Class 10 Science Chapter – 1 Chemical Reactions and Equations Questions and Answers New Updated NCERT Solutions 

Class 10 Science Chapter – 1 Chemical Reactions and Equations

1.1 CHEMICAL  EQUATIONS , QUESTIONS Page no : 6 

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

Answer: A magnesium ribbon must be cleaned before burning in air to remove any oxidation layer or impurities that may have formed on its surface. This is important for the following reasons: 

1. Ensures Pure Reaction: The presence of impurities or oxides can interfere with the burning process, leading to incomplete combustion or a less vigorous reaction. Cleaning the ribbon ensures that the magnesium reacts directly with oxygen in the air.

2. Enhances Visibility of Reaction: When the magnesium ribbon is clean, it burns with a bright white flame, producing magnesium oxide. This clear reaction is essential for observing the characteristics of the chemical change.

3. Safety Precaution: The cleaning process, typically done with sandpaper, also helps to ensure that the reaction proceeds safely and predictably, minimizing the risk of unexpected reactions due to contaminants.

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

(i) Hydrogen + Chlorine → Hydrogen chloride 

Answer: The unbalanced equation is:  H2+Cl2→HCl 

Balanced Equation: H2+Cl2​→2HCl 

(ii) Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride 

Answer: The unbalanced equation is: BaCl2​+Al2(SO4)3→BaSO4+AlCl3

Balanced Equation: 3BaCl2+Al2(SO4​)3​→3BaSO4​+2AlCl3

(iii) Sodium + Water → Sodium hydroxide + Hydrogen 

Answer: The unbalanced equation is: Na+H2O→NaOH+H2

Balanced Equation: 2Na+2H2O→2NaOH+H2

Question: 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. 

Answer:  The unbalanced equation is: BaCl2(aq)+Na2SO4(aq)→BaSO4(s)+NaCl(aq) .

Balanced Equation: BaCl2(aq)+Na2SO4(aq)→BaSO4(s)+2NaCl(aq) .

(ii) Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water. 

Answer: The unbalanced equation is: NaOH(aq)+HCl(aq)→NaCl(aq)+H2O(l) .

Balanced Equation: NaOH(aq)+HCl(aq)→NaCl(aq)+H2O(l) .

1.2 TYPES OF CHEMICAL REACTIONS  , QUESTIONS Page no : 10

Question: 1. A solution of a substance ‘X’ is used for whitewashing. 

(i) Name the substance ‘X’ and write its formula. 

Answer: The substance used for whitewashing is slaked lime.

Chemical Formula: The formula for slaked lime is Ca(OH)₂.

(ii) Write the reaction of the substance ‘X’ named in (i) above with water. 

Answer: When slaked lime is mixed with water, it forms a solution of calcium hydroxide. The reaction can be represented as:

CaO (s)+H2O (l)→Ca(OH)2(aq)+Heat .

Here, CaO (calcium oxide) reacts with water to produce Ca(OH)₂ (calcium hydroxide) and releases heat in the process.

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: In Activity 1.7, when water is electrolyzed, two different gases are produced at the electrodes. The reason why the amount of gas collected in one test tube is double that of the other is due to the stoichiometry of the reaction involved in the electrolysis of water. 

1. Gas Collection: 

– During the electrolysis of water, hydrogen gas (H₂) is produced at the cathode (negative electrode) and oxygen gas (O₂) is produced at the anode (positive electrode).

The reaction can be represented as: 2H2O(l)→2H2(g)+O2(g) . 

– From this equation, it is clear that 2 moles of hydrogen gas are produced for every 1 mole of oxygen gas.

2. Volume Relationship: 

– Since gases occupy the same volume under identical conditions of temperature and pressure, the volume of hydrogen collected is twice that of the oxygen collected. Therefore, if you collect the gases in two separate test tubes, the volume of hydrogen in one test tube will be double that of the volume of oxygen in the other test tube.

3. Name of the Gas: 

– The gas that is collected in the test tube with the larger volume is hydrogen (H₂), while the gas collected in the other test tube is oxygen (O₂).

– This stoichiometric relationship is a fundamental concept in chemistry, illustrating how the ratios of reactants and products in a chemical reaction can predict the amounts of gases produced.

1.3 HAVE YOU OBSERVED THE EFFECTS OF OXIDATION REACTIONS IN EVERYDAY LIFE? , QUESTIONS Page no : 13

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

Answer: When an iron nail is dipped into a copper sulfate solution, a noticeable change occurs in the color of the solution. This is due to a displacement reaction that takes place between the iron and the copper sulfate.

Explanation of the Reaction:

• Chemical Reaction: The iron (Fe) displaces the copper (Cu) from the copper sulfate (CuSO₄) solution. The reaction can be represented as follows:

Fe(s)+CuSO4(aq)→FeSO4(aq)+Cu(s)  .

• Observations:
  • The blue color of the copper sulfate solution fades as the copper ions are displaced and form iron(II) sulfate, which is colorless in solution.
  • The displaced copper precipitates out as a reddish-brown solid on the surface of the iron nail.

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 compounds, resulting in the formation of new compounds. Here’s an example of a double displacement reaction that is commonly observed:

Example of a Double Displacement Reaction

Reaction of Sodium Sulfate with Barium Chloride:

When solutions of sodium sulfate (Na2SO4) and barium chloride (BaCl2) are mixed, a white precipitate of barium sulfate (BaSO4BaSO4) is formed along with sodium chloride (NaCl) remaining in solution. The reaction can be represented as follows:

Na2SO4(aq)+BaCl2(aq)→BaSO4(s)+2NaCl(aq) .

Observations:

• Precipitate Formation: The formation of barium sulfate is indicated by the appearance of a white solid (precipitate) in the solution.
• Color Change: The solution may change color due to the formation of the precipitate.

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

(i) 4Na(s) + O2 (g) → 2Na2 O(s) 

Answer: In this given reaction:  4Na(s) + O2 (g) → 2Na2 O(s)  

We can identify the substances that are oxidized and reduced by analyzing the changes in oxidation states of the elements involved.

Oxidation and Reduction:

1. Oxidation:
   • Sodium (Na): In its elemental form, sodium has an oxidation state of 0. In sodium oxide (Na2O), sodium has an oxidation state of +1.
   • Change: The oxidation state of sodium increases from 0 to +1, indicating that sodium is oxidized.
2. Reduction:
   • Oxygen (O): In its elemental form, oxygen has an oxidation state of 0. In sodium oxide (Na2O), oxygen has an oxidation state of -2.
   • Change: The oxidation state of oxygen decreases from 0 to -2, indicating that oxygen is reduced.

Summary

• Oxidized Substance: Sodium (Na) is oxidized from 0 to +1.
• Reduced Substance: Oxygen (O) is reduced from 0 to -2.

(ii) CuO(s) + H2 (g) → Cu(s) + H2 O(l) 

Answer: In the given reaction: CuO(s) + H2 (g) → Cu(s) + H2 O(l) 

we can identify the substances that are oxidized and reduced by analyzing the changes in oxidation states of the elements involved.

Oxidation and Reduction:

1. Oxidation:

• Hydrogen (H): In molecular hydrogen (H2), hydrogen has an 
• oxidation state of 0. In water (H2O), hydrogen has an oxidation state of +1.
• Change: The oxidation state of hydrogen increases from 0 to +1, indicating that hydrogen is oxidized.

2. Reduction:

• Copper (Cu): In copper(II) oxide (CuO), copper has an oxidation state of +2. In elemental copper (Cu), the oxidation state is 0.
• Change: The oxidation state of copper decreases from +2 to 0, indicating that copper is reduced.

Summary

• Oxidized Substance: Hydrogen (H) is oxidized from 0 to +1.
• Reduced Substance: Copper (Cu) is reduced from +2 to 0.

EXERCISES  Page no:14

Question: 1. 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   

(iv) (c) all 

Answer: The correct choice is: (i) (a) and (b).

Question: 2. Fe2 O3 + 2Al → Al2 O3 + 2Fe The above reaction is an example of a 

(a) combination reaction. 

(b) double displacement reaction. 

(c) decomposition reaction. 

(d) displacement reaction. 

Answer: The correct choice is: (d) displacement reaction.

Question: 3. 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. 

Answer: The correct answer is: (a) Hydrogen gas and iron chloride are produced. 

Page no: 15

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

Answer: A balanced chemical equation is a representation of a chemical reaction that shows the reactants and products along with their respective quantities in such a way that the number of atoms of each element is the same on both sides of the equation. This reflects the law of conservation of mass, which states that matter cannot be created or destroyed in a chemical reaction. 

Example of a Balanced Chemical Equation

For instance, the reaction of hydrogen and oxygen to form water can be represented as:

2H2(g)+O2(g)→2H2O(l)  .

In this equation:

There are 4 hydrogen atoms and 2 oxygen atoms on both sides, indicating that the equation is balanced.

Importance of Balancing Chemical Equations 

1. Conservation of Mass: Balancing ensures that the mass of the reactants equals the mass of the products, adhering to the principle that matter is neither created nor destroyed.

2. Stoichiometry: A balanced equation provides the correct proportions of reactants and products, which is essential for calculating the amounts needed or produced in a reaction.

3. Predicting Reaction Outcomes: Understanding the ratios of reactants to products helps in predicting how much of each substance will be consumed or produced.

4. Chemical Safety: Accurate balancing is crucial in industrial and laboratory settings to ensure safety and efficiency in chemical processes.

Conclusion

In summary, a balanced chemical equation is essential for accurately representing chemical reactions, ensuring compliance with the law of conservation of mass, and facilitating calculations related to the quantities of substances involved.

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

(a) Hydrogen gas combines with nitrogen to form ammonia. 

Answer: Chemical Equation: H2(g)+N2(g)→NH3(g) .

Balanced Equation: To balance this equation, we need to ensure that the number of atoms of each element is the same on both sides. The balanced equation is: 

3H2(g)+N2(g)→2NH3(g) .

(b) Hydrogen sulphide gas burns in air to give water and sulfur dioxide. 

Answer: Chemical Equation: H2S(g)+O2(g)→H2O(g)+SO2(g) .

Balanced Equation: To balance this equation, we need to ensure that the number of atoms of each element is the same on both sides. The balanced equation is: 

2H2​S(g)+3O2(g)→2H2O(g)+2SO2(g) .

(c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate. 

Answer: Chemical Equation: BaCl2(aq)+Al2(SO4)3(aq)→AlCl3(aq)+BaSO4(s) .

Balanced Equation: To balance this equation, we need to ensure that the number of atoms of each element is the same on both sides. The balanced equation is: 

3BaCl2(aq)+Al2(SO4)3(aq)→2AlCl3(aq)+3BaSO4(s) .

(d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas. 

Answer: Chemical Equation: K(s)+H2O(l)→KOH(aq)+H2(g) .

Balanced Equation: To balance this equation, we need to ensure that the number of atoms of each element is the same on both sides. The balanced equation is: 

2K(s)+2H2O(l)→2KOH(aq)+H2(g) .

Question: 6. Balance the following chemical equations. 

(a) HNO3 + Ca(OH)2 → Ca(NO3 )2 + H2O 

Answer: Balanced Equation: 2HNO3+Ca(OH)2→Ca(NO3)2+2H2O .

(b) NaOH + H2 SO4 → Na2 SO4 + H2O 

Answer: Balanced Equation: 2NaOH+H2SO4→Na2SO4+2H2O .

(c) NaCl + AgNO3 → AgCl + NaNO3 

Answer: Balanced Equation: NaCl+AgNO3→AgCl+NaNO3  .

(d) BaCl2 + H2 SO4 → BaSO4 + HCl

Answer: Balanced Equation: BaCl2+H2SO4→BaSO4+2HCl  .

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

Answer: (a) Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water 

Balanced Equation: Ca(OH)2+CO2→CaCO3+H2O  .

(b) Zinc + Silver nitrate → Zinc nitrate + Silver 

Answer: Balanced Equation: Zn+2AgNO3→Zn(NO3)2+2Ag .

(c) Aluminium + Copper chloride → Aluminium chloride + Copper 

Answer: Balanced Equation: 2Al+3CuCl2→2AlCl3+3Cu .

(d) Barium chloride + Potassium sulphate → Barium sulphate + Potassium chloride 

Answer: Balanced Equation: BaCl2+K2SO4→BaSO4+2KCl .

(Note: These equations are already balanced as written.)

Question: 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) → Potassium iodide(aq) + Barium bromide(s)

Answer: Unbalanced Equation: KBr(aq)+Bal2(aq)→KI(aq)+BaBr2(s) .

Balanced Equation: 2KBr(aq)+BaI2(aq)→2KI(aq)+BaBr2(s) .

(b) Zinc carbonate(s) → Zinc oxide(s) + Carbon dioxide(g) 

Answer: Unbalanced Equation: ZnCO3(s)→ZnO(s)+CO2(g) .

Balanced Equation: ZnCO3(s)→ZnO(s)+CO2(g) .

(c) Hydrogen(g) + Chlorine(g) → Hydrogen chloride(g)

Answer: Unbalanced Equation: H2(g)+Cl2(g)→HCl(g) .

Balanced Equation: H2(g)+Cl2(g)→2HCl(g) .

(d) Magnesium(s) + Hydrochloric acid(aq) → Magnesium chloride(aq) + Hydrogen(g) 

Answer: Unbalanced Equation: Mg(s)+HCl(aq)→MgCl2(aq)+H2(g)      .

Balanced Equation: Mg(s)+2HCl(aq)→MgCl2(aq)+H2(g) .

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

Answer: Exothermic Reactions: 

1. Definition: Exothermic reactions are those that release energy in the form of heat or light to the surroundings during the reaction. This results in an increase in the temperature of the surroundings.

2. Characteristics: 

– Energy is released.

– The enthalpy change (ΔH) is negative.

Examples: 

1. Combustion of Fuels: For instance, the burning of natural gas (methane) can be represented as:  

CH4(g)+2O2(g)→CO2(g)+2H2O(g)+Energy   

2. Respiration: The process of respiration in living organisms, where glucose reacts with oxygen to produce carbon dioxide, water, and energy: C6H12O6(aq)+6O2(aq)→6CO2(aq)+6H2O(l)+Energy 

Endothermic Reactions:

Definition: Endothermic reactions are those that absorb energy from the surroundings, usually in the form of heat. This results in a decrease in the temperature of the surroundings.

Characteristics: 

1. Energy is absorbed.

2. The enthalpy change (ΔH) is positive.

Examples: 

– Photosynthesis: The process by which plants convert carbon dioxide and water into glucose and oxygen using sunlight: 

6CO2(g)+6H2O(l)+Energy→C6H12O6(aq)+6O2(g) .

– Dissolving Ammonium Nitrate in Water: When ammonium nitrate is dissolved in water, it absorbs heat, resulting in a cooling effect: 

H2O(l)→NH4+(aq)+NO3−(aq)+Energy (absorbed) .

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

Answer: Definition of Respiration: Respiration is a biochemical process in which glucose (a carbohydrate) is broken down in the presence of oxygen to produce carbon dioxide, water, and energy. This process occurs in living organisms to release energy necessary for various cellular activities.

Chemical Equation for Respiration: The overall reaction for respiration can be represented as: C6H12O6(aq)+6O2​(g)→6CO2​(aq)+6H2​O(l)+Energy

Why is it Exothermic? 

1. Energy Release: 

– During respiration, the chemical bonds in glucose are broken, and new bonds are formed in carbon dioxide and water. The energy stored in the glucose molecule is released during this process.

– The energy released is utilized by cells for various functions such as muscle contraction, maintaining body temperature, and synthesizing new molecules.

2. Temperature Increase: 

– Since respiration releases energy, it results in an increase in the temperature of the surroundings, which is a characteristic of exothermic reactions.

3. Negative Enthalpy Change: 

– The enthalpy change (ΔH) for respiration is negative, indicating that energy is released to the environment.

Summary:

Respiration is considered an exothermic reaction because it involves the breakdown of glucose in the presence of oxygen, resulting in the release of energy, carbon dioxide, and water. This energy is essential for sustaining life processes, and the increase in temperature during respiration further confirms its exothermic nature.

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

Answer: Definition of Combination Reactions:

• In a combination reaction, two or more substances (elements or compounds) combine to form a single product. This process involves the formation of new chemical bonds.
• General Form: 
• A+B→AB

Example of a Combination Reaction: 

– Formation of Water: 2H2(g)+2(g)→2H2O(l)

In this reaction, hydrogen and oxygen combine to form water.

Definition of Decomposition Reactions:

• In a decomposition reaction, a single compound breaks down into two or more simpler substances. This process involves the breaking of chemical bonds.
• General Form: 
• AB→A+B

Example of a Decomposition Reaction: 

– Decomposition of Calcium Carbonate: CaCO3​(s)→CaO(s)+CO2​(g)

In this reaction, calcium carbonate decomposes into calcium oxide and carbon dioxide.

Why They Are Opposites: 

1. Process: 

– Combination Reactions: Involve the formation of a single product from multiple reactants. 

– Decomposition Reactions: Involve the breakdown of a single reactant into multiple products.

2. Bond Formation vs. Bond Breaking:Combination Reactions: New bonds are formed as reactants combine. 

– Decomposition Reactions: Existing bonds are broken to yield simpler substances.

3. Example Comparison: 

– In the combination reaction of hydrogen and oxygen, two elements combine to form water. Conversely, in the decomposition of water, water can be broken down into hydrogen and oxygen gas: 2H2O(l)→2H2(g)+O2(g)

Summary:

Decomposition reactions are considered the opposite of combination reactions because they involve the breakdown of a single compound into simpler substances, while combination reactions involve the formation of a single compound from multiple reactants. The two processes are fundamentally different in terms of bond formation and breaking.