Skip to content
TNPSC Guide
தமிழ்EN

Study note + questions

Chemistry: from the atom to everyday chemicals

By TNPSC Guide editorial team · Last updated:

Key numbers

Key facts

Rutherford's gold-foil experiment1911, the nucleus (Std 9, p. 126)
Chadwick, the neutron1932 (Std 9, p. 128)
Avogadro number / molar volume6.023 × 10²³ / 22.4 litre (Std 10, p. 96)
Periodic table118 elements, 7 periods, 18 groups (Std 10, p. 105)
Sixth and seventh periods32 elements each (Std 10, p. 105)
Hall process900°C - 950°C, 5-6 V (Std 10, p. 112)
Aluminiumdensity 2.7, melting point 660°C (Std 10, p. 113)
Copper pyrites / blister copper76 per cent of copper / 98 per cent copper (Std 10, pp. 113-114)
Blast furnace8:4:1; 1500°C / 1000°C / 400°C (Std 10, p. 115)
Pamban bridge1914, India's first sea bridge (Std 10, p. 117)
pHSørensen (Denmark), 1909; scale 0 to 14 (Std 10, p. 146)
Human blood / tooth decaypH 7.35 to 7.45 / saliva below pH 5.5 (Std 10, p. 147)
Glacial acetic acidboils at 391 K (Std 10, p. 162)
Basicityacetic 1, sulphuric 2, phosphoric 3 (Std 9, p. 164)
World population in farming40 per cent, according to the book (Std 9, p. 196)

Easy to confuse

Zinc blende / tinstonefroth flotation / magnetic separation; "zinc blende by magnetic separation" is wrong
Bayer process / Hall processbauxite to alumina / electrolytic reduction of molten alumina
Matte / blister copperCu₂S + FeS / 98 per cent copper
Magnesite / magnetiteMgCO₃ (also printed with another spelling) / Fe₃O₄, an ore of iron
Isotope / isobar / isotonesame atomic number / same mass number / same number of neutrons
Ore / mineralgives the metal at a profit, bauxite / clay is only a mineral
Phenolphthalein / methyl orangecolourless in acid, pink in base / pink in acid, yellow in base
Washing soda / baking sodaNa₂CO₃, hard water / NaHCO₃, baking powder, fire extinguisher
Bleaching powder / plaster of ParisCaOCl₂, bleaching / CaSO₄.½H₂O, broken bones
pH: Denmark / GermanSørensen's country / the language of the word "potenz"
Avogadro number6.023 × 10²³ in the book; do not answer with another value
The full note

Inside the atom

In 1911 Lord Rutherford found the nucleus by aiming alpha rays at a thin gold foil; he chose gold because it is easily beaten into foil. In 1920 Rutherford held that the nucleus has a neutral particle; in 1932 his student James Chadwick found the neutron by striking beryllium nuclei with alpha rays. The fundamental particles: the electron has charge −1 and mass 1/1837 amu (9.1 × 10⁻²⁸ g); the proton +1, 1 amu (1.6 × 10⁻²⁴ g); the neutron no charge, 1 amu (1.676 × 10⁻²⁴ g). The two books name Rutherford's country differently, so remember only the year and the experiment. Example: you have seen gold beaten into thin foil at a jeweller's; that ease of beating is why Rutherford chose gold.

The mole and Avogadro's number

In the SI system one mole is the amount of a substance with as many basic particles as there are atoms in 12 g (0.012 kg) of the carbon-12 isotope. Avogadro's number, named after the Italian scientist Avogadro, is 6.023 × 10²³ in the book. At standard temperature and pressure (S.T.P: 273.15 K and 1 atmosphere) the standard molar volume is 22.4 litre. Same atomic number, different mass number: isotopes (₁₇Cl³⁵, ₁₇Cl³⁷); same mass number, different atomic number: isobars (₁₈Ar⁴⁰, ₂₀Ca⁴⁰); same number of neutrons: isotones (₆C¹³, ₇N¹⁴). Atomicity = molecular mass / atomic mass; relative molecular mass = 2 × vapour density. Example: as we buy eggs by the tray at the market, chemistry counts particles by the mole.

The periodic table

In Mendeleev's table argon (39.95 amu) was placed before potassium (39.10 amu); this was an anomaly. Henry Moseley found that the atomic number is the best basis for arranging the elements. The modern periodic law: "the physical and chemical properties of the elements depend on their atomic numbers". The modern periodic table has 118 elements, 7 periods and 18 groups. The first period (hydrogen, helium) is the shortest; the sixth (Cs to Rn) and the seventh (atomic numbers 87 to 118) have 32 elements each. Groups: 1 alkali metals, 2 alkaline earth metals, 3-12 transition metals, 13 boron family, 14 carbon family, 15 nitrogen family, 16 oxygen or chalcogen family, 17 halogens, 18 noble gases; the lanthanides and actinides are inner transition elements. Example: as a class register orders pupils by roll number, not by height, Moseley ordered the elements by atomic number, not by atomic mass.

Ore and mineral

Clay (Al₂O₃.2SiO₂.2H₂O) and bauxite (Al₂O₃.2H₂O) are both minerals of aluminium; aluminium is extracted from bauxite at a profit, so bauxite is an ore and clay only a mineral. Every ore is a mineral; not every mineral is an ore. A flux is added to remove the gangue (CaO basic, SiO₂ acidic); gangue + flux → slag. Methods of concentration: gravity separation for oxide ores (haematite Fe₂O₃, an ore of iron); magnetic separation (tinstone SnO₂, an ore of tin); froth flotation for sulphide ores, with pine oil (zinc blende ZnS); the chemical method (bauxite). Types of ores: oxide (bauxite, cuprite Cu₂O, haematite Fe₂O₃); carbonate (marble CaCO₃, magnesite MgCO₃, siderite FeCO₃); halide (cryolite Na₃AlF₆, fluorspar CaF₂, rock salt NaCl); sulphide (galena PbS, iron pyrites FeS₂, zinc blende ZnS). Ores found in Tamil Nadu: limestone in Coimbatore, Cuddalore and Dindigul; gypsum in Tiruchi and Coimbatore; titanium minerals in Kanniyakumari, Nellai and Thoothukudi; chromite in Coimbatore and Salem; magnesite in Dharmapuri, Erode, Salem and Tiruvannamalai; tungsten in Madurai and Dindigul. Example: Salem appears in two lists, chromite and magnesite; Coimbatore appears in three, limestone, gypsum and chromite.

Metals and reduction

All metals except mercury and gallium are solid at room temperature. Sodium and potassium are soft enough to cut with a knife and are less dense than water. The oxides of the very reactive Na, K, Ca, Mg and Al are reduced by electrolysis; Zn, Fe, Pb and Cu by chemical reduction with carbon (coke); Ag and Hg by heating strongly until they decompose. Example: mercury spilled from a broken thermometer rolls about as a liquid at room temperature.

Aluminium

Aluminium is the most abundant metal in the Earth's crust; its ores are bauxite Al₂O₃.2H₂O, cryolite Na₃AlF₆ and corundum Al₂O₃, and the chief ore is bauxite. The Bayer process turns bauxite into alumina; the Hall process reduces molten alumina by electrolysis. Its electrolyte is pure alumina + molten cryolite + fluorspar (fluorspar lowers the melting temperature); the temperature is 900°C - 950°C and the voltage 5-6 V. Aluminium is a light, low-density metal (density 2.7) with melting point 660°C. Dilute and concentrated nitric acid form an oxide layer on aluminium, so it does not react with them. Fe₂O₃ + 2 Al → 2 Fe + Al₂O₃ + heat is the aluminothermic (thermite) reaction. Example: aluminium vessels in the kitchen feel light in the hand because of the metal's low density.

Copper

The Romans called copper cuprum because it came from the island of Cyprus. Its ores are copper pyrites CuFeS₂, cuprite or ruby copper Cu₂O and copper glance Cu₂S; the chief ore is copper pyrites, from which 76 per cent of copper is obtained. Smelting gives matte (Cu₂S + FeS, cuprous sulphide and ferrous sulphide) and slag. Heating matte in a Bessemer converter gives blister copper, with 98 per cent copper and 2 per cent impurities, which is purified by electrolysis. Copper is a reddish-brown metal; in moist air it gets a green coating of copper carbonate; below 1370K it forms CuO (black), above it Cu₂O (red). Example: the green coating on a copper pot left unused at home is copper carbonate.

Iron and corrosion

Iron is the most abundant metal in the crust after aluminium; its ores are haematite Fe₂O₃, magnetite Fe₃O₄ and iron pyrites FeS₂, and the chief ore is haematite. Ore, coke and limestone go into the blast furnace in the ratio 8:4:1. The lower part is the combustion zone (1500°C), the middle the fusion zone (1000°C) and the upper part the reduction zone (400°C); the slag is calcium silicate. Rust is hydrated ferric oxide (Fe₂O₃·xH₂O). Electroplating zinc on iron is galvanisation; aluminium is used for anodising; in cathodic protection the metal that corrodes easily is the sacrificial metal. The Pamban bridge is the railway bridge linking Pamban island at Rameswaram with the Indian mainland; opened in 1914, it is India's first sea bridge, and according to the book it was the longest until the Bandra-Worli sea link opened in 2010. Example: the reddish-brown powder on an iron gate at home in the rainy season is rust; a galvanised iron sheet does not rust like that.

The pH scale

The "p" of pH is the German word "potenz", meaning power. The Danish biochemist S.P.L. Sørensen proposed it in 1909; the scale runs from 0 to 14; pH = – log₁₀ [H+]. Acids have pH below 7, bases above 7, and a neutral solution 7. The ionic product of water K_w = [H₃O⁺][OH⁻] is 1.00 × 10⁻¹⁴ at 25 °C. The book's table: HCl (4%) 0, lemon juice 2, vinegar 3, tomato juice 4.2, sour milk 4.5, coffee 5.6, pure water 7, blood plasma 7.4, sea water 8, baking soda 9, milk of magnesia 10, lime water 12, caustic soda (4% NaOH) 14. The pH of human blood is 7.35 to 7.45. Tooth enamel is calcium phosphate; when the pH of saliva falls below 5.5, teeth decay. Dissolved sulphur dioxide and nitrogen oxides lower the pH of rainwater and cause acid rain. Citrus fruits grow in slightly alkaline soil, paddy in acidic soil and sugarcane in neutral soil. Example: in the delta districts paddy and sugarcane fields do not want the same soil; paddy likes acidic soil, sugarcane neutral soil.

Carbon compounds

Functional groups: alcohol –OH (ethanol CH₃CH₂OH); aldehyde –CHO (acetaldehyde CH₃CHO); ketone –CO– (acetone CH₃COCH₃); carboxylic acid –COOH (acetic acid CH₃COOH); ester –COOR (methyl acetate); ether –O–R (dimethyl ether). Alkanes are saturated; alkenes and alkynes are unsaturated; an alkene changes the colour of bromine water, an alkane does not. Pure ethanoic acid forms ice-like crystals when cooled, so it is glacial acetic acid; it boils at 391 K; dilute acetic acid (vinegar) is a food preservative. Ketones are solvents and stain removers; ethers are anaesthetics and painkillers; every cooking oil and lipid has esters. Soaps are the sodium or potassium salts of long-chain carboxylic acids (fatty acids); detergents are sodium salts of sulphonic acid, with an –SO₃H group in place of –COOH. Example: the vinegar added at home to keep pickles from spoiling is dilute acetic acid, and cooking oil has esters.

Acids

Sources of acids: apple malic, lemon citric, grape tartaric, tomato oxalic, vinegar acetic, curd lactic, orange ascorbic, tea tannic, the acid of the stomach hydrochloric, and the sting of ants and bees formic acid. Acids in plants and animals are organic acids (HCOOH, CH₃COOH); acids from rocks and minerals are mineral acids (HCl, HNO₃, H₂SO₄). Methane (CH₄) and ammonia (NH₃) contain hydrogen but are not acids. Basicity: acetic, formic and nitric acid 1; sulphuric acid 2; phosphoric acid 3. Acetic acid has four hydrogen atoms but can replace only one, so it is monobasic. Example: on the dining table curd has lactic, lemon citric, tomato oxalic and tea tannic acid; keep grape as tartaric and do not mix it with tomato.

Indicators and bases

An acid turns blue litmus red; a base turns red litmus blue. Phenolphthalein is colourless in acid and pink in base; methyl orange is pink in acid and yellow in base. Pink comes in both, so watch the medium. Uses of bases: sodium hydroxide for making soap, calcium hydroxide for whitewashing, magnesium hydroxide for stomach upsets, ammonium hydroxide for removing oil stains from clothes. Flame-test colours: brick red Ca²⁺, golden yellow Na⁺, lilac K⁺, green Zn²⁺. Example: in the school laboratory a drop of phenolphthalein in lime water turns pink, and in lemon juice it shows no colour.

Everyday chemicals

Washing soda (Na₂CO₃) softens hard water. Baking soda (NaHCO₃) is used to make baking powder, in soda-acid fire extinguishers and in antacids; baking powder is a mixture of baking soda and tartaric acid. Bleaching powder, calcium oxychloride (CaOCl₂), is a disinfectant and bleaches cotton and linen. Plaster of Paris is calcium sulphate hemihydrate (CaSO₄.½H₂O); it sets broken bones and makes moulds for statues. Example: the cast put on a broken arm at the hospital is plaster of Paris, and washing soda softens hard well water.

Farming and food

Chemical fertilizers: ammonium nitrate, calcium phosphate, urea and NPK (nitrogen, phosphorus, potassium). Pesticides: chlorinated hydrocarbons, organophosphates and carbamates. According to the book, 40 per cent of today's world population works in farming, the largest single employment in the world. Food additives: preservatives (vinegar, sodium benzoate, benzoic acid, sodium nitrite); colours (carotenoids, anthocyanin, curcumin); artificial sweeteners (saccharin, cyclamate); flavour enhancers (monosodium glutamate); antioxidants (vitamin C, E, carotene). Example: the urea a farmer puts on paddy is a chemical fertilizer, and the carbamate he sprays is a pesticide.

Source: அறிவியல், பத்தாம் வகுப்பு, தமிழ் வழி (Science, Standard X, Tamil medium; Samacheer Kalvi) (10ஆம் வகுப்பு அறிவியல், அலகு 7 (அணுக்களும் மூலக்கூறுகளும்) பக்கம் 96, 101; அலகு 8 (தனிமங்களின் ஆவர்த்தன வகைப்பாடு) பக்கம் 104-105, 110-115, 117; அலகு 10 (வேதிவினைகளின் வகைகள்) பக்கம் 146-147; அலகு 11 (கார்பனும் அதன் சேர்மங்களும்) பக்கம் 155, 162, 166; 9ஆம் வகுப்பு அறிவியல், அலகு 11 (அணு அமைப்பு) பக்கம் 126, 128; அலகு 14 (அமிலங்கள், காரங்கள் மற்றும் உப்புகள்) பக்கம் 164, 168, 170; அலகு 16 (பயன்பாட்டு வேதியியல்) பக்கம் 196-197)

PYQ (2012-2025 papers)

Provisional key: until TNPSC publishes the final key; the answer may change.

Free questions

  1. Question 1

    Which of the following is an example of isotones?

    1. A. ₆C¹³, ₇N¹⁴
    2. B. ₁₇Cl³⁵, ₁₇Cl³⁷
    3. C. ₁₈Ar⁴⁰, ₂₀Ca⁴⁰
    4. D. ₇N¹⁴, ₁₇Cl³⁵
    Answer and explanation

    Correct answer: A. ₆C¹³, ₇N¹⁴

    Isotones have the same number of neutrons: ₆C¹³ has 13 − 6 = 7 and ₇N¹⁴ has 14 − 7 = 7.

    Source: அறிவியல், பத்தாம் வகுப்பு, தமிழ் வழி (Science, Standard X, Tamil medium; Samacheer Kalvi) (அலகு 7 அணுக்களும் மூலக்கூறுகளும், p. 101 (PDF p. 109))

  2. Question 2

    According to the textbook, what is the value of the Avogadro number?

    1. A. 6.023 × 10²³
    2. B. 6.023 × 10⁻²³
    3. C. 22.4
    4. D. 273.15
    Answer and explanation

    Correct answer: A. 6.023 × 10²³

    The Avogadro number, named after the Italian scientist Avogadro, is 6.023 × 10²³.

    Source: அறிவியல், பத்தாம் வகுப்பு, தமிழ் வழி (Science, Standard X, Tamil medium; Samacheer Kalvi) (அலகு 7 அணுக்களும் மூலக்கூறுகளும், p. 96 (PDF p. 104))

  3. Question 3

    By the modern periodic law, the physical and chemical properties of elements depend on what?

    1. A. Atomic mass
    2. B. Number of neutrons
    3. C. Vapour density
    4. D. Atomic number
    Answer and explanation

    Correct answer: D. Atomic number

    The modern periodic law: the physical and chemical properties of elements depend on their atomic numbers.

    Source: அறிவியல், பத்தாம் வகுப்பு, தமிழ் வழி (Science, Standard X, Tamil medium; Samacheer Kalvi) (அலகு 8 தனிமங்களின் ஆவர்த்தன வகைப்பாடு, p. 105 (PDF p. 113))

  4. Question 4

    What are the elements of group 17 of the modern periodic table called?

    1. A. Noble gases
    2. B. The chalcogen family
    3. C. Halogens
    4. D. Alkali metals
    Answer and explanation

    Correct answer: C. Halogens

    Group 17 is the halogens, group 18 the noble gases, group 16 the oxygen (chalcogen) family and group 1 the alkali metals.

    Source: அறிவியல், பத்தாம் வகுப்பு, தமிழ் வழி (Science, Standard X, Tamil medium; Samacheer Kalvi) (அலகு 8 தனிமங்களின் ஆவர்த்தன வகைப்பாடு, p. 105 (PDF p. 113))

The unit's 20 free questions

Share this topic

Full guide: TNPSC Group 4 Part B General Studies: 75 questions, 6 units and how to study them

Sources

TNPSC Guide is not the official TNPSC website. Notices: tnpsc.gov.in · Apply: apply.tnpscexams.in