NCERT Solutions For Class 10 Chemistry: Chemical Reactions and Equations

Intext Questions (Page 7)

Question 1: Why should a magnesium ribbon be cleaned before burning in air?

Answer-

A magnesium ribbon should be cleaned by rubbing it with sandpaper to remove the layer of magnesium oxide and other impurities that may have formed on the surface due to reaction with oxygen in the air. Cleaning ensures that the magnesium metal reacts directly.

Question 2: Write the balanced equation for the following chemical reactions.

Answer-

(i) Hydrogen + Chlorine $\rightarrow$ Hydrogen chloride
H$_{2}$ + Cl$_{2}$ $\rightarrow$ 2HCl

(ii) Barium chloride + Aluminium sulphate $\rightarrow$ Barium sulphate + Aluminium chloride
3BaCl$_{2}$ + Al$_{2}$(SO$_{4}$)$_{3}$ $\rightarrow$ 3BaSO$_{4}$ + 2AlCl$_{3}$

(iii) Sodium + Water $\rightarrow$ Sodium hydroxide + Hydrogen
2Na + 2H$_{2}$O $\rightarrow$ 2NaOH + H$_{2}$

Question 3: Write a balanced chemical equation with state symbols for the following reactions.

Answer-

(i) Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and the solution of sodium chloride.
BaCl$_{2}$(aq) + Na$_{2}$SO$_{4}$(aq) $\rightarrow$ BaSO$_{4}$(s) + 2NaCl(aq)

(ii) Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water.
NaOH(aq) + HCl(aq) $\rightarrow$ NaCl(aq) + H$_{2}$O(l)

Intext Questions (Page 10)

Question 1: A solution of a substance ‘X’ is used for whitewashing. (i) Name the substance ‘X’ and write its formula. (ii) Write the reaction of the substance ‘X’ named in (i) above with water.

Answer-

(i) The substance ‘X’ is Calcium oxide (quick lime), and its formula is CaO.

(ii) The reaction of Calcium oxide (quick lime) with water produces slaked lime (calcium hydroxide) and releases a large amount of heat:
CaO(s) + H$_{2}$O(l) $\rightarrow$ Ca(OH)$_{2}$(aq) + Heat

Question 2: Why is the amount of gas collected in one of the test tubes in Activity 1.7 double of the amount collected in the other? Name this gas.

Answer-

Activity 1.7 describes the electrolysis of water (H$_{2}$O). Since water contains hydrogen and oxygen atoms in a 2:1 ratio, the decomposition reaction yields twice the volume of hydrogen gas as oxygen gas. The gas collected in double the amount is Hydrogen gas (H$_{2}$).

Intext Questions (Page 13)

Question 1: Why does the colour of copper sulphate solution change when an iron nail is dipped in it?

Answer-

The blue colour of the copper sulphate solution fades because iron (Fe) is a more reactive element than copper, causing a displacement reaction. Iron displaces copper from copper sulphate, forming iron sulphate (FeSO$_{4}$), which is lighter in colour, thus changing the solution's colour.
Fe(s) + CuSO$_{4}$(aq) $\rightarrow$ FeSO$_{4}$(aq) + Cu(s)

Question 2: Give an example of a double displacement reaction other than the one given in Activity 1.10.

Answer-

A double displacement reaction involves the exchange of ions between two reactants. An example is the reaction between lead(II) nitrate and potassium iodide:
Pb(NO$_{3}$)$_{2}$(aq) + 2KI(aq) $\rightarrow$ PbI$_{2}$(s) + 2KNO$_{3}$(aq)

Question 3: Identify the substances that are oxidised and the substances that are reduced in the following reactions.

Answer-

(i) 4Na(s) + O$_{2}$(g) $\rightarrow$ 2Na$_{2}$O(s)
Substance oxidised is Na (Sodium) because it gains oxygen.
Substance reduced is O$_{2}$ (Oxygen).

(ii) CuO(s) + H$_{2}$(g) $\rightarrow$ Cu(s) + H$_{2}$O(l)
Substance oxidised is H$_{2}$ (Hydrogen) because it gains oxygen.
Substance reduced is CuO (Copper oxide) because it loses oxygen.

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Exercise Questions

Question 1: Which of the statements about the reaction below are incorrect?
2PbO(s) + C(s) $\rightarrow$ 2Pb(s) + CO$_{2}$(g)
(a) Lead is getting reduced. (b) Carbon dioxide is getting oxidised. (c) Carbon is getting oxidised. (d) Lead oxide is getting reduced.

Answer-

In the reaction, Lead oxide (PbO) is losing oxygen (reduced), and Carbon (C) is gaining oxygen (oxidised). Therefore, the incorrect statements are:
(a) Lead is getting reduced. (The substance being reduced is PbO).
(b) Carbon dioxide is getting oxidised. (Carbon is getting oxidised, and CO$_{2}$ is a product).
The correct option indicating the incorrect statements is (i) (a) and (b).

Question 2: Fe$_{2}$O$_{3}$ + 2Al $\rightarrow$ Al$_{2}$O$_{3}$ + 2Fe
The above reaction is an example of a

Answer-

This reaction is an example of a (d) displacement reaction because Aluminium (Al) displaces Iron (Fe) from iron oxide.

Question 3: What happens when dilute hydrochloric acid is added to iron fillings? Tick the correct answer.

Answer-

When dilute hydrochloric acid is added to iron fillings, a reaction occurs where iron displaces hydrogen. The correct answer is (a) Hydrogen gas and iron chloride are produced.

Question 4: What is a balanced chemical equation? Why should chemical equations be balanced?

Answer-

A balanced chemical equation is one where the number of atoms of each element is equal on the reactant side (LHS) and the product side (RHS) of the arrow. Chemical equations must be balanced to satisfy the Law of Conservation of Mass. This law states that mass can neither be created nor destroyed in a chemical reaction; hence, the total mass of the elements in the reactants must be equal to the total mass in the products.

Question 5: Translate the following statements into chemical equations and then balance them.

Answer-

(a) Hydrogen gas combines with nitrogen to form ammonia: 3H$_{2}$ + N$_{2}$ $\rightarrow$ 2NH$_{3}$.

(b) Hydrogen sulphide gas burns in air to give water and sulphur dioxide: 2H$_{2}$S + 3O$_{2}$ $\rightarrow$ 2H$_{2}$O + 2SO$_{2}$.

(c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate: 3BaCl$_{2}$ + Al$_{2}$(SO$_{4}$)$_{3}$ $\rightarrow$ 2AlCl$_{3}$ + 3BaSO$_{4}$.

(d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas: 2K + 2H$_{2}$O $\rightarrow$ 2KOH + H$_{2}$.

Question 6: Balance the following chemical equations.

Answer-

(a) HNO$_{3}$ + Ca(OH)$_{2}$ $\rightarrow$ Ca(NO$_{3}$)$_{2}$ + H$_{2}$O: 2HNO$_{3}$ + Ca(OH)$_{2}$ $\rightarrow$ Ca(NO$_{3}$)$_{2}$ + 2H$_{2}$O.

(b) NaOH + H$_{2}$SO$_{4}$ $\rightarrow$ Na$_{2}$SO$_{4}$ + H$_{2}$O: 2NaOH + H$_{2}$SO$_{4}$ $\rightarrow$ Na$_{2}$SO$_{4}$ + 2H$_{2}$O.

(c) NaCl + AgNO$_{3}$ $\rightarrow$ AgCl + NaNO$_{3}$: NaCl + AgNO$_{3}$ $\rightarrow$ AgCl + NaNO$_{3}$.

(d) BaCl$_{2}$ + H$_{2}$SO$_{4}$ $\rightarrow$ BaSO$_{4}$ + HCl: BaCl$_{2}$ + H$_{2}$SO$_{4}$ $\rightarrow$ BaSO$_{4}$ + 2HCl.

Question 7: Write the balanced chemical equations for the following reactions.

Answer-

(a) Calcium hydroxide + Carbon dioxide $\rightarrow$ Calcium carbonate + Water: Ca(OH)$_{2}$ + CO$_{2}$ $\rightarrow$ CaCO$_{3}$ + H$_{2}$O.

(b) Zinc + Silver nitrate $\rightarrow$ Zinc nitrate + Silver: Zn + 2AgNO$_{3}$ $\rightarrow$ Zn(NO$_{3}$)$_{2}$ + 2Ag.

(c) Aluminium + Copper chloride $\rightarrow$ Aluminium chloride + Copper: 2Al + 3CuCl$_{2}$ $\rightarrow$ 2AlCl$_{3}$ + 3Cu.

(d) Barium chloride + Potassium sulphate $\rightarrow$ Barium sulphate + Potassium chloride: BaCl$_{2}$ + K$_{2}$SO$_{4}$ $\rightarrow$ BaSO$_{4}$ + 2KCl.

Question 8: Write the balanced chemical equation for the following and identify the type of reaction in each case.

Answer-

(a) Potassium bromide(aq) + Barium iodide(aq) $\rightarrow$ Potassium iodide(aq) + Barium bromide(s)
2KBr(aq) + BaI$_{2}$(aq) $\rightarrow$ 2KI(aq) + BaBr$_{2}$(s). Type: Double Displacement Reaction.

(b) Zinc carbonate(s) $\rightarrow$ Zinc oxide(s) + Carbon dioxide(g)
ZnCO$_{3}$(s) $\rightarrow$ ZnO(s) + CO$_{2}$(g). Type: Decomposition Reaction.

(c) Hydrogen(g) + Chlorine(g) $\rightarrow$ Hydrogen chloride(g)
H$_{2}$(g) + Cl$_{2}$(g) $\rightarrow$ 2HCl(g). Type: Combination Reaction.

(d) Magnesium(s) + Hydrochloric acid(aq) $\rightarrow$ Magnesium chloride(aq) + Hydrogen(g)
Mg(s) + 2HCl(aq) $\rightarrow$ MgCl$_{2}$(aq) + H$_{2}$(g). Type: Displacement Reaction.

Question 9: What does one mean by exothermic and endothermic reactions? Give examples.

Answer-

Exothermic Reactions are chemical reactions in which heat is released (or evolved) along with the formation of products, often making the reaction mixture warm.
Example: Burning of natural gas: CH$_{4}$(g) + 2O$_{2}$(g) $\rightarrow$ CO$_{2}$(g) + 2H$_{2}$O(g).

Endothermic Reactions are chemical reactions in which energy is absorbed. This energy may be supplied in the form of heat, light, or electricity.
Example: Decomposition of silver chloride by light: 2AgCl(s) $\xrightarrow{Sunlight}$ 2Ag(s) + Cl$_{2}$(g).

Question 10: Why is respiration considered an exothermic reaction? Explain.

Answer-

Respiration is considered an exothermic process because energy is released during this reaction to sustain life. Glucose (broken down from carbohydrates) combines with oxygen in the body's cells, providing the necessary energy, which is evolved.
C$_{6}$H$_{12}$O$_{6}$(aq) + 6O$_{2}$(aq) $\rightarrow$ 6CO$_{2}$(aq) + 6H$_{2}$O(l) + energy.

Question 11: Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.

Answer-

Decomposition reactions are the opposite of combination reactions because they involve inverse structural changes. A combination reaction involves two or more substances combining to form a single product. Conversely, a decomposition reaction involves a single reactant breaking down to yield two or more simpler products.

Combination Reaction Example: 2H$_{2}$(g) + O$_{2}$(g) $\rightarrow$ 2H$_{2}$O(l)
Decomposition Reaction Example: CaCO$_{3}$(s) $\xrightarrow{Heat}$ CaO(s) + CO$_{2}$(g)

Question 12: Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.

Answer-

(i) Decomposition by Heat (Thermal decomposition):
2Pb(NO$_{3}$)$_{2}$(s) $\xrightarrow{Heat}$ 2PbO(s) + 4NO$_{2}$(g) + O$_{2}$(g).

(ii) Decomposition by Light (Photolysis):
2AgCl(s) $\xrightarrow{Sunlight}$ 2Ag(s) + Cl$_{2}$(g).

(iii) Decomposition by Electricity (Electrolysis):
2H$_{2}$O(l) $\xrightarrow{Electricity}$ 2H$_{2}$(g) + O$_{2}$(g).

Question 13: What is the difference between displacement and double displacement reactions? Write equations for these reactions.

Answer-

The difference lies in the number of elements or ions exchanged.

Displacement Reaction: A more reactive element displaces or removes another element from its compound. Only one element replacement occurs.
Equation: Fe(s) + CuSO$_{4}$(aq) $\rightarrow$ FeSO$_{4}$(aq) + Cu(s).

Double Displacement Reaction: There is an exchange of two different atoms or groups of atoms (ions) between the two reactant compounds.
Equation: Na$_{2}$SO$_{4}$(aq) + BaCl$_{2}$(aq) $\rightarrow$ BaSO$_{4}$(s) + 2NaCl(aq).

Question 14: In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write down the reaction involved.

Answer-

Since copper is more reactive than silver, copper displaces silver from the silver nitrate solution:
Cu(s) + 2AgNO$_{3}$(aq) $\rightarrow$ Cu(NO$_{3}$)$_{2}$(aq) + 2Ag(s).

Question 15: What do you mean by a precipitation reaction? Explain by giving examples.

Answer-

A precipitation reaction is any reaction that produces an insoluble substance called a precipitate. This occurs typically when two soluble solutions are mixed, resulting in a solid product.

Example: When solutions of sodium sulphate and barium chloride are mixed, the white precipitate of barium sulphate (BaSO$_{4}$) is formed.
Na$_{2}$SO$_{4}$(aq) + BaCl$_{2}$(aq) $\rightarrow$ BaSO$_{4}$(s) + 2NaCl(aq).

Question 16: Explain the following in terms of gain or loss of oxygen with two examples each. (a) Oxidation (b) Reduction

Answer-

(a) Oxidation: A substance is said to be oxidised if it gains oxygen during a reaction.
Example 1: Formation of copper oxide: 2Cu + O$_{2}$ $\xrightarrow{Heat}$ 2CuO (Copper gains oxygen).
Example 2: Reaction of carbon with zinc oxide: ZnO + C $\rightarrow$ Zn + CO (Carbon gains oxygen).

(b) Reduction: A substance is said to be reduced if it loses oxygen during a reaction.
Example 1: Reduction of copper oxide: CuO + H$_{2}$ $\xrightarrow{Heat}$ Cu + H$_{2}$O (Copper oxide loses oxygen).
Example 2: Reaction of zinc oxide with carbon: ZnO + C $\rightarrow$ Zn + CO (Zinc oxide loses oxygen).

Question 17: A shiny brown coloured element ‘X’ on heating in air becomes black in colour. Name the element ‘X’ and the black coloured compound formed.

Answer-

The shiny brown coloured element ‘X’ is Copper (Cu). When copper is heated in air (oxygen), it undergoes oxidation, and the black coloured compound formed is Copper(II) oxide (CuO).
2Cu + O$_{2}$ $\xrightarrow{Heat}$ 2CuO.

Question 18: Why do we apply paint on iron articles?

Answer-

We apply paint on iron articles to prevent corrosion, specifically the rusting of iron. Painting cuts off the iron surface's contact with moisture and air (oxygen), which are necessary for the metal to corrode.

Question 19: Oil and fat containing food items are flushed with nitrogen. Why?

Answer-

Oil and fat containing food items are flushed with nitrogen gas to prevent rancidity. Nitrogen is an unreactive gas that acts as an antioxidant to slow down the oxidation of the fats and oils present in the food, which prevents the change in their smell and taste.

Question 20: Explain the following terms with one example each. (a) Corrosion (b) Rancidity.

Answer-

(a) Corrosion: This is the process where a metal is attacked by substances around it, such as moisture, acids, etc.. This leads to the metal surface getting tarnished or coated.
Example: The rusting of iron, where iron articles exposed to humid atmosphere are coated with a reddish-brown powder.

(b) Rancidity: This occurs when fats and oils in food materials are oxidised when exposed to air. This oxidation changes the food's smell and taste.
Example: Leaving oil/fat containing food materials (like chips) open for a long time, causing them to develop an unpleasant smell and taste.