Class 10 Science Chemical Reactions and Equations Important Questions and Answers
NCERT, CBSE Board Exam, Exemplar and Olympiad Level Questions with Simple Student-Friendly Answers
Introduction
Chemical Reactions and Equations is one of the most important chapters of Class 10 Science. It explains how substances change into new substances, how chemical equations are written and balanced, and how different types of reactions take place in daily life.
This chapter is highly important for CBSE board exams because it includes definitions, reasoning questions, balancing of equations, observation-based questions, activity-based questions, assertion-reason questions and competency-based questions.
Chapter Overview
Chemical Reactions
A process in which one or more substances change into new substances with different properties.
Chemical Equations
Symbolic representation of a chemical reaction using chemical formulae of reactants and products.
Balancing Equations
Chemical equations are balanced to obey the law of conservation of mass.
Types of Reactions
Combination, decomposition, displacement, double displacement, oxidation and reduction reactions.
Important Keywords
Important Very Short Answer Questions
Q1. What is a chemical reaction?
Answer: A chemical reaction is a process in which one or more substances change to form new substances with different chemical properties.
Q2. What are reactants?
Answer: The substances that take part in a chemical reaction are called reactants.
Q3. What are products?
Answer: The new substances formed during a chemical reaction are called products.
Q4. Why should a chemical equation be balanced?
Answer: A chemical equation should be balanced because mass is neither created nor destroyed in a chemical reaction. It follows the law of conservation of mass.
Q5. Write the balanced equation for burning of magnesium ribbon.
Answer: 2Mg + O2 → 2MgO
Q6. What is a combination reaction?
Answer: A reaction in which two or more substances combine to form a single product is called a combination reaction.
Q7. What is a decomposition reaction?
Answer: A reaction in which a single compound breaks down into two or more simpler substances is called a decomposition reaction.
Q8. What is a precipitation reaction?
Answer: A reaction in which an insoluble solid, called precipitate, is formed is known as a precipitation reaction.
Q9. What is oxidation?
Answer: Oxidation is the addition of oxygen or removal of hydrogen from a substance.
Q10. What is reduction?
Answer: Reduction is the removal of oxygen or addition of hydrogen to a substance.
Q11. What is rancidity?
Answer: Rancidity is the oxidation of fats and oils, due to which food develops an unpleasant smell and taste.
Q12. What is corrosion?
Answer: Corrosion is the slow damage of metals due to reaction with air, moisture or chemicals. Rusting of iron is an example.
Important Short Answer Questions
Q1. List four observations that indicate a chemical reaction has taken place.
Answer: A chemical reaction may be identified by:
- Change in colour
- Evolution of gas
- Formation of precipitate
- Change in temperature
- Change in state
Q2. Balance the following equation: Fe + H2O → Fe3O4 + H2
Answer: The balanced chemical equation is:
3Fe + 4H2O → Fe3O4 + 4H2
Q3. What happens when quicklime is added to water?
Answer: Quicklime reacts vigorously with water to form slaked lime and releases a large amount of heat.
CaO + H2O → Ca(OH)2 + Heat
This is a combination reaction and an exothermic reaction.
Q4. Why is respiration considered an exothermic reaction?
Answer: During respiration, glucose is broken down in the presence of oxygen to form carbon dioxide and water. Energy is released during this process, so respiration is considered an exothermic reaction.
C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy
Q5. What is the difference between displacement and double displacement reactions?
Answer:
| Displacement Reaction | Double Displacement Reaction |
|---|---|
| One element displaces another element from its compound. | Two compounds exchange their ions to form two new compounds. |
| Example: Zn + CuSO4 → ZnSO4 + Cu | Example: Na2SO4 + BaCl2 → BaSO4 + 2NaCl |
Q6. Why does copper sulphate solution change colour when an iron nail is dipped into it?
Answer: Iron is more reactive than copper. Therefore, iron displaces copper from copper sulphate solution. The blue colour of copper sulphate fades and a reddish-brown coating of copper is formed on the iron nail.
Fe + CuSO4 → FeSO4 + Cu
Q7. What is a redox reaction? Give one example.
Answer: A reaction in which oxidation and reduction occur simultaneously is called a redox reaction.
CuO + H2 → Cu + H2O
- CuO is reduced to Cu.
- H2 is oxidised to H2O.
Q8. Why are chips packets flushed with nitrogen gas?
Answer: Chips contain fats and oils. They may get oxidised and become rancid. Nitrogen is an inert gas and prevents oxidation, so chips remain fresh for a longer time.
Important Long Answer Questions
Q1. Explain the different types of chemical reactions with examples.
Answer: The important types of chemical reactions are:
-
Combination Reaction: Two or more substances combine to form one product.
CaO + H2O → Ca(OH)2
-
Decomposition Reaction: One compound breaks into two or more simpler substances.
CaCO3 → CaO + CO2
-
Displacement Reaction: A more reactive element displaces a less reactive element from its compound.
Zn + CuSO4 → ZnSO4 + Cu
-
Double Displacement Reaction: Two compounds exchange ions to form new compounds.
AgNO3 + NaCl → AgCl + NaNO3
-
Redox Reaction: Oxidation and reduction occur together.
CuO + H2 → Cu + H2O
Q2. Explain thermal decomposition, electrolytic decomposition and photochemical decomposition.
Answer: Decomposition reactions are classified on the basis of the energy used to break the compound.
| Type | Energy Used | Example |
|---|---|---|
| Thermal decomposition | Heat | CaCO3 → CaO + CO2 |
| Electrolytic decomposition | Electricity | 2H2O → 2H2 + O2 |
| Photochemical decomposition | Light | 2AgCl → 2Ag + Cl2 |
Q3. Explain corrosion and methods to prevent it.
Answer: Corrosion is the slow destruction of metals due to reaction with air, moisture, acids or other chemicals. Rusting of iron is the most common example of corrosion.
Rusting of iron:
Iron + Oxygen + Water → Hydrated iron oxide
Methods to prevent corrosion:
- Painting
- Oiling and greasing
- Galvanisation
- Electroplating
- Alloying
- Keeping metals away from moisture
Q4. What is rancidity? How can it be prevented?
Answer: Rancidity is the oxidation of fats and oils present in food. It causes unpleasant smell and taste.
Methods to prevent rancidity:
- Keeping food in airtight containers
- Flushing food packets with nitrogen gas
- Refrigeration
- Adding antioxidants
- Keeping food away from sunlight
Q5. Balance the following chemical equations.
Answer:
| Unbalanced Equation | Balanced Equation |
|---|---|
| H2 + O2 → H2O | 2H2 + O2 → 2H2O |
| Fe + O2 → Fe2O3 | 4Fe + 3O2 → 2Fe2O3 |
| Na + H2O → NaOH + H2 | 2Na + 2H2O → 2NaOH + H2 |
| Al + O2 → Al2O3 | 4Al + 3O2 → 2Al2O3 |
Case-Study Based Questions
Case Study 1: Burning of Magnesium Ribbon
A student cleans a magnesium ribbon with sandpaper and burns it using a spirit lamp. The magnesium ribbon burns with a dazzling white flame and forms a white powder of magnesium oxide.
Q1. Why is magnesium ribbon cleaned before burning?
Answer: Magnesium ribbon is cleaned to remove the layer of magnesium oxide formed on its surface so that it can burn easily.
Q2. Name the white powder formed after burning magnesium.
Answer: The white powder is magnesium oxide.
Q3. Write the balanced chemical equation for this reaction.
Answer:
2Mg + O2 → 2MgO
Q4. Identify the type of reaction.
Answer: It is a combination reaction because magnesium and oxygen combine to form magnesium oxide.
Case Study 2: Reaction Between Lead Nitrate and Potassium Iodide
When potassium iodide solution is added to lead nitrate solution, a yellow precipitate of lead iodide is formed along with potassium nitrate solution.
Q1. Name the yellow precipitate formed.
Answer: The yellow precipitate is lead iodide.
Q2. Write the balanced chemical equation.
Answer:
Pb(NO3)2 + 2KI → PbI2 + 2KNO3
Q3. Which type of reaction is this?
Answer: This is a double displacement reaction and also a precipitation reaction.
Q4. Why is it called a precipitation reaction?
Answer: It is called a precipitation reaction because an insoluble solid, lead iodide, is formed.
Assertion-Reason Questions
Directions: Choose the correct option:
- A. Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
- B. Both Assertion and Reason are true, but Reason is not the correct explanation of Assertion.
- C. Assertion is true, but Reason is false.
- D. Assertion is false, but Reason is true.
Q1. Assertion: A chemical equation must be balanced. Reason: Atoms are conserved in a chemical reaction.
Answer: A
Explanation: A chemical equation is balanced because the number of atoms of each element remains the same on both sides.
Q2. Assertion: Respiration is an exothermic reaction. Reason: Energy is released during respiration.
Answer: A
Explanation: Glucose breaks down during respiration and releases energy, so it is exothermic.
Q3. Assertion: Decomposition reactions are always exothermic. Reason: Heat is always released in decomposition reactions.
Answer: D
Explanation: Decomposition reactions usually require energy in the form of heat, light or electricity. Therefore, the assertion is false.
Q4. Assertion: Rusting of iron is a redox reaction. Reason: Iron gets oxidised during rusting.
Answer: A
Explanation: Rusting involves oxidation of iron and reduction of oxygen, so it is a redox reaction.
Q5. Assertion: Nitrogen is flushed into chips packets. Reason: Nitrogen prevents oxidation of oils and fats.
Answer: A
Explanation: Nitrogen prevents rancidity by reducing the contact of food with oxygen.
HOTS and Competency-Based Questions
Q1. A student observed that a silver chloride sample turned grey in sunlight. Explain the reason.
Answer: Silver chloride decomposes in the presence of sunlight to form silver and chlorine gas. The grey colour appears due to the formation of silver.
2AgCl → 2Ag + Cl2
Q2. Why does the colour of copper sulphate solution change from blue to green when iron is added?
Answer: Iron displaces copper from copper sulphate because iron is more reactive than copper. Ferrous sulphate is formed, which gives a green colour to the solution.
Fe + CuSO4 → FeSO4 + Cu
Q3. Why do we apply paint on iron gates?
Answer: Paint prevents iron from coming in contact with air and moisture. This prevents rusting and protects the iron gate from corrosion.
Q4. A reaction gives a white insoluble solid when two clear solutions are mixed. What type of reaction is likely to occur?
Answer: It is likely to be a precipitation reaction. It may also be a double displacement reaction because ions are exchanged between two compounds.
Q5. Why are decomposition reactions considered opposite of combination reactions?
Answer: In a combination reaction, two or more substances combine to form one product. In a decomposition reaction, one compound breaks down into two or more simpler substances. Therefore, they are opposite reactions.
Quick Revision Box
Must Remember Equations
- 2Mg + O2 → 2MgO
- CaO + H2O → Ca(OH)2
- Zn + CuSO4 → ZnSO4 + Cu
- 2AgCl → 2Ag + Cl2
- CuO + H2 → Cu + H2O
Important Concepts
- Balanced equations follow conservation of mass.
- Precipitation produces an insoluble solid.
- Oxidation and reduction occur together in redox reactions.
- Rancidity is oxidation of fats and oils.
- Corrosion damages metals slowly.
Exam Tips
- Always balance chemical equations carefully.
- Write physical states only when asked or required.
- Learn common observations such as colour change, gas evolution and precipitate formation.
- Remember examples of each type of chemical reaction.
- Use proper keywords like oxidation, reduction, precipitate, displacement and rancidity.
- For assertion-reason questions, check both statements separately before selecting the option.
- Practice NCERT intext questions, back exercise questions and activity-based questions.
Frequently Asked Questions
Q1. Which is the most important topic in Chemical Reactions and Equations?
Answer: Balancing chemical equations, types of reactions, oxidation-reduction, corrosion and rancidity are the most important topics.
Q2. Is balancing chemical equations important for CBSE exams?
Answer: Yes, balancing chemical equations is very important because CBSE often asks direct, application-based and correction-based questions from this topic.
Q3. What is the easiest way to identify a chemical reaction?
Answer: A chemical reaction can be identified by colour change, gas evolution, precipitate formation, change in temperature or formation of a new substance.
Q4. What is the difference between corrosion and rancidity?
Answer: Corrosion is the oxidation of metals, while rancidity is the oxidation of fats and oils in food.
Q5. How can I score full marks in this chapter?
Answer: Learn definitions, practice balancing equations, revise NCERT activities, write correct observations and remember examples of all types of reactions.
Critical Thinking Questions
Q1. A chemical equation is balanced, but the physical states are not mentioned. Is the equation still correct?
Answer: Yes, the equation can still be chemically correct if the number of atoms of each element is balanced. However, writing physical states such as (s), (l), (g) and (aq) makes the equation more informative.
Q2. Why is a balanced chemical equation considered more reliable than a word equation?
Answer: A balanced chemical equation shows the exact number of atoms and molecules involved in a reaction. It follows the law of conservation of mass, while a word equation only names the reactants and products.
Q3. Can a reaction be both a combination reaction and an exothermic reaction? Explain with an example.
Answer: Yes, a reaction can be both combination and exothermic. For example, quicklime reacts with water to form slaked lime and releases heat.
CaO + H2O → Ca(OH)2 + Heat
Q4. A student says that all decomposition reactions are caused by heat. Is the statement correct?
Answer: No, the statement is not correct. Decomposition reactions can take place due to heat, electricity or light. For example, silver chloride decomposes in sunlight, while water decomposes using electricity.
Q5. Why does iron displace copper from copper sulphate solution, but copper cannot displace iron from iron sulphate solution?
Answer: Iron is more reactive than copper, so it can displace copper from copper sulphate solution. Copper is less reactive than iron, so it cannot displace iron from iron sulphate solution.
Q6. If no colour change is observed in a reaction, can we say that no chemical reaction has occurred?
Answer: No, colour change is only one sign of a chemical reaction. A reaction may also be identified by gas evolution, precipitate formation, change in temperature or formation of a new substance.
Q7. Why is rusting considered a harmful chemical reaction, while respiration is considered useful?
Answer: Rusting damages iron objects and weakens their structure, so it is harmful. Respiration releases energy required for life processes, so it is useful.
Q8. A packet of oily snacks becomes bad faster when left open. Explain the chemical reason.
Answer: When the packet is left open, oils and fats react with oxygen present in air. This causes oxidation and leads to rancidity, producing an unpleasant smell and taste.
Q9. Why is the formation of a precipitate strong evidence of a chemical reaction?
Answer: Formation of a precipitate shows that a new insoluble substance has been formed. Since the products have different properties from the reactants, it indicates a chemical reaction.
Q10. In a redox reaction, can oxidation occur without reduction? Give reason.
Answer: No, oxidation cannot occur alone in a redox reaction. If one substance loses oxygen or gains hydrogen, another substance must gain oxygen or lose hydrogen. Therefore, oxidation and reduction occur simultaneously.
Final Conclusion
Chemical Reactions and Equations builds the foundation of Class 10 Chemistry. Students should focus on writing balanced chemical equations, understanding reaction types and learning real-life examples like rusting, rancidity and respiration. Regular practice of NCERT and competency-based questions can help students score excellent marks in CBSE board exams.
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