neutrons = mass number − atomic number, never the mass number alone.
- Assuming any two elements bond covalently, or any metal-nonmetal pair bonds only ionically without checking — ionic bonds form between a metal and a non-metal (electron transfer); covalent bonds form between two non-metals (electron sharing).
- Misclassifying a reaction's type, especially calling a decomposition a combination (they are opposite directions) or a single displacement a double displacement (only one element changes partners in a displacement reaction, not two).
- Leaving an equation unbalanced, or using the wrong compound formula — always check atom counts on both sides, and confirm formulas via the valency crisscross rule rather than guessing.
- Swapping baking soda and washing soda's formulas, or misremembering bleaching powder's — NaHCO₃ (baking soda) and Na₂CO₃ (washing soda) are related but different compounds; CaOCl₂ is bleaching powder, not either of the sodium compounds.
- Reading the pH scale backwards — a lower pH means more acidic, not more basic; pH 1 is strongly acidic, pH 13 is strongly basic, and only pH 7 is neutral.
- Mixing up LPG and CNG's main constituents — LPG is mainly butane/propane; CNG is mainly methane. Swapping these is a recurring NDA distractor.
12. Revision protocol
GK Chemistry rewards a clean, memorised checklist far more than problem-solving — there are no multi-step derivations here, only a fixed set of definitions, formulas and reaction-type rules repeated across differently-worded questions. Rebuild the subatomic-particle table, the atomic-number/mass-number relationship, the four reaction types with one example each, the pH scale's direction, and the everyday compound formulas (baking soda, washing soda, bleaching powder, plaster of Paris) from memory until each takes under five seconds to recall. Then drill the valency crisscross rule and equation-balancing on a handful of common compounds as a five-minute daily exercise; at 15% of GK, this chapter is a smaller share than Physics or Geography, but every one of its questions is just as fast a four-mark gain for a well-drilled candidate, and its closed, factual nature means it decays fast from memory and refreshes just as fast right before the exam.
