A chemical reaction changes one or more substances into new substances with different properties. This chapter covers writing and balancing chemical equations and the main types of reaction — combination, decomposition, displacement, double displacement, oxidation and reduction — along with corrosion and rancidity.
Balance the equation for the reaction between iron and steam, and name the type of reaction.
The unbalanced (skeletal) equation is:
Fe + H₂O → Fe₃O₄ + H₂
Balance iron first: there are 3 Fe atoms on the right, so put 3 before Fe.
3Fe + H₂O → Fe₃O₄ + H₂
Now balance oxygen: 4 O atoms on the right, so put 4 before H₂O.
3Fe + 4H₂O → Fe₃O₄ + H₂
Finally balance hydrogen: the left now has 8 H atoms, so put 4 before H₂.
3Fe + 4H₂O → Fe₃O₄ + 4H₂
Every element now has equal atoms on both sides.
Iron displaces hydrogen from water, so this is a displacement reaction (it is also a redox reaction).
Formula: Balance in the order: metal → oxygen → hydrogen
Final Answer: 3Fe + 4H₂O → Fe₃O₄ + 4H₂ — a displacement (redox) reaction.
Common Mistake: Changing the formula of a compound (e.g. writing H₂O₂) instead of adjusting the coefficient in front of it.
Exam Tip: Never change subscripts inside a formula — only the numbers written before it.
Q2. Question 2 — Quicklime and Water
What is observed when quicklime reacts with water? Write the balanced equation and name the type of reaction.
When water is added to quicklime (calcium oxide), the mixture becomes very hot and hisses, and a white suspension of slaked lime is formed.
The balanced equation is:
CaO(s) + H₂O(l) → Ca(OH)₂(aq)
Two reactants combine to form a single product, so this is a combination reaction.
Because a large amount of heat is released along with the product, it is also an exothermic reaction.
Final Answer: CaO + H₂O → Ca(OH)₂ — a combination reaction that is also exothermic (a lot of heat is given out).
Common Mistake: Naming it a decomposition reaction — here substances join together rather than break apart.
Exam Tip: If heat is released, add the word 'exothermic' — it is often worth an extra mark.
Q3. Question 3 — Zinc in Copper Sulphate
Describe what you observe when a zinc strip is dipped in copper sulphate solution, and explain why it happens.
Observations: the blue colour of the copper sulphate solution slowly fades, and a reddish-brown deposit of copper collects on the zinc strip.
Reason: zinc is more reactive than copper, so it displaces copper from its salt solution:
Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s)
The blue colour fades because the blue Cu²⁺ ions are replaced in solution by colourless Zn²⁺ ions, and the displaced copper settles on the zinc as a reddish-brown layer.
This is a displacement reaction.
Final Answer: The blue colour fades and reddish-brown copper deposits on the zinc: Zn + CuSO₄ → ZnSO₄ + Cu (displacement reaction).
Common Mistake: Writing that zinc dissolves without mentioning that the copper is displaced and deposited.
Exam Tip: Give the observation AND the reason — questions like this always carry both marks.
More NCERT solutions for this chapter are being added by One Roof Education.