NCERT Solution Class 10 Science

Chapter 1: Chemical Reactions and Equations

This NCERT Solution is for students of Class 10. Here we will be discussing the Solution of Chapter 1, Chemical Reactions and Equations, which is the first chapter of NCERT Class 10 Science textbook. Through these solutions, students can understand the concepts easily within a short time. All answers are properly explained with examples so that students do not have to face any difficulty and the language is very simple and easy to understand. Students preparing for class 10 board exams can get a great benefit from these simple and easy solutions.

NCERT Solution of Science Class 10 Chapter 1 Chemical Reactions and Equations, questions and answers will help students to revise and complete their syllabus so that they can easily achieve good marks in their board exams with ease. Also if the students are getting assignments then these solutions will quickly help them to solve their problems so that they can save their time for self-study.

Chapter1: Chemical Reactions and Equations Class 10 Questions Solutions

Page Number 6:

This section contains solutions to the questions given on Page Number 6 of the NCERT Science Book for Class 10 Chapter 1.

Question 1: Magnesium ribbon should be cleaned before burning in the air. Give Reason.

Answer: Magnesium is a very reactive element and when this element is brought in contact with air it forms a white layer of magnesium oxide due to the presence of oxygen in the air. As we all know air takes up all the empty space so there might be a possibility that some portion of the magnesium ribbon which will be used in experiments has already reacted with air and it already formed the magnesium oxide layer on the ribbon. So the magnesium oxide deposit will either slow down or prevent the magnesium ribbon from burning. In order to get the pure magnesium ribbon, it should be properly cleaned. Cleaning the magnesium ribbon will also help to remove any kind of unwanted impurity deposit on the magnesium ribbon and only pure magnesium ribbon will be used for the reaction.

Question 2: Write a balanced equation for the below chemical reactions.

i.) Hydrogen + Chlorine → Hydrogen Chloride

ii.) Barium Chloride + Aluminium Sulphate → Barium Sulphate + Aluminium Chloride

iii.) Sodium + Water → Sodium Hydroxide + Hydrogen

Answer: i.) H₂ + Cl₂→2HCl is the correct answer.

Explanation: If we take a look at the first chemical reaction then we see that the chemical formula of the reactants and the products are not written rather they are written in plain English. Here Hydrogen and Chlorine are the reactants and Hydrogen Chloride is the product. So our first step is to write the chemical formulas for reactants and products.

Chemical Formulas of Reactants and Products:

For Hydrogen, we write it as H₂. For Chlorine it is Cl₂ and for the product i.e. Hydrogen Chloride the formula is HCl.

Now take a look at this unbalanced reaction,

H₂ + Cl₂→ HCl

Now see the numbers on reactants and the products. You will notice that Hydrogen holds the value of 2 on the reactant side while 1 on the product side and the same thing goes for chlorine also. Now in order to make this chemical equation balance, we have to add 2 on the product side. So our new balanced chemical equation will look like this.

H₂ + Cl₂→ 2HCl

Now Hydrogen and Chlorine both have attained the value 2 on the reactant side as well as on the product side, thus our reaction is complete and balanced. 2HCl is equivalent to 2H2Cl.

Remember that we can write any number according to our wish but reactants and products should be properly balanced. Also, we cannot write numbers between chemical formulas.

For Example: If someone balances the above equation like this, H₂ + Cl₂→ H₂Cl₂, then this is completely wrong. You can write numbers only in front of the chemical formulas and not in between the chemical formulas.

ii.) 3BaCl₂ + Al₂(SO₄)₃→3BaSO₄ + 2AlCl₃, is the correct answer.

Explanation: Here also we will follow the same steps as explained above. The first task is to write the chemical formulas for reactants and products.

Chemical Formulas of Reactants and Products:

Barium Chloride - BaCl₂

Aluminium Sulphate - Al₂(SO₄)₃

Barium Sulphate - BaSO₄

Aluminium Chloride - AlCl₃

Now the reaction will look like the following,

BaCl₂ + Al₂(SO₄)₃→ BaSO₄ + AlCl₃

In this chemical equation, we have a total of 4 elements i.e. Barium, Chlorine, Aluminium and Sulfate. Now let's balance the chemical equation.

Initially, barium holds a value of 1 on both sides so no problem with this element right now. Chlorine holds a value of 2 on the reactant side however on the product side it has got a value of 3. So to balance chlorine we need to put a 3 before barium chloride on the reactant side. So the reaction will become something like this.

3BaCl₂ + Al₂(SO₄)₃→ BaSO₄ + AlCl₃

Now barium has got a value of 3 on the reactant side while 1 on the product side. Now chlorine has attained the value of 6 on the reactant side however it is still 3 on the product side. So we add 3 to barium and 2 to chlorine on the product side and the equation becomes.

3BaCl₂ + Al₂(SO₄)₃→ 3BaSO₄ + 2AlCl₃

Chlorine has a value of 6 on the reactant side because, in 3BaCl₂, 3 is distributed over both barium and chlorine. To be more specific 3BaCl₂ is equivalent to 3Ba3Cl₂. So 3Cl₂ is (3x2=6). We have multiplied them because it's a rule we follow.

Look that barium and chlorine have been balanced. Barium has got a value of 3 and Chlorine has a value of 6 on both sides.

Take a look at aluminium; you will notice that it has got the value 2 on both sides automatically. Also, sulphate has a value of 12 on both sides, so it is already balanced.

On the reactant side, sulfate has the value 12 because (SO₄)₃ is equivalent to (SO_4x3) = (SO₁₂). It's also a rule we follow for balancing chemical equations. On the product side, 3BaSO₄ is equivalent to 3Ba3SO₄ and 3SO₄ has the number (3x4=12).

The final balanced chemical equation is,

3BaCl₂ + Al₂(SO₄)₃→ 3BaSO₄ + 2AlCl₃

iii.) 2Na+ 2H₂O→2NaOH + H₂↑, is the correct answer.

Explanation: Chemical Formulas of Reactants and Products:

Sodium - Na

Water - H₂O

Sodium Hydroxide -NaOH

Hydrogen - H₂

Now the given chemical equation will look like this,

Na+ H₂O→NaOH + H₂↑

Important: H₂↑ is a hydrogen gas which is evolved during the reaction.

Initially, sodium is balanced and has a value of 1 on both sides and the same applies to oxygen also but the hydrogen is unbalanced. Hydrogen has got a value of 2 on the reactant side and 3 on the product side. So we need to add 2 before sodium hydroxide to get the hydrogen element balanced. The new chemical equation will be.

Na+ H₂O→2NaOH + H₂↑

2NaOH means 2 is distributed over all the elements i.e. 2NaOH is equivalent to 2Na2O2H. So sodium and oxygen also have the value of 2 on the product side. In order to balance them, we need to add 2 to both sodium and oxygen and the new equation form will be.

2Na+ 2H₂O→2NaOH + H₂↑

Now sodium has the value of 2 on both sides, hydrogen has the value of 4 on both sides and oxygen also has the value of 2 on both sides. Thus our chemical equation is properly balanced and the final result is.

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.

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

Answer i: BaCl₂ (aq) + Na₂SO₄ (aq) → BaSO₄ (s) + 2NaCl (aq), is the correct answer.

Explanation: First write the chemical formulas of reactants and products.

Chemical Formulas of Reactants and Products:

Barium Chloride - BaCl₂

Sodium Sulphate - Na₂SO₄

Barium Sulphate - BaSO₄

Sodium Chloride - NaCl

The given chemical equation will be.

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

The reactants are in water therefore (aq) has been written to specify that they are in an aqueous solution. Barium Sulphate is an insoluble product formed therefore (s) is written to specify that it's solid. Sodium Chloride we all know that it is salt which dissolves in water therefore it has (aq).

Now comes the balancing part. First, we see that barium has a value of 1 on the reactant side as well as on the product side, so it's already balanced. Sulphate is also balanced and has a value of 4. Sodium and chlorine have a value of 2 on the reactant side while 1 on the product side. In order to balance them, we just need to add a 2 before sodium chloride on the product side and our equation becomes.

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

Answer ii: NaOH (aq) + HCl (aq) → NaCl (aq) + H₂O, is the correct answer.

Explanation: First write the chemical formulas of reactants and products.

Sodium Hydroxide - NaOH

Hydrochloric Acid - HCl

Sodium Chloride - NaCl

Water - H₂O

The given chemical equation will be.

NaOH (aq) + HCl (aq) → NaCl (aq) + H₂O

The (aq) thing as already told refers to an aqueous solution. This chemical equation is already balanced. Sodium, Oxygen and Chlorine have a value of 1 on both sides while Hydrogen has a value of 2. So it will be the final answer.

Page Number 10:

This section contains solutions to the questions given on Page Number 10 of the NCERT Science Book for Class 10 Chapter 1.

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 name of substance 'X' is Calcium Oxide or Quick Lime. The chemical formula for Calcium Oxide or Quick Lime is CaO.

Give Reason: Why Calcium Hydroxide is used for Whitewashing walls?

Answer: Calcium Hydroxide is used for whitewashing walls because it slowly reacts with the carbon dioxide in the air to generate a thin layer of calcium carbonate. After two or three days, calcium carbonate forms and provides a shiny finish to the walls. Ca(OH)₂ (aq) + CO₂ (g) → CaCo₃ (s) + H₂O (l).

Answer ii: When substance 'X' reacts with water it forms calcium hydroxide.

CaO (s) + H₂O (l) → 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 the amount collected in the other? Name this gas.

Answer: In activity 1.7, 2 molecules of water on electrolysis produce 2 molecules of Hydrogen and 1 molecule of Oxygen. Thus Hydrogen gas is produced in more quantity.

Explanation: Water is electrolysed to give two gases in two test tubes. One of the gases is Oxygen and other is the Hydrogen. To understand why hydrogen is produced in more quantity we need to take a look at the chemical reaction which occurs in the electrolysis of water.

H₂O (l) → H₂ (g) + O₂ (g)

The above chemical equation is unbalanced. On balancing the equation it becomes.

2H₂O (l) → 2H₂ (g) + O₂ (g)

So clearly we can see that 2 molecules of water produce on electrolysis 2 molecules of hydrogen and 1 molecule of oxygen.

Page Number 13:

This section contains solutions to the questions given on Page Number 13 of the NCERT Science Book for Class 10 Chapter 1.

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

OR

An iron nail is dipped in the solution of copper sulphate for about 30 minutes. State the change in colour observed. Give a reason for the change.

Answer: When iron is dipped in a solution of copper sulphate then a decomposition reaction takes place due to which the blue copper sulphate solution is decomposed and it fades and forms a light green solution of iron sulphate.

Fe (s) + CuSO₄ (aq) → FeSO₄ (aq) + Cu (s)

Explanation: Through the above reaction we can say that iron is more reactive than copper because it displaces copper from the copper sulphate solution. In this reaction Copper is reduced while the iron is oxidized. This reaction is also an example of a double displacement reaction as well as a redox reaction.

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

Answer: Zn (s) + CuSO₄ (aq) → ZnSO₄ (aq) + Cu (s)

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

i.) 4Na(s) + O₂ (g) → 2Na₂O (s)

ii.) CuO (s) + H₂ (g) → Cu (s) + H₂O (l)

Answer: i) Sodium (Na) is Oxidised while Oxygen (O) is reduced.

Explanation: The above reaction is an example of a redox reaction because oxidation and reduction are taking place simultaneously. If a substance loses its electrons then it is said to be oxidised while reduction means the gain of electrons.

4Na⁰ - 4e^- → 4Na¹ (Oxidation)

2O⁰ + 4e^- → 2O^-2 (Reduction)

Answer: ii) Copper (Cu) is reduced while Hydrogen (H) is oxidised.

Cu²⁺ + 2e^- → Cu⁰ (Reduction)

2H⁰ - 2e^- → 2H¹ (Oxidation)

Chemical Reactions and Equations Exercises Solution

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 c

iv.) All

Answer:

i.) a and b

Explanation:

In very simple words, oxidation means gaining oxygen or losing hydrogen and reduction means just the opposite, i.e. loss of oxygen or gain of hydrogen.

In the above example, we notice that PbO is losing oxygen and getting reduced to Pb, therefore we are saying that lead oxide is getting reduced not lead. The carbon is gaining oxygen and is getting changed to Co_2, therefore we are saying that carbon is getting oxidised.

Question 2: 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

Answer: d.) Displacement Reaction.

Explanation: In the above example aluminium is more reactive than iron and hence displaces iron from ferric oxide.

Question 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.

Answer: a

Explanation: When diluting hydrochloric acid is added to iron fillings then a reaction takes place where hydrogen gas and iron chloride are produced.

Chemical Reaction: 2HCl + Fe → FeCl₂ + H₂

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

Answer: A balanced chemical equation is an equation where an equal number of atoms are present on the reactant side as well as on the product side of the chemical reaction.

Eg: HCl + Fe → FeCl₂ + H₂ (Unbalanced Equation)

Reactant: HCl and Fe; Hydrogen, Iron, and Chlorine have a value of 1 on the reactant side.

Products: FeCl₂ and H₂; Iron has a value of 1 while hydrogen and chlorine have a value of 2.

So we see that the reactant side is not equal to the product side. On balancing the equation it becomes, 2HCl + Fe → FeCl₂ + H₂.

According to the law of conservation of mass, we know that mass can neither be created nor be destroyed. So the net mass on both sides should always be equal. If it is not equal then it means that some mass has been lost which will go against the law of conservation of mass. We must balance the chemical equation to satisfy the law of conservation of mass.

Question 5: 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 the air to give water and sulphur 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.

Answer:

a.) 3H₂ (g) + N₂ (g) → 2NH₃ (g)

b.) H₂S (g) + 3O₂ (g) → SO₂ (g) + 2H₂O(l)

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

d.) 2K(s) + 2H₂O(l) → 2KOH (aq) + H₂(g)

Question 6: 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

Answer:

a.) 2HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + 2H₂O

b.) 2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O

c.) NaCl + AgNO₃ → AgCl + NaNO₃

d.) BaCl₂ + H₂SO₄ → BaSO₄ + 2HCl

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

a.) Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water

b.) Zinc + Silver nitrate → Zinc nitrate + Silver

c.) Aluminium + Copper chloride → Aluminium chloride + Copper

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

Answer:

a.) Ca(OH)₂ + CO₂ → CaCO₃ + H₂O

b.) Zn + 2AgNO₃ → Zn(NO₃)₂ + 2Ag

c.) 2Al + 3CuCl₂ → 2AlCl₃ + 3Cu

d.) BaCl₂ + K₂SO₄ → BaSO₄ + 2KCl

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)

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

c.) Hydrogen (g) + Chloride (g) → Hydrogen chloride (g)

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

Answer:

(a) 2KBr (aq) + Bal₂ (aq) → 2Kl (aq) + BaBr₂(s)

Type: Double displacement reaction that involves the exchange of ions between reactants.

(b) ZnCO₃ (s) → ZnO (s) + CO₂ (g)

Type: Decomposition reaction

(c) H₂ (g) + Cl₂ (g) → 2HCl (g)

Type: Combination reaction

(d) Mg (s) + 2HCl (aq) → MgCl₂ (aq) + H₂ (g)

Type: Displacement reaction

Question 9: What do you mean by exothermic and endothermic reactions? Give examples for a proper explanation.

Answer:

Exothermic reactions: When the heat is released during any chemical reaction, that reaction is said to be an exothermic reaction. "+ Heat" is written to demonstrate that the reaction is exothermic and heat is getting released.

Example:

(i) C + O₂ → CO₂ + Heat

(ii) N₂ + 3H₂ → 2NH₃ + Heat

Endothermic reactions: When the heat is absorbed during any chemical reaction, that reaction is said to be an endothermic reaction. An endothermic reaction is usually indicated by writing "- Heat" on the product side of a chemical equation.

Examples:

(i) C + 2S → CS₂ - Heat

(ii) N₂ + O₂ → 2NO - Heat