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RPF Constable Chemical Reactions

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This page covers RPF Constable Chemical Reactions with complete concept notes, 12 graded practice MCQs, key points and exam-specific tips. Free to study.

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Concept Notes

Chemical Reactions— Rules & Concept

Core ConceptRead this first — the foundation of the topic

Chemical reactions are processes where substances (reactants) transform into new substances (products) with different properties. In RRB Group D exams, this topic appears in 2-3 questions every year, making it extremely important. Core Concept: A chemical reaction involves breaking old bonds and forming new bonds. The atoms remain the same, but their arrangement changes. For example, when iron rusts, iron atoms combine with oxygen atoms to form iron oxide.

Key RulesCore rules you must know cold
1

Mass is conserved - total mass of reactants equals total mass of products (Law of Conservation of Mass)

2

Atoms are neither created nor destroyed, only rearranged

3

Energy changes always occur - reactions either release heat (exothermic) or absorb heat (endothermic)

4

Chemical equations must be balanced

Formula BlockMemorise — at least one formula appears in every paper

- Balancing: Count atoms on both sides, make them equal

- Molecular weight = Sum of atomic weights of all atoms
- Mole concept: 1 mole = 6.022 × 10^23 particles = Atomic/Molecular weight in grams
Exam PatternsWhat examiners ask — read before attempting PYQs

RRB Group D typically asks about balancing equations (40%), types of reactions (30%), and real-life examples (30%). Questions often involve identifying reaction types or completing word equations. Shortcut for Balancing Equations: Start with the most complex molecule first. Balance metals, then non-metals, then hydrogen and oxygen last. Use the 'LCM method' - find LCM of atoms appearing on both sides.

Worked ExampleSolve this step-by-step before moving on
1
Step 1

Count Fe atoms - Left: 1, Right: 2 (unbalanced)

2
Step 2

Put coefficient 2 before Fe: 2Fe + O2 → Fe2O3

3
Step 3

Count O atoms - Left: 2, Right: 3 (unbalanced)

4
Step 4

Put coefficient 3 before O2 and 2 before Fe2O3: Fe + 3O2 → 2Fe2O3

5
Step 5

Balance Fe: 4Fe + 3O2 → 2Fe2O3 Answer: 4Fe + 3O2 → 2Fe2O3 Worked Example 2: Identify reaction type - CaCO3 → CaO + CO2

1
Step 1

Count reactants - One compound (CaCO3)

2
Step 2

Count products - Two substances (CaO and CO2)

3
Step 3

One reactant breaks into multiple products = Decomposition reaction

4
Step 4

Heat is required, so it's thermal decomposition Answer: Thermal decomposition reaction

ShortcutsUse these to save 30–60 seconds per question

for Reaction Types: Remember 'CORD' - Combination (A+B→AB), Oxidation-reduction (electron transfer), Replacement (single/double), Decomposition (AB→A+B). Most RRB questions test these four types.

Exam TrapsCommon mistakes students make — avoid these

#1: Students forget to balance equations and write wrong coefficients. Always verify by counting atoms on both sides after balancing. Another frequent error is confusing physical changes (like melting) with chemical changes (like burning).

Chemical changes form new substances; physical changes don't. Another exam trick: When identifying precipitation reactions, remember that most carbonates, sulfides, and hydroxides are insoluble except those of Group 1 metals and ammonium. This helps predict which combinations will form precipitates. For energy changes, remember: Combustion and respiration are always exothermic. Photosynthesis and electrolysis are always endothermic.

This pattern appears frequently in RRB papers.

Key Points to Remember

  • Chemical reactions rearrange atoms to form new substances with different properties
  • Law of Conservation of Mass: Total mass of reactants = Total mass of products
  • Quick formula: Molecular weight = Sum of all atomic weights in the compound
  • CORD method: Combination, Oxidation-reduction, Replacement, Decomposition reaction types
  • Balancing shortcut: Start with most complex molecule, balance metals first, then H and O
  • Exothermic reactions release heat (combustion, respiration), endothermic absorb heat (photosynthesis)
  • Precipitation rule: Most carbonates, sulfides, hydroxides are insoluble except Group 1 metals
  • 1 mole = 6.022 × 10^23 particles = Atomic/molecular weight in grams
  • Chemical change forms new substances, physical change only changes appearance/state
  • Double displacement: AB + CD → AD + CB (exchange of ions between compounds)

Exam-Specific Tips

  • Avogadro's number is 6.022 × 10^23 particles per mole
  • Atomic weight of Carbon = 12, Oxygen = 16, Hydrogen = 1, Nitrogen = 14
  • Lime water is Ca(OH)2, turns milky with CO2 due to CaCO3 formation
  • Rusting formula: 4Fe + 3O2 + 6H2O → 4Fe(OH)3 (hydrated iron oxide)
  • Photosynthesis equation: 6CO2 + 6H2O → C6H12O6 + 6O2 (in presence of sunlight)
  • Respiration equation: C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy
  • Common acids: HCl (Hydrochloric), H2SO4 (Sulphuric), HNO3 (Nitric)
  • pH scale ranges from 0-14, acids below 7, bases above 7, neutral at 7
Practice MCQs

Chemical Reactions — Practice Questions

12graded MCQs · easy to hard · full solution & trap analysis

All MCQs →
Practice 1easy

Which of the following is a characteristic feature of a chemical reaction?

Practice 2easy

In the reaction: 2H₂ + O₂ → 2H₂O, how many moles of water are produced from 4 moles of hydrogen gas?

Practice 3easy

Which type of chemical reaction occurs when a single compound breaks down into two or more simpler substances?

Practice 4easy

When iron reacts with oxygen to form iron oxide, this type of reaction is classified as:

Practice 5medium

When magnesium ribbon burns in oxygen, it forms magnesium oxide. This is an example of which type of chemical reaction?

Practice 6medium

When calcium carbonate is heated strongly, it decomposes into calcium oxide and carbon dioxide. What is the balanced chemical equation for this reaction?

Practice 7medium

In the reaction: Zn + CuSO₄ → ZnSO₄ + Cu, which substance is being oxidized?

Practice 8medium

When silver nitrate solution reacts with sodium chloride solution, a white precipitate forms. This is an example of which type of chemical reaction?

Practice 9medium

Which of the following is a sign that a chemical reaction has occurred?

Practice 10hard

When 2.3 g of sodium reacts with oxygen, which type of chemical reaction occurs and what is the primary product formed?

Practice 11hard

In a neutralization reaction between 40 mL of 1 M HCl and 20 mL of 2 M NaOH, which statement correctly describes the outcome?

Practice 12hard

When lead nitrate solution is mixed with potassium iodide solution, a yellow precipitate forms. What type of reaction is this and what is the precipitate?

60-Second Revision — Chemical Reactions

  • Remember CORD: Combination, Oxidation-reduction, Replacement, Decomposition for reaction types
  • Formula: Molecular weight = Sum of atomic weights, 1 mole = 6.022 × 10^23 particles
  • Balancing trick: Start with complex molecule, count metals first, H and O last
  • Trap: Always check if atoms are equal on both sides after balancing equations
  • Energy: Combustion/respiration = exothermic, Photosynthesis/electrolysis = endothermic
  • Precipitation: Carbonates, sulfides, hydroxides insoluble except Group 1 metals and NH4+
  • Key weights: C=12, O=16, H=1, N=14 for molecular weight calculations
←Periodic TableAcids, Bases & Salts→
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