A real qpaper, start to finish

Answer Keyincluded with every paper

1. Q1. double displacement reaction

   A double displacement reaction involves exchange of ions between two compounds, forming two new compounds.

2. Q2. double displacement

   In this reaction, the ions of the reactants exchange partners: Ba2+ pairs with SO42- and H+ pairs with Cl-.

3. Q3. 4:1

   Balanced decomposition: 2Pb(NO3)2 → 2PbO + 4NO2 + O2. Thus, 4 moles of NO2 are produced for every 1 mole of O2.

4. Q4. Sodium hydrogencarbonate

   Baking soda is sodium hydrogencarbonate with the chemical formula NaHCO₃. It is a mild, edible base used in cooking and as an antacid.

5. Q5. Baking soda

   Water of crystallization is the fixed number of water molecules present in one formula unit of a salt. Gypsum (CaSO₄·2H₂O), washing soda (Na₂CO₃·10H₂O), and blue vitriol (CuSO₄·5H₂O) all contain water of crystallization. Baking soda (NaHCO₃) has no such water molecules.

6. Q6. Its chemical formula is CaSO₄·2H₂O.

   Plaster of Paris is calcium sulphate hemihydrate (CaSO₄·½H₂O). The formula CaSO₄·2H₂O corresponds to gypsum. When heated at 373 K, gypsum loses water of crystallisation and forms Plaster of Paris. It absorbs water to set into a hard mass and is widely used in moulding.

7. Q7. A chemical reaction is a process in which one or more substances (reactants) are converted into new substances (products) with different chemical properties. Example: 2H₂ + O₂ → 2H₂O.

   A chemical reaction involves rearrangement of atoms leading to new substances. The example shows hydrogen combining with oxygen to form water, with two molecules of hydrogen reacting with one molecule of oxygen to give two molecules of water, satisfying atom conservation.

8. Q8. The white ash is magnesium oxide (MgO) formed by the combination of magnesium with oxygen. Equation: 2Mg + O₂ → 2MgO. It is a combination reaction because two reactants combine to form a single product.

   Magnesium burns in oxygen to produce magnesium oxide. Since two elements combine to give one compound, it is classified as a combination reaction. The white ash is pure MgO.

9. Q9. Substance oxidized: HCl (Cl⁻ ions). Substance reduced: MnO₂ (Mn⁴⁺ ions). Oxidizing agent: MnO₂. Reducing agent: HCl.

   In HCl, chlorine has oxidation number -1; in Cl₂, it is 0. So Cl⁻ loses electrons, gets oxidized. In MnO₂, Mn has +4; in MnCl₂, Mn has +2, so Mn gains electrons, gets reduced. Hence MnO₂ oxidizes HCl (oxidizing agent) and HCl reduces MnO₂ (reducing agent).

10. Q10. NaHCO₃ + HCl → NaCl + H₂O + CO₂. The gas evolved (CO₂) is acidic in nature.

    Sodium bicarbonate reacts with hydrochloric acid to form sodium chloride, water, and carbon dioxide. Carbon dioxide turns lime water milky and is an acidic oxide.

11. Q11. A reaction in which both oxidation and reduction take place simultaneously is called a redox reaction. Example: CuO + H₂ → Cu + H₂O. In ZnO + C → Zn + CO: Zinc oxide (ZnO) is reduced to zinc because it loses oxygen. Carbon (C) is oxidised to carbon monoxide (CO) because it gains oxygen. Thus, ZnO is the oxidising agent, and C is the reducing agent.

    Oxidation is gain of oxygen/loss of hydrogen; reduction is loss of oxygen/gain of hydrogen. In the given reaction, ZnO loses oxygen (reduction) and C gains oxygen (oxidation). Hence, it's a redox reaction.

12. Q12. Balanced equation: MnO₂ + 4HCl → MnCl₂ + Cl₂ + 2H₂O In this reaction: - MnO₂ (manganese dioxide) is reduced to MnCl₂ (manganese in +4 oxidation state reduces to +2). It acts as an oxidising agent. - HCl (hydrochloric acid) is oxidised to Cl₂ (chlorine from -1 oxidation state to 0). It acts as a reducing agent. Since both oxidation and reduction occur, it is a redox reaction.

    Balancing: Count atoms on each side: Mn:1, O:2, H:1, Cl:1 on left; Mn:1, Cl:2+2=4, H:2 on right. Need 4 HCl to get 4 H and 4 Cl, then H₂O coefficient 2. Reaction: MnO₂+4HCl→MnCl₂+Cl₂+2H₂O. Oxidation: Cl⁻ → Cl₂ (loss of electrons); Reduction: Mn⁴⁺ → Mn²⁺ (gain of electrons). Hence, redox.

13. Q13. (a) Silver reacts with hydrogen sulphide (H₂S) present in the air to form silver sulphide (Ag₂S), which is black. 2Ag(s) + H₂S(g) → Ag₂S(s) + H₂(g) (b) Copper reacts with moist carbon dioxide (CO₂) and water to form basic copper carbonate, which is green. 2Cu(s) + H₂O(l) + CO₂(g) + O₂(g) → Cu(OH)₂ · CuCO₃(s) (Sometimes written as CuCO₃·Cu(OH)₂) (c) Both are oxidation reactions leading to corrosion. The compounds formed are silver sulphide and basic copper carbonate (or copper carbonate hydroxide).

    These are examples of corrosion, a slow oxidation of metals by atmospheric gases. Silver tarnish is due to Ag₂S formation. Green patina on copper is basic copper carbonate, commonly called verdigris.

14. Q14. (a) 2H2 + O2 → 2H2O; Combination reaction (two reactants combine to form a single product). (b) 2KClO3 → 2KCl + 3O2; Decomposition reaction (single reactant breaks down into two or more products). (c) Zn + 2HCl → ZnCl2 + H2; Displacement reaction (zinc displaces hydrogen from hydrochloric acid because it is more reactive).

    To balance equations, ensure equal number of atoms of each element on both sides. For (a): 2 H2 and 1 O2 give 2 H2O. Type: Combination as two substances combine. For (b): 2 KClO3 → 2 KCl + 3 O2. Type: Decomposition as one compound breaks into two. For (c): Zn + 2 HCl → ZnCl2 + H2. Type: Displacement because a more reactive metal (Zn) displaces hydrogen from acid.

15. Q15. The white solid is lead nitrate (Pb(NO3)2). The balanced equation is: 2Pb(NO3)2 → 2PbO + 4NO2 + O2. This is a thermal decomposition reaction. To confirm the gas (NO2): the brown fumes turn moist blue litmus paper red due to acidic nature, or they have a pungent irritating smell. Alternatively, pass the gas through potassium iodide solution; it will turn brown due to liberation of iodine.

    Lead nitrate is a white crystalline solid that decomposes on strong heating to give yellow lead oxide, brown nitrogen dioxide fumes, and oxygen. The reaction is decomposition, specifically thermal decomposition. NO2 can be identified by its colour, smell, and acidic reaction with litmus.