State the law used while balancing a chemical equation.
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Law of conservation of mass: mass (and number of each atom) on the reactant side equals the product side.
Never change formulae — only change coefficients.
Q.12Long answermediumImportantTypes of reactions
Explain four types of chemical reactions with one example equation each.
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1) Combination: A + B → AB, e.g. 2H₂ + O₂ → 2H₂O.
2) Decomposition: AB → A + B, e.g. 2HgO → 2Hg + O₂.
3) Displacement: A + BC → AC + B, e.g. Zn + CuSO₄ → ZnSO₄ + Cu.
4) Double displacement: AB + CD → AD + CB, e.g. AgNO₃ + NaCl → AgCl↓ + NaNO₃.
Many of these can also be redox if electrons are transferred.
Board answers score better when each type has a labelled equation.
Q.13Long answerhardImportantOxidation and reduction
How can you tell oxidation and reduction in terms of oxygen, hydrogen and electrons? Use 2H₂S + SO₂ → 3S + 2H₂O as an example.
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Oxidation: gain of oxygen / loss of hydrogen / loss of electrons.
Reduction: loss of oxygen / gain of hydrogen / gain of electrons.
In 2H₂S + SO₂ → 3S + 2H₂O, H₂S loses hydrogen (oxidised) and SO₂ loses oxygen (reduced). Electrons move from sulphur in H₂S toward sulphur in SO₂.
Name the oxidising agent (the one that gets reduced) in the last line.
These practice sets use original GKSchools wording — they are not a verbatim dump of the NCERT textbook. “Important” means high-weight practice for the chapter, not a claim that these are previous-year exam questions.
