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The first chapter in the NCERT Class 10 Science Book Solutions revolves around Chemical Reactions and Chemical Equations. This chapter will teach you how to accurately write chemical equations and balance them. You will also learn about the various types of chemical reactions, including Combination, Decomposition, Displacement, Double Displacement, Oxidation, and Reduction Reactions. The chapter also explains Endothermic and Exothermic reactions and Precipitation Reaction. You will also become familiar with the common effects of oxidation reactions viz. Corrosion and Rancidity.
Chemical Reaction Definition: When two or more substances react and form some new substance, it is called a chemical reaction.
Q.1: Why does the colour of copper sulphate solution change when an iron nail is dipped in it?
Ans: Iron is more reactive than copper. So, when an iron nail is dipped in a copper sulphate solution, iron displaces copper from its solution to form iron sulphate, which is green in color. Fe (s) + CuSO4 (aq) → FeSO4 (aq) + Cu (s). Hence, the blue color of copper sulphate solution changes into green color because of this displacement reaction.
Q.2: Give an example of the double displacement reaction other than the one given in Activity 1.10.
Ans: When lead (II) nitrate is mixed with potassium iodide, potassium nitrate and lead iodide are formed.
Pb(NO3)2 + 2KI → 2KNO3 + PbI2
Na2CO3(aq) + CaCl2(aq) → CaCO3(s) + 2NaCl(aq)
Here sodium carbonate and calcium chloride exchange ions to form two new compounds.
Q.3: Identify the substances that are oxidized and the substances that are reduced in the following reactions:
(i) 4Na(s) + O2(g) → 2Na2O(s)
(ii) CuO(s) + H2(g) → Cu(s) + H2O(liq)
Ans:
(i) Sodium (Na) is oxidized to Na2O.
(ii) CuO (Copper oxide) is reduced to Cu, while H2 gas is oxidized to H2O
Q.4: A solution of a substance 'X' is used for white washing.
(i) Name the substance 'X' and write its formula.
(ii) Write the reaction of the substance 'X' named in (i) above with water.
Ans:
(i) The substance 'X' is calcium oxide (lime). Its chemical formula is CaO.
(ii) Calcium oxide reacts vigorously with water to form calcium hydroxide (slaked lime).
Q.5: Why is the amount of gas collected in one of the test tubes in Activity double of the amount collected in the other? Name this gas.
Ans: Water (H2O) contains two parts of hydrogen and one part of oxygen. Therefore, the amount of hydrogen and oxygen produced during electrolysis of water is in a 2:1 ratio. During electrolysis, since hydrogen goes to one test tube and oxygen gas goes to another hence, the amount of gas collected in one of the test tubes is double the amount collected in the other. The gas collected in double the amount is hydrogen and the other gas is oxygen.
Q.6: Why does the colour of copper sulphate solution change when an iron nail is dipped in it?
Ans: When an iron nail is placed in a copper sulphate solution, iron displaces copper from the blue colored copper sulphate solution forming light green colored ferrous sulphate solution and copper metal.
Q.7: Which of the statements about the reaction below are incorrect?
2PbO(s) + C(s) → 2Pb(s) + CO2(g)
(a) Lead is getting reduced.
(b) Carbon dioxide is getting oxidised.
(c) Carbon is getting oxidised.
(d) Lead oxide is getting reduced.
(i) (a) and (b)
(ii) (a) and (c)
(iii) (a), (b) and (c)
(iv) All
Ans: (i) (a) and (b)
Q.8: Fe2O3 + 2Al → Al2O3 + 2Fe
The above reaction is an example of a:
(a) combination reaction
(b) double displacement reaction
(c) decomposition reaction
(d) displacement reaction
Ans: (d) displacement reaction
Q.9: What happens when dilute hydrochloric acid is added to iron fillings? Tick the correct answer.
(a) Hydrogen gas and iron chloride are produced
(b) Chlorine gas and iron hydroxide are produced
(c) No reaction takes place
(d) Iron salt and water are produced
Ans: (a) Hydrogen gas and iron chloride are produced.
Q.10: What is a balanced chemical equation? Why should chemical equations be balanced?
Ans: A chemical equation is balanced when the numbers of atoms of each type involved in a chemical reaction are same on both the reactant and product sides of the equation. The chemical equations must always be balanced to satisfy the law of conservation of mass which states that "Mass can neither be created nor destroyed in a chemical reaction." This means that the total mass of the elements present in the products of a chemical reaction has to be equal to the total mass of the elements present in the reactants.
Q.11: Translate the following statements into chemical equations and then balance them.
(a) Hydrogen gas combines with nitrogen to form ammonia.
(b) Hydrogen sulphide gas burns in air to give water and sulpur dioxide.
(c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.
(d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.
Ans:
(a) 3H2 + N2 → 2NH3
(b) 2H2S + 3O2 → 2H2O + 2SO2
(c) 3BaCl2 + Al2(SO4)3 → 2AlCl3 + 3BaSO4
(d) 2K + 2H2O → 2KOH + H2
Q.12: Balance the following chemical equations.
(a) HNO3 + Ca(OH)2 → Ca(NO3)2 + H2O
(b) NaOH + H2SO4 → Na2SO4 + H2O
(c) NaCl + AgNO3 → AgCl + NaNO3
(d) BaCl2 + H2SO4 → BaSO4 + HCl
Ans:
(a) 2HNO3 + Ca(OH)2 → Ca(NO3)2 + 2H2O
(b) 2NaOH + H2SO4 → Na2SO4 + 2H2O
(c) NaCl + AgNO3 → AgCl + NaNO3
(d) BaCl2 + H2SO4 → BaSO4 + 2HCl
Q.26: Explain the following terms with one example each.
(a) Corrosion
(b) Rancidity
Ans:
(a) Corrosion: Corrosion is the process of eating away of metals by the attack of substances such as moisture, acids, etc. Example - the black coating on silver and the green coating on copper are other examples of corrosion.
(b) Rancidity: When food items containing fats and oils are kept for a long time, they get oxidized and their smell and taste change. This process is known as rancidity. Example: Butter when kept in open for long time, tastes and smells bad because of rancidity.
Q.29: Write balanced chemical equations with state symbols for the following reactions:
(i) Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and the solution of sodium chloride.
(ii) Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water.
Ans:
(i) BaCl2(aq) + Na2SO4(aq) → BaSO4(s) + 2NaCl(aq)
(ii) NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)
