General Science - Chemistry

Chemistry is a major component of General Science in SSC CGL General Awareness. Every year 2 to 4 questions come directly from Chemistry covering atomic structure, periodic table, acids and bases, metals, organic chemistry and chemical reactions. Students who master these topics consistently score well in this section. Chemistry is closely connected to everyday life - understanding common substances like baking soda, vinegar, bleaching powder and acids found in food helps answer questions easily.

1. Chemistry - An Introduction

1.1 Dalton's Atomic Theory

• John Dalton proposed the Atomic Theory in the early 19th century
• Key idea: Every chemical element is made of a unique kind of atom and atoms of different elements differ in their mass
• In 1808, Dalton presented the physical picture of how two or more different kinds of atoms combine in a definite ratio to form compounds
• Example: Oxygen and Hydrogen combine in 1:2 ratio (by number of atoms) to form water

1.2 Metals, Non-metals and Metalloids

• Elements that show properties of both metals and non-metals are called Metalloids
• Metalloids: Boron (B), Silicon (Si), Antimony (Sb)
• Aluminium, Mercury, Copper are metals - NOT metalloids

1.3 Change in State of Matter

• Evaporation rate increases when atmospheric humidity decreases
• Process of liquid to gas conversion: Evaporation (Vaporization)
• Solution: A homogeneous mixture of two or more components

1.4 Avogadro's Law

• At the same temperature and pressure, all gases have the same number of molecules in equal volumes
• Avogadro's number = 6.023 × 10²³
• One mole of an ideal gas occupies 22.4 litres at STP (Standard Temperature and Pressure)

1.5 Gas Laws

2. Atomic Structure

2.1 Atom and Its Fundamental Components

• Electron was discovered by J.J. Thomson in 1897
• Proton was discovered by Rutherford
• Neutron was discovered by James Chadwick in 1932

2.2 Atomic Number and Mass Number

2.3 Isotopes

• Isotopes: Atoms of the same element that have the same atomic number but different mass numbers
• Hydrogen has three naturally occurring isotopes:
  • Protium (¹H¹): 1 proton, 0 neutrons
  • Deuterium (¹H²): 1 proton, 1 neutron
  • Tritium (¹H³): 1 proton, 2 neutrons
• Scandium has 13 isotopes

3. Periodic Classification of Elements

3.1 Mendeleev's Periodic Table

• Dmitri Mendeleev (Russian chemist) created the Periodic Table in 1869
• Elements arranged by increasing atomic weight
• Mendeleev could NOT give proper position to Hydrogen in the periodic table

3.2 Modern Periodic Table

• 7 periods and 18 groups
• Period 1: 2 elements; Periods 2 and 3: 8 each; Periods 4 and 5: 18 each; Period 6: 32 elements; Period 7: incomplete
• Last element of Period 5: Xenon (Xe)

3.3 Groups of Periodic Table

• Noble/Inert gases: Do not participate in chemical reactions - have 8 electrons in outermost shell
• Noble gases in light bulbs: Argon is used in light bulbs - reduces oxidation of tungsten filament extending bulb life
• Hydrogen matches properties of both Group 1 and Group 17
• Silicon (Si) belongs to Period 3 - not related to Period 6
• Lanthanides are also called d-block elements - Actually Lanthanides are f-block elements called Rare Earth Elements (atomic numbers 57-71)

3.4 Newlands' Law of Octaves

• John Newlands (British scientist) arranged known elements in increasing atomic mass in 1866
• Started from Hydrogen - ended at the 56th element Thorium
• Found that every 8th element had similar properties to the first - compared this to musical octaves
• Called "Newlands' Law of Octaves"

3.5 Important Elements and their Symbols

4. Valency and Chemical Bonding

• Argentic (Silver) valency: Usually +1, sometimes +2 when it shows transition properties
• Silver atomic number: 47

5. Oxidation and Reduction

5.1 Rusting of Iron

• Three components are necessary for rusting:
  1. Iron
  2. Oxygen
  3. Moisture (Water)
• When iron comes in contact with moisture and oxygen, brown-coloured iron oxide (Fe₂O₃) forms
• Galvanization: Process of coating iron with zinc to prevent rusting

6. Electrochemical Activity Series

• Metals arranged in decreasing order of reactivity:
• Li > K > Ba > Sr > Ca > Na > Mg > Al > Mn > Zn > Cr > Fe > Cd > Co > Ni > Sn > Pb > Cu > Hg
• Most reactive metal among common ones: Calcium (Ca)

7. Acids, Bases and Salts

7.1 Acids

• Substances that produce hydrogen ions (H⁺) in their aqueous solutions
• Acids turn blue litmus RED
• Bases turn red litmus BLUE

Important Acids and their Sources:

Acid-Food Match (Frequently Asked):

Cutting Onions and Tears:

• When we cut onions, the enzyme Synthase converts amino acid sulphoxide of onion into Sulphenic Acid
• Sulphenic acid reacts with air to form propanethial-S-oxide which reaches eyes causing lachrymation (tears)

7.2 Salts

• Water with low salinity floats on top of water with high salinity

8. Periodic Table - Non-metals and Compounds

8.1 Hydrogen

• Hydrogen matches properties of Group 1 and Group 17
• Discovered by Henry Cavendish in 1766
• Called Fuel of the Future
• Only element whose nucleus has no neutron (Protium isotope)
• Used in: Fuel, production of vegetable ghee, rocket fuel

Removal of Permanent Hardness of Water:

• Permanent hardness of water is caused by dissolved sulphates, chlorides and nitrates of Calcium and Magnesium
• Cannot be removed by boiling
• Removed by adding Washing Soda (Sodium Carbonate - Na₂CO₃)

8.2 Nitrogen

• Catenation property is strongest in Carbon
• Nitrogen is a diatomic element (N₂)
• Nitrous Oxide (N₂O) chemical name is Laughing Gas

8.3 Inert Gases

• Argon is the inert gas commonly found in light bulbs

8.4 Carbon

• Dry Ice is the solid form of Carbon Dioxide (CO₂)
• Called dry ice because it does not convert to liquid - directly sublimates from solid to gas at atmospheric pressure and -78.7°C
• Used as refrigerant in food preservation
• Buckminsterfullerene: An allotrope of Carbon (C₆₀)
  • Carbon atoms join in hexagonal or pentagonal shapes forming a cage-like structure
  • Most common form: Buckminsterfullerene (C₆₀)

9. Metals and Their Compounds

9.1 Sodium Compounds

• Sodium Benzoate is the most popular preservative used in sauces, salad dressings and beverages

9.2 Calcium Compounds

• Plaster of Paris: Used in bone fractures, making toys, decorative materials, smoothing wall surfaces
• Limestone: Sedimentary rock made mainly of various crystalline forms of Calcium Carbonate (CaCO₃)

9.3 Aluminium Compounds

• Aluminium Chloride (AlCl₃) is NOT used as a chemical coagulant in water treatment
• Aluminium Sulphate IS used as coagulant in water treatment

9.4 Silver

• Modern Olympic gold medals: Primary component is Silver (Ag) - only 1.34% pure gold; 92.05% silver and 6.61% copper

9.5 Gold

• Most malleable and ductile metal: Gold
• Second best conductor of heat and electricity after silver

9.6 Copper

• Only antibacterial metal: Copper (Cu) - atomic number 29
• Used in electrical coating, electrical cells and alloy making

9.7 Zinc and Galvanization

• In Galvanization (electroplating): A layer of zinc is deposited on iron
• This stops corrosion of the metal

9.8 Tin

• Lowest melting point among: Tin (231.9°C) < Lead (327.5°C) < Zinc (419.5°C) < Silver (961.8°C)
• Tin has the lowest melting point

9.9 Magnesium

• Epsom Salt: Magnesium Sulphate (MgSO₄·7H₂O)
• Colourless crystalline solid - used in cotton industry, soap and paint industry

10. Alloys

11. Ores and Metallurgy

Ores of Iron:

• Magnetite: Fe₃O₄
• Haematite: Fe₂O₃
• Siderite: FeCO₃
• Cuprite (Cu₂O) is an ore of Copper - NOT iron

12. Fuels

• First person to isolate Methane gas: Alessandro Volta who discovered that a mixture of methane and air could be exploded by an electric spark
• Methane is a colourless, odourless, non-toxic but flammable gas
• Common name: Marsh Gas (found in marshes/wetlands)
• Methane is also a greenhouse gas
• Hydrogenation: Conversion of unsaturated hydrocarbons to saturated using Nickel or Palladium as catalyst
  • Used to convert vegetable oils to vegetable ghee

13. Soap and Detergents

• Detergent powder contains Sodium Carbonate (Na₂CO₃) as the main chemical
• Sodium Carbonate also known as Washing Soda, Soda Ash, Soda Crystal
• Sodium Alkyl Sulphate is also present in detergents

14. Glass and Cement

• Silica content in Cement: Approximately 17% to 25%
• Removing green colour from glass: Manganese Dioxide (MnO₂)
• Sulphur Dioxide used as germicide and bleaching agent

Colours in Glass:

15. Organic Chemistry

15.1 Nomenclature

• Functional group -OH: Compound belongs to Alcohol class
• Ethanol (CH₃CH₂OH): Formula CH₃CH₂OH
• Chloroform: Chemical name Trichloromethane (CHCl₃)
• Acetic Acid: CH₃COOH
• Methane: CH₄

Important Formulas:

15.2 Hydrocarbons

• Alkanes: CₙH₂ₙ₊₂ (saturated - single bonds)
• Alkenes: CₙH₂ₙ (one double bond)
• Alkynes: CₙH₂ₙ₋₂ (one triple bond) - highest boiling point due to more energy needed to break triple bonds
• Catenation: Ability of an atom to bond with atoms of the same element - Carbon has maximum catenation ability

15.3 Alcohols

• Organic compounds containing functional group -OH
• Types: Mono, Di, Tri, Poly hydrolic alcohols (based on number of OH groups)

15.4 Important Acids in Food

Vinegar = Synonym for Ethanoic Acid (Acetic Acid) - used in kitchens worldwide as acidic seasoning

15.5 Aldehydes

• Organic compounds with functional group -CHO
• General formula: CₙH₂ₙO

15.6 Esters

• Ethyl Acetate (Ethyl Ester): Has a very strong fruity smell - colourless organic compound
• Made by: C₂H₅OH + CH₃COOH → CH₃COOC₂H₅ + H₂O (in presence of conc. H₂SO₄)

15.7 Benzene

• Hydrocarbon with formula C₆H₆
• 6 carbon atoms form a ring like a hexagonal cage
• Each carbon has 1 hydrogen atom
• Bond angle in Benzene = 120°
• Discovered by Michael Faraday in 1825 in illuminating gas

16. Chemical Reactions

• CH₃CH₂OH + O₂ → CO₂ + H₂O + Heat and Light (combustion reaction)

17. Miscellaneous Chemistry

17.1 Important Discoveries

• Hermann Kolbe (German chemist) published a method for synthesis of a naturally occurring product Acetic Acid (2 carbon) in 1845
• Erich Hückel (German chemist) in 1931 gave the (4n+2) rule for aromatic compounds - number of electrons in the pi system must be 4n+2 where n = 0, 1, 2, 3...
• Antoine Lavoisier published "Methods of Chemical Nomenclature" in 1787 - included naming rules for chemical compounds still in use today
• Benzene discovered by Michael Faraday in 1825

17.2 Combustion

• Fireworks exploding is an example of Combustion
• Three requirements for combustion:
  1. Combustible material (fuel like coal, LPG, diesel, petrol)
  2. Sustaining material (Oxygen)
  3. Ignition temperature

17.3 Fluoridation

• Using water to protect teeth from decay and prevent it effectively is called Fluoridation

17.4 Flocculation in Water Treatment

• In flocculation water treatment method - positively charged chemical is added to impure water
• Impure water has negatively charged suspended solid particles
• Positively charged coagulant like Aluminium Sulphate is added
• This neutralises the particles which settle to the bottom

18. SSC CGL Important One-Liners - Chemistry

1. John Dalton proposed the Atomic Theory in the early 19th century
2. Elements showing properties of both metals and non-metals are called Metalloids
3. Metalloids: Boron, Silicon, Antimony
4. Avogadro's Number = 6.023 × 10²³
5. One mole of ideal gas occupies 22.4 litres at STP
6. Charles' Law (1787): Volume ∝ Temperature at constant pressure
7. Electron discovered by J.J. Thomson in 1897
8. Neutron discovered by James Chadwick in 1932
9. Lithium nucleus has 3 protons
10. Oxygen atomic mass = 16; Atomic number = 8; Nitrogen atomic number = 7
11. Protium, Deuterium, Tritium are isotopes of Hydrogen
12. Scandium has 13 isotopes
13. Dmitri Mendeleev created Periodic Table in 1869
14. Mendeleev could NOT place Hydrogen correctly in his periodic table
15. Modern Periodic Table has 7 periods and 18 groups
16. Last element of Period 5: Xenon (Xe)
17. Group 0 elements are called Noble/Inert Gases - He, Ne, Ar, Kr, Xe, Rn
18. Argon is used in light bulbs (inert gas)
19. Hydrogen matches properties of Group 1 and Group 17
20. Newlands' Law of Octaves - every 8th element similar properties - started from Hydrogen, ended at Thorium (56th element)
21. Most reactive metal among given options: Calcium
22. Activity Series: Li > K > Ba > Ca > Na > Mg > Al > Zn > Fe > Cu
23. Rusting requires: Iron + Oxygen + Moisture
24. Galvanization = coating iron with Zinc to prevent rusting
25. Acids turn blue litmus RED; Bases turn red litmus BLUE
26. Apple contains Malic Acid
27. Vinegar contains Acetic (Ethanoic) Acid
28. Ant/Bee sting contains Formic (Methanoic) Acid
29. Lemon/Orange contains Citric Acid
30. Milk/Curd contains Lactic Acid
31. Dry Ice = solid form of Carbon Dioxide (CO₂)
32. Buckminsterfullerene is an allotrope of Carbon
33. Catenation ability is strongest in Carbon
34. Nitrogen is a diatomic molecule (N₂)
35. Nitrous Oxide (N₂O) = Laughing Gas
36. Washing Soda removes permanent hardness of water
37. Baking Soda chemical name: Sodium Hydrogen Carbonate (NaHCO₃)
38. Plaster of Paris: CaSO₄·½H₂O - used in bone fractures
39. Limestone = mainly Calcium Carbonate (CaCO₃)
40. Epsom Salt = Magnesium Sulphate (MgSO₄·7H₂O)
41. Copper is the only antibacterial metal
42. Gold is the most malleable and ductile metal
43. Tin has the lowest melting point (231.9°C) among common metals
44. Modern Olympic gold medals main component is Silver
45. Constantan (Eureka) = Copper (55%) + Nickel (45%)
46. Cuprite (Cu₂O) is an ore of Copper - NOT iron
47. Methane = Marsh Gas - discovered isolation by Alessandro Volta
48. Hydrogenation converts vegetable oils to vegetable ghee - catalyst: Nickel/Palladium
49. Propene formula = C₃H₆
50. Benzene formula = C₆H₆ - Bond angle = 120°
51. Benzene discovered by Michael Faraday in 1825
52. Ethanol formula = CH₃CH₂OH
53. Chloroform = Trichloromethane = CHCl₃
54. Propanal is an Aldehyde (Propionaldehyde)
55. Ethyl Acetate has a fruity smell - it is an Ester
56. Alkyne has highest boiling point among hydrocarbons of same carbon chain
57. Antoine Lavoisier published chemical nomenclature rules in 1787
58. Erich Hückel gave (4n+2) rule for aromatic compounds in 1931
59. Hermann Kolbe synthesized acetic acid from inorganic components in 1845
60. Silica content in cement: 17% to 25%

19. Chapter Summary

Chemistry is a scoring section in SSC CGL. Focus on:

Top Priority:

• Atomic Theory - Dalton, electrons, protons, neutrons and their discoverers
• Periodic Table - Mendeleev 1869, noble gases, important atomic numbers
• Acids and Bases - litmus test, acids in food (malic, citric, lactic, acetic, formic)
• Important compounds - baking soda, washing soda, plaster of paris, dry ice
• Metals - copper (antibacterial), gold (most malleable), galvanization with zinc
• Organic Chemistry - methane (marsh gas), benzene (Faraday 1825), ethanol formula