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RRB Group D · General Science

Chemistry

In the official syllabus: RRB — CEN No. 09/2025, Para 14.1 'Question Type and Syllabus', Computer Based Test, Question Type and Syllabus, item c: General Science (read 29 Sept 2026).

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Matter and Its StatesAtomic StructurePeriodic ClassificationChemical BondingChemical Reactions and EquationsAcids, Bases and SaltsMetals and Non-MetalsCarbon and Its CompoundsCombustion and FuelsSynthetic Fibres and PlasticsElectrolysis

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Chemistry — Railway Group D Study Notes

Overview

Understanding chemistry is essential not just for direct questions but also for comprehending everyday phenomena—from rusting of railway tracks to cleaning agents used in maintenance.

The exam tests fundamental concepts rather than complex calculations. Questions focus on classification of matter, basic atomic structure, the periodic table, chemical reactions, acids-bases-salts, metals-non-metals, carbon compounds, and fuels. Familiarity with common household chemicals, industrial processes, and the chemistry of daily-use materials gives you an edge. Memorization of key definitions, chemical formulas, and reaction types is equally important as conceptual clarity.

Success in this section requires consistent revision of NCERT Class 8–10 chemistry chapters, with special emphasis on real-world applications—how chemistry relates to railway operations, environmental issues, and technological developments.

Key Concepts

  • Matter exists in three primary states (solid, liquid, gas) plus plasma; state changes are physical processes (melting, boiling, sublimation) while chemical changes alter composition permanently.
  • Atoms are the smallest units of elements; molecules form when atoms combine; elements contain only one type of atom while compounds contain two or more different atoms chemically bonded.
  • The Modern Periodic Table arranges elements by increasing atomic number in 18 groups (vertical) and 7 periods (horizontal); elements in the same group share similar chemical properties.
  • Chemical bonds form through transfer of electrons (ionic bonding in NaCl) or sharing of electrons (covalent bonding in H₂O); metallic bonding involves a "sea" of free electrons.
  • Chemical reactions involve breaking old bonds and forming new ones; types include combination, decomposition, displacement, double displacement, and redox (oxidation-reduction) reactions.
  • Acids release H⁺ ions in water (pH < 7), taste sour, turn blue litmus red; bases release OH⁻ ions (pH > 7), taste bitter, feel slippery, turn red litmus blue; salts form from acid-base neutralization.
  • Metals are lustrous, malleable, ductile, good conductors; tend to lose electrons forming cations; non-metals are poor conductors, brittle, gain electrons forming anions.
  • Carbon forms millions of compounds due to its tetravalency and catenation ability; hydrocarbons (compounds of C and H) are classified as alkanes, alkenes, and alkynes.

Formulas / Key Facts

Common Chemical Formulas:

  • Water: H₂O | Carbon dioxide: CO₂ | Sodium chloride (table salt): NaCl
  • Hydrochloric acid: HCl | Sulphuric acid: H₂SO₄ | Nitric acid: HNO₃
  • Sodium hydroxide (caustic soda): NaOH | Calcium hydroxide (slaked lime): Ca(OH)₂
  • Calcium carbonate (limestone): CaCO₃ | Methane: CH₄ | Ethanol (alcohol): C₂H₅OH
  • Ammonia: NH₃ | Glucose: C₆H₁₂O₆

pH Scale: Ranges from 0 to 14; 7 is neutral, <7 acidic, >7 basic; each unit represents 10× change in H⁺ concentration.

Reactivity Series (high to low): K > Na > Ca > Mg > Al > Zn > Fe > Pb > H > Cu > Hg > Ag > Au

Valency Rules: Hydrogen = 1, Oxygen = 2, Nitrogen = 3, Carbon = 4, Aluminium = 3, Chlorine = 1

Allotropes of Carbon: Diamond (hardest natural substance), graphite (conducts electricity), fullerenes (C₆₀), graphene.

Law of Conservation of Mass: Total mass of reactants = Total mass of products in a chemical reaction.

Avogadro's Number: 6.022 × 10²³ particles per mole.

Worked Examples

Example 1: Balancing Chemical Equations

Unbalanced: Fe + O₂ → Fe₂O₃

Step 1: Count atoms — Left: Fe=1, O=2; Right: Fe=2, O=3

Step 2: Balance Fe by putting 2 before Fe on left: 2Fe + O₂ → Fe₂O₃

Step 3: Balance O by putting 3/2 before O₂: 2Fe + (3/2)O₂ → Fe₂O₃

Step 4: Multiply entire equation by 2 to remove fraction: 4Fe + 3O₂ → 2Fe₂O₃

Example 2: pH Calculation Application

Rain water has pH 5.6 (slightly acidic due to dissolved CO₂). Acid rain has pH < 5. If a sample has pH 4, it is:

Solution: pH 4 is less than 5, so this is acid rain. Since pH scale is logarithmic, pH 4 is 10 times more acidic than pH 5. This happens when industrial SO₂ and NO₂ dissolve in rain forming H₂SO₄ and HNO₃.

Example 3: Metal Reactivity

Will copper displace zinc from zinc sulphate solution?

Solution: Check reactivity series: Zn is above Cu. Only more reactive metals displace less reactive ones. Since Cu is less reactive than Zn, no reaction occurs. Reverse is true: Zn can displace Cu from CuSO₄ solution.

Common Mistakes

Mistake 1: Confusing physical and chemical change → Fix: Physical changes are reversible and don't form new substances (ice melting); chemical changes produce new substances with different properties (iron rusting).

Mistake 2: Writing incorrect chemical formulas by ignoring valency → Fix: Always cross-multiply valencies. For aluminium oxide: Al³⁺ and O²⁻, cross gives Al₂O₃, not AlO.

Mistake 3: Thinking all acids are dangerous and all bases are safe → Fix: Concentrated bases like NaOH are extremely corrosive; dilute acids like citric acid (lemon) are safe. Concentration matters more than type.

Mistake 4: Believing metals are always solid → Fix: Mercury (Hg) is liquid at room temperature; gallium melts in hand (29.8°C melting point).

Mistake 5: Assuming pH = 0 is neutral because it's zero → Fix: pH 7 is neutral (pure water). pH 0 represents extremely strong acid (like battery acid).

Quick Reference

  • Three states of matter: Solid (fixed shape and volume), Liquid (fixed volume, variable shape), Gas (variable shape and volume)
  • Atomic number = number of protons = number of electrons in neutral atom; mass number = protons + neutrons
  • Ionic compound properties: High melting point, conduct electricity in solution, crystalline structure
  • Covalent compound properties: Low melting point, poor conductors, exist as molecules
  • Indicators: Litmus (natural), phenolphthalein (synthetic), methyl orange, turmeric, red cabbage
  • Common salt (NaCl) sources: Sea water evaporation, rock salt mining; used in food preservation, soap making (with NaOH)
  • Combustion types: Complete (blue flame, more heat, CO₂), incomplete (yellow flame, less heat, CO/soot)
  • Calorific value: Heat energy produced per kg of fuel; LPG (55 MJ/kg) > Petrol (45) > Coal (25–35) > Wood (17)

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Which of the following is the correct electronic configuration of an element with atomic number 13?

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4 practice questions on Chemistry for RRB Group D, with answers

Shishya's practice questions, written with AI. Each answer was checked by an automated second pass, not by a person.

  1. 1.A solution has a pH value of 3. What is the concentration of hydrogen ions [H+] in moles per litre in this solution?

    • (A)0.001 M
    • (B)0.01 M
    • (C)0.0001 M
    • (D)0.1 M
    Show the answer and solution

    Answer: (A) 0.001 M

    Solution: Step 1: pH is defined as pH = -log[H+], where [H+] is the hydrogen ion concentration in moles per litre. Step 2: Given pH = 3. Step 3: Rearranging the formula: [H+] = 10^(-pH). Step 4: [H+] = 10^(-3) = 1/1000 = 0.001 M. Step 5: Therefore, the hydrogen ion concentration is 0.001 M.

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  2. 2.In the reactivity series of metals, which metal is placed below hydrogen and does not react with dilute acids?

    • (A)Zinc
    • (B)Iron
    • (C)Copper
    • (D)Aluminium
    Show the answer and solution

    Answer: (C) Copper

    Solution: Step 1: The reactivity series arranges metals in order of their decreasing reactivity. Step 2: Metals placed above hydrogen in the reactivity series react with dilute acids to liberate hydrogen gas. Step 3: Metals placed below hydrogen do not react with dilute acids. Step 4: The reactivity series (partial): Potassium > Sodium > Calcium > Magnesium > Aluminium > Zinc > Iron > Hydrogen > Copper > Silver > Gold. Step 5: Among the given options, Copper is the only metal placed below hydrogen. Step 6: Therefore, Copper does not react with dilute acids.

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  3. 3.What is the percentage of carbon by mass in methane (CH4)? (Given: Atomic mass of C = 12, H = 1)

    • (A)60%
    • (B)75%
    • (C)80%
    • (D)85%
    Show the answer and solution

    Answer: (B) 75%

    Solution: Step 1: The molecular formula of methane is CH4, which means 1 atom of carbon and 4 atoms of hydrogen. Step 2: Calculate the molecular mass of CH4: Mass of carbon = 1 × 12 = 12 u; Mass of hydrogen = 4 × 1 = 4 u; Total molecular mass = 12 + 4 = 16 u. Step 3: Percentage of carbon by mass = (Mass of carbon / Total molecular mass) × 100. Step 4: Percentage of carbon = (12 / 16) × 100 = 0.75 × 100 = 75%. Step 5: Therefore, carbon constitutes 75% of the mass of methane.

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  4. 4.During electrolysis of acidulated water using platinum electrodes, the volume ratio of hydrogen gas to oxygen gas liberated at the cathode and anode respectively is:

    • (A)1 : 1
    • (B)1 : 2
    • (C)2 : 1
    • (D)3 : 1
    Show the answer and solution

    Answer: (C) 2 : 1

    Solution: Step 1: During electrolysis of acidulated water, water molecules decompose into hydrogen and oxygen gases. Step 2: The chemical equation for the electrolysis of water is: 2H2O → 2H2 + O2. Step 3: From the balanced equation, 2 molecules of water produce 2 molecules of hydrogen and 1 molecule of oxygen. Step 4: At the cathode (negative electrode), hydrogen gas is liberated: 2 volumes. At the anode (positive electrode), oxygen gas is liberated: 1 volume. Step 5: Therefore, the volume ratio of hydrogen to oxygen is 2 : 1. This can also be verified experimentally using Hoffman's voltameter.

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Notes generated on 10 May 2026