Below are 20 Important CBSE Class 10 Science (Chapter 1: Chemical Reactions and Equations) Questions and Answers for the 2026–27 academic session. These are designed and cover all major topics such as balancing equations, types of reactions, oxidation-reduction, corrosion, and rancidity. The chapter focuses on writing and balancing chemical equations, identifying reaction types, and understanding oxidation effects in daily life.
1. What is a chemical reaction? Mention any four indications of a chemical reaction.
Answer:
A chemical reaction is a process in which one or more substances called reactants are converted into new substances called products with different properties. Chemical reactions occur continuously around us, such as rusting of iron, digestion of food, and burning of fuels. The common indications of a chemical reaction are: (i) change in colour, (ii) evolution of a gas, (iii) formation of a precipitate, and (iv) change in temperature. Sometimes a change in state may also occur. These observations indicate that new substances have been formed. Therefore, a chemical reaction involves both breaking of old chemical bonds and formation of new bonds.
2. Why should a chemical equation be balanced?
Answer:
A chemical equation must be balanced to satisfy the Law of Conservation of Mass. According to this law, matter can neither be created nor destroyed during a chemical reaction. Therefore, the number of atoms of each element must remain the same on both sides of the equation. A balanced equation gives the correct ratio of reactants and products involved in the reaction. For example, in the reaction of hydrogen and oxygen, the balanced equation is:
2H₂ + O₂ → 2H₂O
Here, the number of hydrogen and oxygen atoms is equal on both sides. Thus, balancing ensures the equation accurately represents the chemical change occurring.
3. What is a combination reaction? Give one example.
Answer:
A combination reaction is a chemical reaction in which two or more substances combine to form a single product. Such reactions are generally exothermic because heat is often released during the process. Combination reactions are commonly observed in everyday life and industrial processes. A common example is the reaction between quicklime and water:
CaO + H₂O → Ca(OH)₂ + Heat
In this reaction, calcium oxide combines with water to form calcium hydroxide, also known as slaked lime. The reaction releases a large amount of heat. Combination reactions help in understanding how simple substances join together to produce more complex compounds.
4. What is a decomposition reaction? Explain with an example.
Answer:
A decomposition reaction is a reaction in which a single compound breaks down into two or more simpler substances. Such reactions usually require energy in the form of heat, electricity, or light. Depending on the source of energy, decomposition reactions may be thermal, electrolytic, or photochemical. For example:
CaCO₃ → CaO + CO₂
When calcium carbonate is heated, it decomposes into calcium oxide and carbon dioxide gas. This is known as thermal decomposition. Decomposition reactions are important in industries such as cement manufacturing and extraction of metals. They demonstrate how complex compounds can be broken down into simpler substances under suitable conditions.
5. What is a displacement reaction? Give an example.
Answer:
A displacement reaction is a chemical reaction in which a more reactive element displaces a less reactive element from its compound. These reactions are based on the reactivity series of metals. The more reactive metal replaces the less reactive metal from its salt solution. For example:
Fe + CuSO₄ → FeSO₄ + Cu
When an iron nail is dipped in copper sulphate solution, iron displaces copper from the solution and forms iron sulphate. The blue colour of copper sulphate gradually changes to green due to the formation of iron sulphate. Displacement reactions help determine the relative reactivity of different metals.
6. What is a double displacement reaction? Give an example.
Answer:
A double displacement reaction is a reaction in which two compounds exchange their ions to form two new compounds. Such reactions often result in the formation of a precipitate. For example:
Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl
In this reaction, barium sulphate is formed as a white insoluble precipitate. Double displacement reactions are also called precipitation reactions when an insoluble substance is produced. These reactions are useful in chemical analysis and industrial processes. They demonstrate how ions can exchange partners to produce entirely different compounds with new properties.
7. Differentiate between exothermic and endothermic reactions.
Answer:
Exothermic reactions are chemical reactions that release heat energy into the surroundings. As a result, the temperature of the surroundings increases. Examples include respiration and burning of fuels. Endothermic reactions are reactions that absorb heat energy from the surroundings. Consequently, the surroundings become cooler. Examples include thermal decomposition of calcium carbonate and photosynthesis. In exothermic reactions, energy is given out, whereas in endothermic reactions, energy is taken in. Understanding these reactions helps explain many natural and industrial processes. Energy changes play an important role in determining the feasibility and usefulness of chemical reactions.
8. What is oxidation? Give one example.
Answer:
Oxidation is the process in which a substance gains oxygen or loses hydrogen during a chemical reaction. Oxidation reactions are very common in everyday life and industrial processes. For example:
2Cu + O₂ → 2CuO
In this reaction, copper combines with oxygen to form copper oxide. Since oxygen is added to copper, the reaction is called oxidation. Oxidation is responsible for many phenomena such as rusting of iron, burning of fuels, and respiration. Understanding oxidation helps in studying corrosion, energy production, and various biological processes that occur in living organisms.
9. What is reduction? Explain with an example.
Answer:
Reduction is the process in which a substance loses oxygen or gains hydrogen during a chemical reaction. Reduction always occurs simultaneously with oxidation in a redox reaction. For example:
CuO + H₂ → Cu + H₂O
In this reaction, copper oxide loses oxygen and gets converted into copper metal. Therefore, copper oxide undergoes reduction. Reduction reactions are important in metallurgy, where metals are extracted from their ores. They also play a significant role in biological processes and energy transformations. Understanding reduction helps students identify electron transfer and oxygen-hydrogen changes occurring during chemical reactions.
10. What is a redox reaction? Give an example.
Answer:
A redox reaction is a chemical reaction in which oxidation and reduction occur simultaneously. One substance gets oxidised while another gets reduced. For example:
CuO + H₂ → Cu + H₂O
Here, hydrogen gains oxygen and gets oxidised to water, while copper oxide loses oxygen and gets reduced to copper. Since both oxidation and reduction occur together, it is called a redox reaction. Redox reactions are extremely important in daily life, including respiration, combustion, corrosion, and industrial metal extraction. They help explain how substances exchange oxygen, hydrogen, or electrons during chemical transformations.
11. What is corrosion?
Answer:
Corrosion is the gradual destruction of metals due to chemical reactions with the environment. The most common example is rusting of iron. Rusting occurs when iron reacts with oxygen and moisture present in the air to form hydrated iron oxide. Corrosion weakens metals and causes economic losses in industries. Bridges, vehicles, pipelines, and machinery are often affected by corrosion. It can be prevented by painting, galvanisation, oiling, greasing, or electroplating. Corrosion not only damages metal structures but also reduces their efficiency and lifespan. Therefore, protective measures are essential to minimize its harmful effects.
12. What is rancidity? How can it be prevented?
Answer:
Rancidity is the process in which fats and oils are oxidised, causing food to develop an unpleasant smell and taste. Foods containing fats become spoiled when exposed to air for a long period. Oxidation changes their chemical composition and reduces food quality. Rancidity can be prevented by storing food in airtight containers, refrigeration, adding antioxidants, and flushing packets with nitrogen gas. Chips packets are often filled with nitrogen to prevent oxidation. Preventing rancidity helps maintain food freshness, nutritional value, and taste. Thus, controlling oxidation is important in food preservation and storage.
13. Why does the colour of copper sulphate solution change when an iron nail is dipped in it?
Answer:
When an iron nail is dipped in copper sulphate solution, a displacement reaction takes place. Iron is more reactive than copper, so it displaces copper from copper sulphate solution. The reaction is:
Fe + CuSO₄ → FeSO₄ + Cu
As iron sulphate is formed, the blue colour of copper sulphate changes to green. A reddish-brown layer of copper is deposited on the iron nail. This experiment demonstrates the concept of displacement reactions and the reactivity series of metals. The colour change indicates that a new substance has been formed during the reaction.
14. What are the characteristics of a chemical equation?
Answer:
A chemical equation is a symbolic representation of a chemical reaction. It shows reactants on the left side and products on the right side separated by an arrow. A proper chemical equation should be balanced and may include state symbols such as (s), (l), (g), and (aq). It may also indicate reaction conditions like temperature, pressure, or catalyst. A balanced chemical equation follows the law of conservation of mass and provides quantitative information about the reaction. Chemical equations make it easier to understand and communicate chemical changes occurring during reactions.
15. Why are state symbols used in chemical equations?
Answer:
State symbols indicate the physical state of reactants and products involved in a chemical reaction. They help make chemical equations more informative and accurate. The symbols used are: (s) for solid, (l) for liquid, (g) for gas, and (aq) for aqueous solution. For example:
Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g)
These symbols provide additional information about the substances and help scientists understand reaction conditions more clearly. State symbols are especially useful in predicting precipitation, gas evolution, and changes in physical state during reactions. Therefore, they are an essential part of writing chemical equations. (EduTech)
16. Explain the importance of balancing chemical equations.
Answer:
Balancing chemical equations is important because it ensures that the number of atoms of each element remains equal on both sides of the equation. This satisfies the Law of Conservation of Mass. A balanced equation provides the correct stoichiometric relationship between reactants and products. It helps chemists calculate the quantities of substances involved in reactions. Unbalanced equations give an incorrect representation of the reaction and may lead to errors in calculations. Therefore, balancing is essential for understanding chemical processes, conducting experiments, and solving numerical problems accurately in chemistry.
17. Why is respiration considered an exothermic reaction?
Answer:
Respiration is considered an exothermic reaction because it releases energy during the breakdown of glucose. The reaction is:
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy
In living organisms, glucose reacts with oxygen to produce carbon dioxide, water, and energy. This released energy is used to perform various life processes such as movement, growth, and maintaining body temperature. Since heat and energy are produced, respiration is classified as an exothermic reaction. It is one of the most important biological reactions occurring continuously in all living cells to sustain life.
18. What is a precipitation reaction? Give an example.
Answer:
A precipitation reaction is a type of double displacement reaction in which an insoluble solid called a precipitate is formed. When two aqueous solutions react, ions combine to form an insoluble compound that settles down. For example:
BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq)
Here, barium sulphate appears as a white precipitate. Precipitation reactions are useful in water purification, chemical analysis, and industrial applications. They help identify specific ions present in solutions and demonstrate how insoluble compounds are formed from soluble reactants.
19. Explain rusting of iron.
Answer:
Rusting is a slow oxidation process in which iron reacts with oxygen and moisture present in the atmosphere. The product formed is hydrated iron oxide, commonly known as rust. Rust is reddish-brown in colour and weakens the metal surface. Both oxygen and water are necessary for rusting to occur. Rusting damages iron structures such as bridges, vehicles, tools, and machinery. It can be prevented by painting, galvanisation, greasing, and electroplating. Since rusting causes economic losses and structural damage, preventing corrosion is an important practical application of chemistry.
20. What is the difference between physical and chemical changes?
Answer:
A physical change is a change in which no new substance is formed. Only the shape, size, or state of a substance changes. Examples include melting of ice and boiling of water. In contrast, a chemical change results in the formation of one or more new substances with different properties. Examples include rusting of iron and burning of magnesium. Physical changes are usually reversible, while chemical changes are often irreversible. Chemical changes involve breaking and forming chemical bonds. Therefore, the formation of new substances is the main feature that distinguishes chemical changes from physical changes.