Chemical Reactions and Equations - Class 10 Science Chapter 1 - NCERT solution

Chemical Reactions and Equations - Class 10 Science Chapter 1- NCERT Solution

Class 10 Science Chapter 1: Chemical Reactions and Equations - Detailed Solutions

Short Answer Questions

1. Why should a magnesium ribbon be cleaned before it is burnt in air?

Magnesium is a highly reactive metal. When exposed to air, it readily forms a stable layer of magnesium oxide on its surface. This oxide layer acts as a protective barrier, preventing further reaction with oxygen. Cleaning the magnesium ribbon with sandpaper removes this layer, allowing the underlying pure metal to be exposed and react effectively with air during burning.

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

(i) Hydrogen + Chlorine $\rightarrow$ Hydrogen chloride
(ii) Barium chloride + Aluminium sulphate $\rightarrow$ Barium sulphate + Aluminium chloride
(iii) Sodium + Water $\rightarrow$ Sodium hydroxide + Hydrogen

(i) $H_2(g) + Cl_2(g) \rightarrow 2HCl(g)$
(ii) $3BaCl_2(aq) + Al_2(SO_4)_3(aq) \rightarrow 3BaSO_4(s) + 2AlCl_3(aq)$
(iii) $2Na(s) + 2H_2O(l) \rightarrow 2NaOH(aq) + H_2(g)$

3. Write a balanced chemical equation 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.

(i) $BaCl_2(aq) + Na_2SO_4(aq) \rightarrow BaSO_4(s) + 2NaCl(aq)$
(ii) $NaOH(aq) + HCl(aq) \rightarrow NaCl(aq) + H_2O(l)$

1. 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' with water.

(i) The substance 'X' is calcium oxide. Its chemical formula is $CaO$ (Also known as quick lime).
(ii) Calcium oxide reacts vigorously with water to form calcium hydroxide (slaked lime). $CaO(s) + H_2O(l) \rightarrow Ca(OH)_2(aq)$

2. Why is the amount of gas collected in one of the test tubes in Active Chemistry 7 double the amount collected in the other? Name this gas.

Water ($H_2O$) contains two parts of hydrogen and one part of oxygen. Therefore, the amount of hydrogen and oxygen produced during the electrolysis of water is in a ratio of 2:1. During electrolysis, since hydrogen goes to one test tube and oxygen goes to another, the amount of gas collected in one of the test tubes is double the amount collected in the other.

The reaction is: $2H_2O(l) \rightarrow 2H_2(g) + O_2(g)$

The amount of hydrogen gas ($H_2$) collected would be double that of oxygen gas ($O_2$).

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

When an iron nail is placed in a copper sulphate solution, iron displaces copper from the copper sulphate solution, forming iron sulphate, which is green in colour.

The reaction is: $Fe(s) + CuSO_4(aq) \rightarrow FeSO_4(aq) + Cu(s)$

Therefore, the blue color of the copper sulfate solution fades, and a green color appears.

2. Give an example of a double displacement reaction other than the one given in Active Chemistry 11.

Sodium carbonate reacts with calcium chloride to form calcium carbonate and sodium chloride.

The reaction is: $Na_2CO_3(aq) + CaCl_2(aq) \rightarrow CaCO_3(s) + 2NaCl(aq)$

In this reaction, sodium carbonate and calcium chloride exchange their ions mutually to form two new compounds. Hence, it is a double displacement reaction.

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

(i) $4Na(s) + O_2(g) \rightarrow 2Na_2O(s)$
(ii) $CuO(s) + H_2(g) \rightarrow Cu(s) + H_2O(l)$

(i) Sodium ($Na$) is oxidized as it gains oxygen, and oxygen ($O_2$) is reduced.
(ii) Copper oxide ($CuO$) is reduced to copper ($Cu$), while hydrogen ($H_2$) is oxidized to water ($H_2O$).

1. Which of the statements about the reaction below are incorrect?

2PbO(s) + C(s) \rightarrow 2Pb(s) + CO_2(g)

(1) Lead is getting reduced.
(2) Carbon dioxide is getting oxidized.
(3) Carbon is getting oxidized.
(4) Lead oxide is getting reduced.

(a) (1) and (2)
(b) (1) and (3)
(c) (1), (2), and (3)
(d) All of these

(a) (1) and (2)

Carbon is getting oxidized into $CO_2$, while $PbO$ is getting reduced into $Pb$.

2. $Fe_2O_3 + 2Al \rightarrow Al_2O_3 + 2Fe$

The above reaction is an example of a:

(a) combination reaction.
(b) double displacement reaction.
(c) decomposition reaction.
(d) displacement reaction.

(d) The given reaction is an example of a displacement reaction.

3. What happens when dilute hydrochloric acid is added to iron filings? 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.

(a) Hydrogen gas and iron chloride are produced. The reaction is: $Fe(s) + 2HCl(aq) \rightarrow FeCl_2(aq) + H_2(g) \uparrow$

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

A reaction which has an equal number of atoms of each element on both sides of the chemical equation is called a balanced chemical equation.

The law of conservation of mass states that mass can neither be created nor destroyed. Hence, in a chemical reaction, the total mass of reactants should be equal to the total mass of the products. It means that the total number of atoms of each element should be equal on both sides of a chemical equation. Hence, it is for this reason that chemical equations should be balanced.

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

(a) Hydrogen gas combines with nitrogen to form ammonia.
(b) Hydrogen sulfide gas burns in air to give water and sulfur dioxide.
(c) Barium chloride reacts with aluminum sulfate to give aluminum chloride and a precipitate of barium sulfate.
(d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.

(a) $3H_2(g) + N_2(g) \rightarrow 2NH_3(g)$
(b) $2H_2S(g) + 3O_2(g) \rightarrow 2H_2O(l) + 2SO_2(g)$
(c) $3BaCl_2(aq) + Al_2(SO_4)_3(aq) \rightarrow 2AlCl_3(aq) + 3BaSO_4(s)$
(d) $2K(s) + 2H_2O(l) \rightarrow 2KOH(aq) + H_2(g)$

6. Balance the following chemical equations.

(a) $HNO_3 + Ca(OH)_2 \rightarrow Ca(NO_3)_2 + H_2O$
(b) $NaOH + H_2SO_4 \rightarrow Na_2SO_4 + H_2O$
(c) $NaCl + AgNO_3 \rightarrow AgCl + NaNO_3$
(d) $BaCl_2 + H_2SO_4 \rightarrow BaSO_4 + HCl$

(a) $2HNO_3 + Ca(OH)_2 \rightarrow Ca(NO_3)_2 + 2H_2O$
(b) $2NaOH + H_2SO_4 \rightarrow Na_2SO_4 + 2H_2O$
(c) $NaCl + AgNO_3 \rightarrow AgCl + NaNO_3$
(d) $BaCl_2 + H_2SO_4 \rightarrow BaSO_4 + 2HCl$

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

(a) Calcium hydroxide + Carbon dioxide $\rightarrow$ Calcium carbonate + Water
(b) Zinc + Silver nitrate $\rightarrow$ Zinc nitrate + Silver
(c) Aluminum + Copper chloride $\rightarrow$ Aluminum chloride + Copper
(d) Barium chloride + Potassium sulfate $\rightarrow$ Barium sulfate + Potassium chloride

(a) $Ca(OH)_2 + CO_2 \rightarrow CaCO_3 + H_2O$
(b) $Zn + 2AgNO_3 \rightarrow Zn(NO_3)_2 + 2Ag$
(c) $2Al + 3CuCl_2 \rightarrow 2AlCl_3 + 3Cu$
(d) $BaCl_2 + K_2SO_4 \rightarrow BaSO_4 + 2KCl$

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

(a) Potassium bromide(aq) + Barium iodide(aq) $\rightarrow$ Potassium iodide(aq) + Barium bromide(s)
(b) Zinc carbonate(s) $\rightarrow$ Zinc oxide(s) + Carbon dioxide(g)
(c) Hydrogen(g) + Chlorine(g) $\rightarrow$ Hydrogen chloride(g)
(d) Magnesium(s) + Hydrochloric acid(aq) $\rightarrow$ Magnesium chloride(aq) + Hydrogen(g)

(a) $2KBr(aq) + BaI_2(aq) \rightarrow 2KI(aq) + BaBr_2(s)$; Double displacement reaction
(b) $ZnCO_3(s) \rightarrow ZnO(s) + CO_2(g)$; Decomposition reaction
(c) $H_2(g) + Cl_2(g) \rightarrow 2HCl(g)$; Combination reaction
(d) $Mg(s) + 2HCl(aq) \rightarrow MgCl_2(aq) + H_2(g)$; Displacement reaction

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

Chemical reactions that release energy in the form of heat are called exothermic reactions.

Example: $Na(s) + \frac{1}{2}Cl_2(s) \rightarrow NaCl(s) + 411 \, kJ \text{ of energy}$

Reactions that absorb energy or require energy in order to proceed are called endothermic reactions.

Example: $6CO_2(g) + 6H_2O(l) \xrightarrow{Sunlight} C_6H_{12}O_6(aq) + 6O_2(g)$

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

Energy is released during respiration, a process where glucose combines with oxygen in cells to produce carbon dioxide, water, and energy.

The reaction is: $C_6H_{12}O_6(aq) + 6O_2(g) \rightarrow 6CO_2(g) + 6H_2O(l) + Energy$

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

Decomposition reactions break down a compound into simpler substances using energy, while combination reactions combine substances to form a new compound, releasing energy.

Decomposition: $AB + Energy \rightarrow A + B$ (e.g., $2H_2O(l) \rightarrow 2H_2(g) + O_2(g)$)

Combination: $A + B \rightarrow AB + Energy$ (e.g., $2H_2(g) + O_2(g) \rightarrow 2H_2O(l)$)

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

(a) Thermal decomposition: $2FeSO_4(s) \rightarrow Fe_2O_3(s) + SO_2(g) + SO_3(g)$
(b) Decomposition by light: $2AgCl(s) \rightarrow 2Ag(s) + Cl_2(g)$
(c) Decomposition by electricity: $2Al_2O_3(aq) \rightarrow 4Al(s) + 3O_2(g)$

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

Displacement: $A + BX \rightarrow AX + B$ (e.g., $CuSO_4(aq) + Zn(s) \rightarrow ZnSO_4(aq) + Cu(s)$)

Double displacement: $AB + CD \rightarrow AD + CB$ (e.g., $Na_2SO_4(aq) + BaCl_2(aq) \rightarrow BaSO_4(s) + 2NaCl(aq)$)

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

$2AgNO_3(aq) + Cu(s) \rightarrow Cu(NO_3)_2(aq) + 2Ag(s)$

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

A reaction in which an insoluble solid (precipitate) is formed is called a precipitation reaction.

Example: $Na_2SO_4(aq) + BaCl_2(aq) \rightarrow BaSO_4(s) \downarrow + 2NaCl(aq)$

In this reaction, barium sulphate is obtained as a precipitate.

17. A Shiny brown Coloured Element 'X' is copper (Cu) and the black-coloured compound formed is copper oxide (CuO). The equation of the reaction involved in heating copper is given below.

$2Cu (Shiny \, brown) + O_2 \xrightarrow{Heat} 2CuO (Black)$

18. Why do we apply paint on iron articles?

Iron articles are painted because it prevents them from rusting. When painted, the contact of iron articles with moisture and air is cut off. Hence, rusting is prevented. Their presence is essential for rusting to take place.

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

Nitrogen is an inert gas and does not easily react with these substances. On the other hand, oxygen reacts with food substances and makes them rancid. Thus, bags used in packing food items are flushed with nitrogen gas to remove oxygen inside the pack. When oxygen is not present inside the pack, rancidity of oil and fat containing food items is prevented.

20. Explain the following terms with one example of each.

(a) Corrosion
(b) Rancidity

(a) Corrosion- Corrosion is defined as a process where materials, usually metals, deteriorate as a result of a chemical reaction with air, moisture, chemicals, etc.

For example, iron, in the presence of moisture, reacts with oxygen to form hydrated iron oxide.

4Fe + 3O_2 + nH_2O \rightarrow 2Fe_2O_3 \cdot nH_2O

This hydrated iron oxide is rust.

(b) Rancidity - The process of oxidation of fats and oils that can be easily noticed by the change in taste and smell is known as rancidity.

For example, the taste and smell of butter changes when kept for long. Rancidity can be avoided by

(1) Storing food in airtight containers.
(2) Storing food in refrigerators.
(3) Adding antioxidants.
(4) Storing food in an environment of nitrogen.

Multiple Choice Questions

1. Physical changes are generally ________.

(1) temporary
(2) permanent
(3) irreversible
(4) endothermic

2. The symbol H stands for ________ of hydrogen.

(1) one atom
(2) one molecule
(3) one ion
(4) two atoms

3. Which is the correct symbol for manganese?

(1) M
(2) Ma
(3) Mn
(4) Mg

4. The correct formula for nitrogen dioxide is ________.

(1) NO
(2) N₂O
(3) NO₂
(4) N₂O₅

5. Which of the following is an incorrect formula?

(1) NaCl₂
(2) BaSO₄
(3) H₂CO₃
(4) P₂O₅

6. The chemical formula of lead sulphate is

(1) Pb₂SO₄
(2) Pb(SO₄)₂
(3) PbSO₄
(4) Pb₂(SO₄)₃

7. One of the following processes does not involve a chemical reaction. That is:

(1) Melting of candle wax when heated
(2) Burning of candle wax when heated
(3) Digestion of food in our stomach
(4) Ripening of banana

8. The correct formula for ammonium sulphate is ________.

(1) NH₄SO₄
(2) (NH₄)₂SO₄
(3) (NH₃)₂SO₄
(4) (NH₄)₂S

9. In the following equation $Na_2CO_3 + x HCl \rightarrow 2 NaCl + CO_2 + H_2O$, the value of x is

(1) 1
(2) 2
(3) 3
(4) 4

10. Consider the reaction: $Na(s) + O_2(g) \rightarrow Na_2O(s)$. Number of sodium needed to balance the equation would be

(1) 1
(2) 2
(3) 3
(4) 4

11. Consider the reaction: $Al(s) + O_2(g) \rightarrow Al_2O_3$. Number of Al(s) needed to balance the equation are

(1) 1
(2) 2
(3) 3
(4) 4

12. It is necessary to balance a chemical equation in order to satisfy the law of

(1) Conservation of motion
(2) Conservation of momentum
(3) Conservation of energy
(4) Conservation of mass

13. The correctly balanced equation for S is ________.

(1) $2FeS + O_2 \rightarrow Fe_2O_3 + 4SO_2$
(2) $2FeS + 3O_2 \rightarrow 2Fe_2O_3 + 4SO_2$
(3) $4FeS + 4O_2 \rightarrow 2Fe_2O_3 + 2SO_2$
(4) $4FeS + 7O_2 \rightarrow 2Fe_2O_3 + 4SO_2$

14. Consider the reaction: $S(s) + O_2(g) \rightarrow SO_2$. The state of $SO_2$ in this reaction is

(1) liquid
(2) solid
(3) gaseous
(4) all of these

15. Which of the following symbols is not a correct indication of the phase of a substance in a reaction?

(1) (s) = solid
(2) (g) = grams
(3) (aq) = aqueous
(4) (ℓ) = liquid

16. The reaction $H_2 + Cl_2 \rightarrow 2HCl$ is a

(1) decomposition reaction
(2) combination reaction
(3) double displacement reaction
(4) displacement reaction

17. The reaction $C + O_2 \rightarrow CO_2 + Heat$, is a

(1) combination reaction
(2) oxidation reaction
(3) exothermic reaction
(4) all of these

18. The electrolytic decomposition of water gives $H_2$ and $O_2$ in the ratio of

(1) 1:2 by volume
(2) 2:1 by volume
(3) 8:1 by mass
(4) 1:2 by mass

19. When hydrated ferrous sulphate is heated strongly it undergoes decomposition to form ferric oxide as a main product accompanied by a change in colour from

(1) blue to green.
(2) green to blue.
(3) green to brown.
(4) green to yellow.

20. Breaking of lead bromide into lead and bromine is an example of ________.

(1) decomposition reaction
(2) synthesis reaction
(3) displacement reaction
(4) neutralisation reaction

21. Which of the following is not a decomposition reaction?

(1) $CaCO_3 \rightarrow CaO + CO_2$
(2) $2KClO_3 \rightarrow 2KCl + 3O_2$
(3) Digestion of food in the body
(4) $H_2 + Cl_2 \rightarrow 2HCl$

22. Which of the following is a decomposition reaction?

(1) $NaOH + HCl \rightarrow NaCl + H_2O$
(2) $NH_4CNO \rightarrow H_2NCONH_2$
(3) $2KClO_3 \rightarrow 2KCl + 3O_2$
(4) $H_2 + I_2 \rightarrow 2HI$

23. Conversion of $CaCO_3$ into $CaO$ as per the following reaction is an example of

CaCO_3 \rightarrow CaO + CO_2

(1) decomposition reaction
(2) reduction reaction
(3) oxidation reaction
(4) none of these

24. Which of the following is a displacement reaction?

(1) $CaCO_3 \rightarrow CaO + CO_2$
(2) $CaO + 2HCl \rightarrow CaCl_2 + H_2O$
(3) $Fe + CuSO_4 \rightarrow FeSO_4 + Cu$
(4) $NaOH + HCl \rightarrow NaCl + H_2O$

25. $Fe_2O_3 + 2Al \rightarrow Al_2O_3 + 2Fe$

This reaction is an example of

(1) combination reaction
(2) double displacement reaction
(3) decomposition reaction
(4) displacement reaction

26. When iron nails are added to an aqueous solution of copper sulphate, a chemical change occurs. Which of the following is not true about this reaction?

(1) The blue colour of the solution fades.
(2) Iron nails become brownish in colour.
(3) It is a displacement reaction.
(4) Iron nails dissolve completely.

27. In the reaction : $BaCl_2 + ZnSO_4 \rightarrow ZnCl_2 + BaSO_4$, the white precipitate seen is due to ________.

(1) $ZnCl_2$
(2) $BaSO_4$
(3) $BaCl_2$
(4) $ZnSO_4$

28. Which of the following represents a double displacement reaction?

(1) $2H_2 + O_2 \rightarrow 2H_2O$
(2) $2Mg + O_2 \rightarrow 2MgO$
(3) $AgNO_3 + NaCl \rightarrow AgCl(\downarrow) + NaNO_3$
(4) $H_2 + Cl_2 \rightarrow 2HCl$

29. Which of the following reactions is a double displacement reaction?

(1) $FeCl_3 + 3NaOH \rightarrow Fe(OH)_3 + 3NaCl$
(2) $Zn + H_2SO_4 \rightarrow ZnSO_4 + H_2$
(3) $2CO + O_2 \rightarrow 2CO_2$
(4) $N_2 + O_2 \rightarrow 2NO$

30. Which of the following is a redox reaction?

(1) $CaCO_3 \rightarrow CaO + CO_2$
(2) $CuO + H_2 \rightarrow Cu + H_2O$
(3) $CaO + 2HCl \rightarrow CaCl_2 + H_2O$
(4) $NaOH + HCl \rightarrow NaCl + H_2O$

31. Which of the following statements is correct about the reaction?

2PbO(s) + C(s) \rightarrow 2Pb(s) + CO_2(g)

(1) Lead is getting reduced
(2) Carbon is getting reduced
(3) Lead oxide is getting reduced
(4) All of these

32. $Mg + O_2 \rightarrow MgO$ which is correct about this reaction?

(1) Mg is oxidized
(2) Oxygen is oxidized
(3) MgO is reduced
(4) MgO is oxidized

33. Corrosion of iron is known as

(1) rusting
(2) rancid
(3) antioxidant
(4) reduction

34. Which of the following gases is used in the storage of fat and oil containing foods for a long time?

(1) Carbon dioxide gas
(2) Nitrogen gas
(3) Oxygen gas
(4) Neon gas

35. The main cause of rancidity in foods is

(1) bacteria
(2) proteins
(3) antioxidants
(4) oxidation of the fatty acid molecule

Fill in the Blanks

1. Formation of nitric oxide from nitrogen and oxygen is a combination reaction.

2. Reaction in which energy is absorbed is known as endothermic reaction.

3. The reaction in which heat is given out along with products is known as exothermic reaction.

4. Digestion of food in our body is an example of decomposition reaction.

5. The reaction $CaCO_3 \xrightarrow{Heat} CaO + CO_2$ is a decomposition reaction.

6. The reaction in which oxygen is added to the substance is called oxidation reaction.

7. The reaction in which hydrogen is added to a substance is called reduction reaction.

8. The potato chips manufacturers use nitrogen gas to fill the chips bags to prevent the chips from getting oxidised.

9. When calcium carbonate is heated, it decomposes to give calcium oxide and carbon dioxide.

10. Rancidity of food is a chemical change.

True or False

1. Rusting of iron is a physical change. False

2. In general, combination reactions are exothermic. True

3. A physical change is usually reversible. True

4. When a magnesium ribbon burns brilliantly in air, a physical change takes place. False

5. $3Fe + H_2O \rightarrow Fe_3O_4 + 4H_2$ is a balanced reaction. False

6. Ammonium chloride dissolves in water with the absorption of heat. This is an Exothermic reaction. False

7. At least one of the products or reactants must be solid. False

8. Iron nails become brownish in copper sulphate solution. True

9. Precipitate is an insoluble substance formed during a reaction. True

10. If a substance gains oxygen during a reaction, it is said to be, not reduced. True

NCERT Solution- Chemical Reaction and Equations- Class 10 Science