Balancing of Chemical Reactions and Equations : Concepts, Examples, FAQs

Anand Classes provides detailed explanations and step-by-step solutions for writing and balancing chemical equations as per NCERT Class 10 Science, with solved examples and state symbols for better understanding.

Rules For Writing Chemical Equations

❓ Q1: What is a chemical equation?

A chemical equation is a short-hand representation of a chemical reaction using chemical symbols and formulae of the substances involved.

Instead of writing the full sentence for a chemical reaction, we can express it briefly and scientifically.

For example: Zinc metal reacts with dilute sulphuric acid to form zinc sulphate and hydrogen gas.

✅ Word equation:

Zinc + Sulphuric acid → Zinc sulphate + Hydrogen

✅ Chemical equation:

Zn + H₂SO₄ → ZnSO₄ + H₂

So, the chemical equation gives us:

• What substances react (reactants)
• What new substances are formed (products)
• In what proportions they combine (stoichiometry)

❓ Q2: What are reactants and products in a chemical equation?

• Reactants: Substances that participate in the chemical reaction.
  👉 Always written on the left-hand side (LHS) of the arrow.
• Products: Substances that are formed as a result of the reaction.
  👉 Always written on the right-hand side (RHS) of the arrow.

✅ Example:

Zn + H₂SO₄ → ZnSO₄ + H₂

Here:

• Reactants = Zn (Zinc), H₂SO₄ (Sulphuric Acid)
• Products = ZnSO₄ (Zinc Sulphate), H₂ (Hydrogen)

❓ Q3: What does the arrow (→) in a chemical equation represent?

The arrow (→) signifies the direction of the chemical reaction.

• It shows that the reactants transform into products.
• It separates the left side (reactants) from the right side (products).

❓ Q4: What is a word equation? How is it converted into a chemical equation?

A word equation uses the names of reactants and products.
A chemical equation replaces those names with symbols and formulae.

✅ Word Equation:

Zinc + Sulphuric acid → Zinc sulphate + Hydrogen

✅ Chemical Equation:

Zn + H₂SO₄ → ZnSO₄ + H₂

This step is essential in chemistry because it helps you:

• Identify the chemical species involved
• Understand the reaction quantitatively

❓ Q5: What is a balanced chemical equation?

A balanced chemical equation has equal numbers of atoms of each element on both sides of the equation.

✅ Example:

Zn + H₂SO₄ → ZnSO₄ + H₂

Let’s count atoms on both sides:

Since atoms of all elements are equal, this is a balanced equation.

❓ Q6: What is an unbalanced chemical equation?

An unbalanced equation has unequal numbers of atoms of one or more elements on both sides.

✅ Example:

H₂ + O₂ → H₂O

Count atoms:

This equation is unbalanced because oxygen atoms are not equal.

❓ Q7: Why is balancing a chemical equation necessary?

Chemical equations must follow the Law of Conservation of Mass, which states:

“Mass can neither be created nor destroyed in a chemical reaction.“

So the total number of atoms (mass) on both sides must be equal.

If not, the equation would imply that atoms are being created or destroyed, which is scientifically incorrect.

❓ Q8: What is the correct way to balance a chemical equation?

To balance a chemical equation:

1. Write correct chemical formulae of all reactants and products.
2. List the number of atoms of each element on both sides.
3. Adjust coefficients (numbers in front of compounds/elements), not subscripts.
4. Start balancing:
   • First, metals
   • Then, non-metals
   • Then, hydrogen
   • Finally, oxygen

❓ Q9: What are the common mistakes to avoid while balancing?

• ❌ Never change the subscripts in a chemical formula.
  Eg: Don’t change H₂O to H₂O₂ just to add more oxygen.
• ✅ Only change the coefficients (e.g., 2H₂ + O₂ → 2H₂O)
• ❌ Don’t forget that some elements exist as diatomic molecules in nature: Diatomic Elements Formula Hydrogen H₂ Oxygen O₂ Nitrogen N₂ Chlorine Cl₂ Fluorine F₂ Bromine Br₂ Iodine I₂

To Make Chemical Equations More Informative

❓ Q1: What is meant by a “more informative chemical equation”?

Answer:
A more informative chemical equation includes:

• Physical states of reactants/products,
• Heat changes (exothermic or endothermic),
• Conditions like temperature, pressure, or catalyst.

This provides a complete detail of the chemical reaction.

🌡️ 1. Indicating Physical States

❓ Q2: Which symbols represent the physical states in a chemical equation?

Answer:

• Solid: (s)
• Liquid: (l)
• Gas: (g)
• Aqueous (dissolved in water): (aq)

❓ Q3: Write the reaction of zinc with dilute sulphuric acid showing physical states.

Answer:

$$\text{Zn (s) + H}_2\text{SO}_4\text{ (aq)} \rightarrow \text{ZnSO}_4\text{ (aq) + H}_2\text{ (g)}$$

Explanation:

• Zinc is a solid (s)
• Sulphuric acid is in aqueous form (aq)
• Zinc sulphate forms in solution (aq)
• Hydrogen gas is evolved (g)

❓ Q4: How is a precipitate represented?

Answer:
A precipitate is shown using (s) or (ppt) for solid that settles down.

Example:

$$\text{Ca(OH)}_2\text{ (aq) + CO}_2\text{ (g)} \rightarrow \text{CaCO}_3\text{ (s) + H}_2\text{O (l)}$$

Here, CaCO₃ (s) is the white precipitate.

🔥 2. Indicating Heat Changes

❓ Q5: What are exothermic and endothermic reactions?

Answer:

• Exothermic reaction: Releases heat $$\text{C (s) + O}_2\text{ (g)} \rightarrow \text{CO}_2\text{ (g) + Heat}$$
• Endothermic reaction: Absorbs heat $$\text{N}_2\text{ (g) + O}_2\text{ (g) + Heat} \rightarrow \text{2NO (g)}$$

❓ Q6: Example of exothermic reaction (Combustion of methane)

Answer: $$\text{CH}_4\text{ (g) + 2O}_2\text{ (g)} \rightarrow \text{CO}_2\text{ (g) + 2H}_2\text{O (g) + Heat}$$

❓ Q7: Example of endothermic reaction (Thermal decomposition)

Answer: $$\text{CaCO}_3\text{ (s)} \xrightarrow{\text{Heat}} \text{CaO (s) + CO}_2\text{ (g)}$$

❓ Q8: Is respiration exothermic? Write its equation.

Answer:
Yes, respiration is exothermic. $$\text{C}_6\text{H}_{12}\text{O}_6\text{ (aq) + 6O}_2\text{ (g)} \rightarrow \text{6CO}_2\text{ (g) + 6H}_2\text{O (l) + Energy}$$

⚙️ 3. Indicating Reaction Conditions

❓ Q9: How are temperature, pressure, and catalysts shown in equations?

Answer:

They are written above the arrow (→) in the chemical equation.

Example (Decomposition of potassium chlorate): $$\text{2KClO}_3\text{ (s)} \xrightarrow{\text{MnO}_2, \Delta} \text{2KCl (s) + 3O}_2\text{ (g)}$$

• Δ (delta) → Heat
• MnO₂ → Catalyst

❓ Q10: Example with temperature, pressure, and catalyst.

Answer:

Methanol synthesis: $$\text{CO (g) + 2H}_2\text{ (g)} \xrightarrow[\text{ZnO + CrO}_3]{300^\circ\text{C}, 300 \text{ atm}} \text{CH}_3\text{OH (l)}$$

❓ Q11: Write the photosynthesis equation with conditions.

Answer: $$\text{6CO}_2\text{ (g) + 6H}_2\text{O (l)} \xrightarrow[\text{chlorophyll}]{\text{sunlight}} \text{C}_6\text{H}_{12}\text{O}_6\text{ (aq) + 6O}_2\text{ (g)}$$

Rules to Write a Balanced Chemical Equation

✅ Q1: What are the steps to write a balanced chemical equation?

Ans: The following four steps should be followed to write a balanced chemical equation:

1. Step 1: Write the word equation, placing reactants on the left and products on the right.
2. Step 2: Replace the names of substances with their chemical symbols and formulae.
3. Step 3: Balance the equation by adjusting the coefficients without changing any chemical formula.
4. Step 4 (Optional): Make the equation more informative by:
   • Indicating the physical states of substances (s),(l),(g),(aq)
   • Mentioning heat changes (if any),
   • Stating the conditions under which the reaction takes place.

✅ Q2: How do you write and balance the chemical equation for methane burning in oxygen?

Ans:
Word Equation:
Methane + Oxygen → Carbon dioxide + Water

Step-by-step conversion to a chemical equation:

• Methane: $\mathrm{CH_4}$
• Oxygen: $\mathrm{O_2}$
• Carbon dioxide: $\mathrm{CO_2}$
• Water: $\mathrm{H_2O}$

Unbalanced equation:
$$\mathrm{CH_4 + O_2 \rightarrow CO_2 + H_2O}$$

Balancing:

• Balance H: $$\mathrm{CH_4 + O_2 \rightarrow CO_2 + 2H_2O}$$
• Balance O: $$\mathrm{CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O}$$

Balanced equation with physical states and heat:
$$\mathrm{CH_4 (g) + 2O_2 (g) \rightarrow CO_2 (g) + 2H_2O (g) + \text{Heat}}$$

This is an exothermic reaction.

✅ Q3: How do you write a balanced equation when sodium hydroxide reacts with copper sulfate?

Ans:
Word Equation:
Copper sulfate + Sodium hydroxide → Copper hydroxide + Sodium sulfate

Formulas:

• Copper sulfate: $\mathrm{CuSO_4}$
• Sodium hydroxide: $\mathrm{NaOH}$
• Copper hydroxide: $\mathrm{Cu(OH)_2}$
• Sodium sulfate: $\mathrm{Na_2SO_4}$

Unbalanced equation:
$$\mathrm{CuSO_4 + NaOH \rightarrow Cu(OH)_2 + Na_2SO_4}$$

Balancing:

• Balance Na, O, H:
  $$\mathrm{CuSO_4 + 2NaOH \rightarrow Cu(OH)_2 + Na_2SO_4}$$

Balanced equation with physical states:
$$\mathrm{CuSO_4 (aq) + 2NaOH (aq) \rightarrow Cu(OH)_2 (s) + Na_2SO_4 (aq)}$$

This is a precipitation reaction forming a blue precipitate of copper hydroxide.

✅ Q4: What is the balanced equation when iron reacts with steam?

Ans:
Word Equation:
Iron + Steam → Iron (II, III) oxide + Hydrogen

Formulas:

• Iron: $\mathrm{Fe}$
• Steam (water vapor): $\mathrm{H_2O}$
• Iron (II, III) oxide: $\mathrm{Fe_3O_4}$
• Hydrogen: $\mathrm{H_2}$

Unbalanced equation:
$$\mathrm{Fe + H_2O \rightarrow Fe_3O_4 + H_2}$$

Balancing:

• Balance Fe: $$\mathrm{3Fe + H_2O \rightarrow Fe_3O_4 + H_2}$$
• Balance O: $$\mathrm{3Fe + 4H_2O \rightarrow Fe_3O_4 + H_2}$$
• Balance H: $$\mathrm{3Fe + 4H_2O \rightarrow Fe_3O_4 + 4H_2}$$

Final balanced equation with states:
$$\mathrm{3Fe (s) + 4H_2O (g) \rightarrow Fe_3O_4 (s) + 4H_2 (g)}$$

This is a redox reaction where iron is oxidized, and water (steam) is reduced.

Sample Problems

✅ Sample Problem 1: Write the balanced chemical equation for the reaction:

Q: Hydrogen + Chlorine → Hydrogen chloride
(NCERT Book Question)

A:
This reaction shows that hydrogen combines with chlorine to form hydrogen chloride.

Step 1 – Word Equation:
Hydrogen + Chlorine → Hydrogen chloride

Step 2 – Write chemical formulae:
Hydrogen: $\mathrm{H_2}$
Chlorine: $\mathrm{Cl_2}$
Hydrogen chloride: $\mathrm{HCl}$

Step 3 – Write unbalanced chemical equation:
$$\mathrm{H_2 + Cl_2 \rightarrow HCl}$$

Step 4 – Balance the equation:
To balance hydrogen and chlorine atoms: $$\mathrm{H_2 + Cl_2 \rightarrow 2HCl}$$

Step 5 – Add physical states: $$\mathrm{H_2 (g) + Cl_2 (g) \rightarrow 2HCl (g)}$$

This is a balanced chemical equation where equal numbers of atoms of each element appear on both sides.

✅ Sample Problem 2: Translate and balance this statement:

Q: Hydrogen gas combines with nitrogen to form ammonia.
(NCERT Book Question)

A:
Step 1 – Word Equation:
Hydrogen + Nitrogen → Ammonia

Step 2 – Write formulae:
Hydrogen: $\mathrm{H_2}$
Nitrogen: $\mathrm{N_2}$
Ammonia: $\mathrm{NH_3}$

Step 3 – Unbalanced equation:
$$\mathrm{H_2 + N_2 \rightarrow NH_3}$$

Step 4 – Balance the equation:
To balance nitrogen and hydrogen atoms: $$\mathrm{3H_2 + N_2 \rightarrow 2NH_3}$$

Step 5 – Add physical states: $$\mathrm{3H_2 (g) + N_2 (g) \rightarrow 2NH_3 (g)}$$

This is a balanced equation representing the formation of ammonia.

✅ Sample Problem 6: Write the balanced chemical equation for the reaction:

Q: Sodium metal reacts with water to give sodium hydroxide and hydrogen.
(NCERT Book Question)

A:
Step 1 – Word Equation:
Sodium + Water → Sodium hydroxide + Hydrogen

Step 2 – Write formulae:
Sodium: $\mathrm{Na}$
Water: $\mathrm{H_2O}$
Sodium hydroxide: $\mathrm{NaOH}$
Hydrogen: $\mathrm{H_2}$

Step 3 – Unbalanced equation:
$$\mathrm{Na + H_2O \rightarrow NaOH + H_2}$$

Step 4 – Balance the equation:

• To balance H atoms:
  $$\mathrm{Na + 2H_2O \rightarrow 2NaOH + H_2}$$
• Balance Na:
  $$\mathrm{2Na + 2H_2O \rightarrow 2NaOH + H_2}$$

Step 5 – Add physical states: $$\mathrm{2Na (s) + 2H_2O (l) \rightarrow 2NaOH (aq) + H_2 (g)}$$

This is a balanced chemical equation for the reaction of sodium with water.

✅ Sample Problem 3: Write the balanced chemical equation for the reaction:

Q: Magnesium burns in oxygen to form magnesium oxide.
(NCERT Book Question)

A:
Step 1 – Word Equation:
Magnesium + Oxygen → Magnesium oxide

Step 2 – Write formulae:
Magnesium: $\mathrm{Mg}$
Oxygen: $\mathrm{O_2}$
Magnesium oxide: $\mathrm{MgO}$

Step 3 – Unbalanced equation:
$$\mathrm{Mg + O_2 \rightarrow MgO}$$

Step 4 – Balance the equation:

• Balance O:
  $$\mathrm{Mg + O_2 \rightarrow 2MgO}$$
• Balance Mg:
  $$\mathrm{2Mg + O_2 \rightarrow 2MgO}$$

Step 5 – Add physical states: $$\mathrm{2Mg (s) + O_2 (g) \rightarrow 2MgO (s)}$$

This is a balanced chemical equation showing magnesium burning in oxygen.

Multiple-Choice Questions (MCQs) “Balancing of Chemical Reactions and Equations”

🧪 Multiple Choice Questions (MCQs)

1. What does the arrow (→) in a chemical equation signify?
A) It indicates the end of the equation.
B) It separates the products from the reactants.
C) It shows the direction of the chemical reaction.
D) It denotes equilibrium in the reaction.
Answer: C) It shows the direction of the chemical reaction.

2. In the reaction:
Zn + H₂SO₄ → ZnSO₄ + H₂
What are the reactants?
A) ZnSO₄ and H₂
B) Zn and H₂SO₄
C) Zn and H₂
D) H₂SO₄ and H₂
Answer: B) Zn and H₂SO₄

3. Which of the following is a balanced chemical equation?
A) H₂ + O₂ → H₂O
B) 2H₂ + O₂ → 2H₂O
C) H₂ + O₂ → 2H₂O
D) 2H₂ + 2O₂ → 2H₂O
Answer: B) 2H₂ + O₂ → 2H₂O

4. Why is balancing a chemical equation necessary?
A) To satisfy the Law of Conservation of Mass.
B) To make the equation look symmetrical.
C) To ensure the equation has equal numbers on both sides.
D) To follow the Law of Definite Proportions.
Answer: A) To satisfy the Law of Conservation of Mass.

5. Which of the following is NOT a correct step in balancing a chemical equation?
A) Adjusting coefficients to balance atoms.
B) Changing subscripts in chemical formulas.
C) Counting atoms of each element on both sides.
D) Starting with balancing metals, then non-metals.
Answer: B) Changing subscripts in chemical formulas.

6. What is the product of the following reaction?
2H₂ + O₂ → 2H₂O
A) Hydrogen gas
B) Oxygen gas
C) Water
D) Hydrogen peroxide
Answer: C) Water

7. What is the state symbol for hydrogen chloride gas in the reaction:
H₂ (g) + Cl₂ (g) → 2HCl (g)
A) (s)
B) (l)
C) (g)
D) (aq)
Answer: C) (g)

8. Which law is used to balance chemical equations?
A) Law of Constant Composition
B) Law of Mass Action
C) Law of Conservation of Mass
D) Law of Definite Proportions
Answer: C) Law of Conservation of Mass

9. Which of the following reactions is a synthesis reaction?
A) CaCO₃ → CaO + CO₂
B) H₂ + Cl₂ → 2HCl
C) NaOH + HCl → NaCl + H₂O
D) H₂O → H₂ + O₂
Answer: B) H₂ + Cl₂ → 2HCl

10. In the reaction:
2Na + 2H₂O → 2NaOH + H₂
What type of reaction is this?
A) Combination Reaction
B) Decomposition Reaction
C) Displacement Reaction
D) Double Displacement Reaction
Answer: C) Displacement Reaction

11. In a combustion reaction, which of the following is always produced?
A) Carbon dioxide and water
B) Oxygen and water
C) Oxygen and carbon monoxide
D) Carbon monoxide and water
Answer: A) Carbon dioxide and water

12. What is the coefficient of NaOH in the balanced equation:
2Na + 2H₂O → 2NaOH + H₂?
A) 1
B) 2
C) 3
D) 4
Answer: B) 2

13. Which of the following compounds is formed when magnesium burns in oxygen?
A) Magnesium oxide (MgO)
B) Magnesium hydroxide (MgOH)
C) Magnesium chloride (MgCl)
D) Magnesium carbonate (MgCO₃)
Answer: A) Magnesium oxide (MgO)

14. What is the product when sodium reacts with water?
A) Sodium hydroxide and hydrogen gas
B) Sodium chloride and oxygen gas
C) Sodium peroxide and water
D) Sodium hydroxide and oxygen gas
Answer: A) Sodium hydroxide and hydrogen gas

15. Which of the following is the correct representation of a decomposition reaction?
A) H₂ + O₂ → H₂O
B) NaCl → Na + Cl₂
C) NaOH + HCl → NaCl + H₂O
D) H₂O → H₂ + O₂
Answer: B) NaCl → Na + Cl₂

Assertion-Reason based questions on chemical equations for Class 10

Assertion-Reason Based Questions

1.
Assertion: The balanced chemical equation for the reaction between hydrogen and chlorine is:
H₂ + Cl₂ → 2HCl.
Reason: The equation is balanced because there are equal numbers of hydrogen and chlorine atoms on both sides.
A) Both Assertion and Reason are correct, and Reason is the correct explanation of Assertion.
B) Both Assertion and Reason are correct, but Reason is not the correct explanation of Assertion.
C) Assertion is correct, but Reason is incorrect.
D) Both Assertion and Reason are incorrect.
Answer: A) Both Assertion and Reason are correct, and Reason is the correct explanation of Assertion.

2.
Assertion: Magnesium oxide is formed when magnesium burns in oxygen.
Reason: Magnesium burns with a bright flame when it reacts with oxygen.
A) Both Assertion and Reason are correct, and Reason is the correct explanation of Assertion.
B) Both Assertion and Reason are correct, but Reason is not the correct explanation of Assertion.
C) Assertion is correct, but Reason is incorrect.
D) Both Assertion and Reason are incorrect.
Answer: A) Both Assertion and Reason are correct, and Reason is the correct explanation of Assertion.

3.
Assertion: The reaction between sodium and water is an example of a combination reaction.
Reason: Sodium reacts with water to form sodium hydroxide and hydrogen gas.
A) Both Assertion and Reason are correct, and Reason is the correct explanation of Assertion.
B) Both Assertion and Reason are correct, but Reason is not the correct explanation of Assertion.
C) Assertion is correct, but Reason is incorrect.
D) Both Assertion and Reason are incorrect.
Answer: C) Assertion is correct, but Reason is incorrect.

4.
Assertion: In a displacement reaction, one element displaces another from a compound.
Reason: In the reaction, the element being displaced always forms a precipitate.
A) Both Assertion and Reason are correct, and Reason is the correct explanation of Assertion.
B) Both Assertion and Reason are correct, but Reason is not the correct explanation of Assertion.
C) Assertion is correct, but Reason is incorrect.
D) Both Assertion and Reason are incorrect.
Answer: C) Assertion is correct, but Reason is incorrect.

5.
Assertion: A combustion reaction produces carbon dioxide and water.
Reason: Combustion is a reaction where a substance reacts with oxygen to release heat and light.
A) Both Assertion and Reason are correct, and Reason is the correct explanation of Assertion.
B) Both Assertion and Reason are correct, but Reason is not the correct explanation of Assertion.
C) Assertion is correct, but Reason is incorrect.
D) Both Assertion and Reason are incorrect.
Answer: A) Both Assertion and Reason are correct, and Reason is the correct explanation of Assertion.

6.
Assertion: In the reaction between sodium and water, sodium hydroxide and hydrogen are produced.
Reason: The equation for the reaction is:
2Na + 2H₂O → 2NaOH + H₂.
A) Both Assertion and Reason are correct, and Reason is the correct explanation of Assertion.
B) Both Assertion and Reason are correct, but Reason is not the correct explanation of Assertion.
C) Assertion is correct, but Reason is incorrect.
D) Both Assertion and Reason are incorrect.
Answer: A) Both Assertion and Reason are correct, and Reason is the correct explanation of Assertion.

7.
Assertion: Decomposition reactions are the reverse of combination reactions.
Reason: In a decomposition reaction, a compound breaks down into two or more simpler substances.
A) Both Assertion and Reason are correct, and Reason is the correct explanation of Assertion.
B) Both Assertion and Reason are correct, but Reason is not the correct explanation of Assertion.
C) Assertion is correct, but Reason is incorrect.
D) Both Assertion and Reason are incorrect.
Answer: A) Both Assertion and Reason are correct, and Reason is the correct explanation of Assertion.

8.
Assertion: The reaction between hydrogen and chlorine is an example of a combination reaction.
Reason: Two reactants combine to form a single product.
A) Both Assertion and Reason are correct, and Reason is the correct explanation of Assertion.
B) Both Assertion and Reason are correct, but Reason is not the correct explanation of Assertion.
C) Assertion is correct, but Reason is incorrect.
D) Both Assertion and Reason are incorrect.
Answer: A) Both Assertion and Reason are correct, and Reason is the correct explanation of Assertion.

Case Study based question related to chemical equations for Class 10

Case Study 1:

During a chemistry experiment in a laboratory, students were asked to observe the reaction between magnesium metal and oxygen. A piece of magnesium ribbon was burned in air, and they observed that the magnesium burns with a bright white flame to form a white powdery substance. The reaction is as follows:

Magnesium + Oxygen → Magnesium oxide
Mg + O₂ → MgO

The students were asked to answer the following questions based on their observations:

Question 1:
What is the type of reaction taking place in the above case?
A) Combination reaction
B) Decomposition reaction
C) Displacement reaction
D) Combustion reaction

Answer:
A) Combination reaction
Explanation:
In the given case, two reactants, magnesium and oxygen, combine to form a single product, magnesium oxide. This is characteristic of a combination reaction.

Question 2:
How many atoms of magnesium and oxygen are involved in the balanced equation of the reaction?
A) 1 atom of magnesium and 1 atom of oxygen
B) 2 atoms of magnesium and 2 atoms of oxygen
C) 1 atom of magnesium and 2 atoms of oxygen
D) 2 atoms of magnesium and 1 atom of oxygen

Answer:
B) 2 atoms of magnesium and 2 atoms of oxygen
Explanation:
The balanced equation is:
2Mg + O₂ → 2MgO.
There are 2 atoms of magnesium and 2 atoms of oxygen on both sides of the equation.

Question 3:
What is the physical state of magnesium and oxygen in the reaction?
A) Magnesium is a gas, and oxygen is a solid
B) Magnesium is a solid, and oxygen is a gas
C) Both magnesium and oxygen are gases
D) Both magnesium and oxygen are solids

Answer:
B) Magnesium is a solid, and oxygen is a gas
Explanation:
Magnesium is a solid metal, and oxygen is a diatomic gas (O₂) at room temperature.

Question 4:
What is the nature of the product formed in the reaction?
A) A liquid
B) A solid
C) A gas
D) A solution

Answer:
B) A solid
Explanation:
Magnesium oxide (MgO) formed in the reaction is a white solid, which is a basic oxide.

Question 5:
Which of the following is true about the reaction between magnesium and oxygen?
A) It is an endothermic reaction.
B) It is an exothermic reaction.
C) It requires a catalyst.
D) It is a reversible reaction.

Answer:
B) It is an exothermic reaction.
Explanation:
The reaction between magnesium and oxygen releases a large amount of heat and light, indicating it is an exothermic reaction.

Case Study 2:

Case Study:
Sodium metal reacts vigorously with water to form sodium hydroxide and hydrogen gas. The reaction is highly exothermic and is observed as bubbles of hydrogen gas being released.

The chemical equation for the reaction is:
2Na + 2H₂O → 2NaOH + H₂

Question 1:
What type of chemical reaction is taking place when sodium reacts with water?
A) Decomposition reaction
B) Double displacement reaction
C) Combination reaction
D) Redox reaction

Answer:
D) Redox reaction
Explanation:
In this reaction, sodium is oxidized (loses electrons) and hydrogen is reduced (gains electrons), which is characteristic of a redox reaction.

Question 2:
What gas is produced when sodium reacts with water?
A) Oxygen
B) Nitrogen
C) Hydrogen
D) Carbon dioxide

Answer:
C) Hydrogen
Explanation:
Hydrogen gas (H₂) is produced during the reaction of sodium with water.

Question 3:
What is the physical state of sodium hydroxide formed in the reaction?
A) Solid
B) Liquid
C) Gas
D) Aqueous

Answer:
D) Aqueous
Explanation:
Sodium hydroxide (NaOH) dissolves in water to form an aqueous solution.

Question 4:
What safety precautions should be taken during this reaction?
A) Keep sodium in an open container.
B) The reaction should be performed under water or kerosene.
C) Use an open flame to initiate the reaction.
D) The reaction should be performed without any protective gear.

Answer:
B) The reaction should be performed under water or kerosene.
Explanation:
Sodium reacts violently with water, producing heat and hydrogen gas. To prevent accidents, sodium should be stored in kerosene and the reaction should be carried out with proper precautions.

Question 5:
What would happen if the reaction between sodium and water occurs in an enclosed container?
A) No reaction will occur.
B) Hydrogen gas will escape, and the reaction will stop.
C) Pressure will build up, possibly causing an explosion.
D) Sodium will burn without forming hydrogen.

Answer:
C) Pressure will build up, possibly causing an explosion.
Explanation:
Since the reaction produces hydrogen gas and heat, if the reaction occurs in an enclosed container, pressure will build up and can cause an explosion.

Class 10 Chemistry Test Paper: Chemical Equations

This test paper covers the essential aspects of Chemical Equations for Class 10 and assesses students on their understanding, ability to balance equations, and apply the laws of chemical reactions.

Time: 1 hour : Total Marks: 40 : Marking Scheme:

• Section A (MCQs) – 5 x 1 = 5 marks
• Section B (Short Answer) – 5 x 2 = 10 marks
• Section C (Long Answer) – 3 x 3 = 9 marks
• Section D (Case Study) – 5 marks

General Instructions:

1. All questions are compulsory.
2. The paper consists of 4 sections – A, B, C, and D.
3. Section A contains 5 MCQs of 1 mark each.
4. Section B contains 5 short answer questions of 2 marks each.
5. Section C contains 3 long answer questions of 3 marks each.
6. Section D contains 1 case study-based question of 5 marks.

Section A: Multiple Choice Questions (1 Mark Each)

Q1: Which of the following is the correct representation of a combination reaction?
A) H₂ + O₂ → H₂O
B) H₂O → H₂ + O₂
C) NaCl → Na + Cl₂
D) CuSO₄ + Zn → Cu + ZnSO₄

Q2: What is the state symbol of hydrogen gas?
A) (s)
B) (l)
C) (g)
D) (aq)

Q3: In the reaction:
2Na + 2H₂O → 2NaOH + H₂,
What is the product formed along with sodium hydroxide?
A) Oxygen gas
B) Nitrogen gas
C) Hydrogen gas
D) Chlorine gas

Q4: Which of the following reactions is a decomposition reaction?
A) 2H₂ + O₂ → 2H₂O
B) CaCO₃ → CaO + CO₂
C) NaCl + AgNO₃ → NaNO₃ + AgCl
D) Mg + O₂ → MgO

Q5: What type of reaction occurs when magnesium burns in oxygen?
A) Decomposition reaction
B) Combination reaction
C) Redox reaction
D) Double displacement reaction

Section B: Short Answer Questions (2 Marks Each)

Q6: Write the balanced chemical equation for the reaction between hydrogen and chlorine to form hydrogen chloride.

Q7: What happens when zinc reacts with hydrochloric acid? Write the chemical equation for the reaction.

Q8: Define the term “chemical equation.” Why is it important to balance chemical equations?

Q9: Write the chemical equation for the reaction between sodium and water, and balance it.

Q10: What is the law of conservation of mass? How is it verified in a chemical reaction?

Section C: Long Answer Questions (3 Marks Each)

Q11: Describe the process of balancing a chemical equation. Balance the following equation:
C₄H₁₀ + O₂ → CO₂ + H₂O

Q12: Explain the difference between a physical change and a chemical change with suitable examples. Also, give the general characteristics of a chemical reaction.

Q13: A student burns a magnesium ribbon in oxygen, and a bright white flame is observed. What is the product formed, and how would you write the balanced chemical equation for this reaction? Also, state the type of reaction involved.

Section D: Case Study-Based Question (5 Marks)

Case Study 1:

During a chemistry practical, a group of students reacted magnesium with oxygen. The students observed that a bright white flame produced a white powdery substance. The reaction is as follows:

Magnesium + Oxygen → Magnesium oxide
Mg + O₂ → MgO

Based on this, answer the following questions:

Q14:
(i) What is the type of reaction occurring in the above case?
(ii) Write the balanced chemical equation for the reaction.
(iii) What is the state of magnesium and oxygen in the reaction?
(iv) What safety precautions should be taken when performing this experiment?
(v) Why is the reaction exothermic in nature?

🧠 Do You Know?

• All combustion reactions are exothermic.
• Most decomposition reactions are endothermic.
• Respiration releases energy, but photosynthesis requires it.
• 🚀 Hydrogen and oxygen are used in space shuttles as fuel. They react to form water, releasing huge energy.
• 🧪 Zinc + Sulphuric acid is a common lab reaction to produce hydrogen gas.
• ⚖️ The Law of Conservation of Mass was given by Antoine Lavoisier.

📝 Quick Revision Points

✅ A chemical equation represents a reaction using formulae.
✅ Reactants are substances that react; products are formed.
✅ The arrow (→) shows the direction of reaction.
✅ A balanced equation has equal atoms on both sides.
✅ An unbalanced equation violates the law of conservation of mass.
✅ Always balance chemical equations by adjusting coefficients, not subscripts.

• Use: (s), (l), (g), (aq) to show physical states.
• + Heat on product side → Exothermic
• + Heat on reactant side → Endothermic
• Use Δ and catalysts above arrows to show conditions.
• Always balance your equations.