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
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
When zinc metal reacts with dilute sulfuric acid, hydrogen gas is produced. Which type of chemical reaction is this?
Practice 2medium
When zinc metal is added to dilute hydrochloric acid, hydrogen gas is evolved and zinc chloride is formed. Which type of chemical reaction is this, and what is the oxidation state change of zinc?
Practice 3hard
When 2.3 g of sodium metal reacts with water, hydrogen gas is produced. If the reaction is: 2Na + 2H₂O → 2NaOH + H₂↑, how many moles of H₂ gas are produced? (Atomic mass: Na = 23)
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