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Key Concepts and Exam Tips for Chemical Reactions and Equations
Master Class 10 Science: Chemical Reactions and Equations (2025-26)
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FAQs on Master Class 10 Science: Chemical Reactions and Equations (2025-26)
1. What are the most important topics from Chapter 1, Chemical Reactions and Equations, for the CBSE Class 10 Board Exam 2025-26?
For the CBSE Class 10 Board Exam 2025-26, the most important topics from this chapter that students must focus on are:
Balancing chemical equations: This is a fundamental skill and frequently appears in 2 or 3-mark questions.
Types of chemical reactions: You must be able to identify, define, and provide examples for Combination, Decomposition (especially thermal, electrolytic, and photolytic), Displacement, and Double Displacement reactions.
Redox reactions: Identifying the substance oxidised and the substance reduced, as well as the oxidising and reducing agents, is a high-yield topic.
Application-based questions: Concepts like corrosion and rancidity are often tested through real-world scenarios or as short-note questions.
2. What type of 1-mark or MCQ questions are frequently asked from the Chemical Reactions and Equations chapter?
For 1-mark questions, including MCQs and assertion-reasoning questions, you can expect questions based on:
Identifying the type of reaction from a given chemical equation.
The colour of reactants or products, for example, the colour of copper sulphate solution or lead iodide precipitate.
Identifying the oxidising or reducing agent in a simple redox reaction.
The chemical formula for common compounds like quicklime, slaked lime, or rust.
The gas evolved in a reaction, such as hydrogen gas when a metal reacts with an acid.
3. How can you differentiate between a displacement and a double displacement reaction? Why is one often a precipitation reaction?
A key difference lies in what is being exchanged. In a displacement reaction, a more reactive element displaces a less reactive element from its compound (e.g., Fe + CuSO₄ → FeSO₄ + Cu). In a double displacement reaction, there is an exchange of ions between two aqueous compounds to form two new compounds (e.g., Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl).
A double displacement reaction is often a precipitation reaction because the exchange of ions frequently results in the formation of an insoluble solid (the precipitate), like Barium Sulphate (BaSO₄) in the example above, which is a white solid.
4. What is a balanced chemical equation and why is it essential to balance it for board exams?
A balanced chemical equation is one where the number of atoms of each element is equal on both the reactant and product sides. It is essential to balance equations because it adheres to the Law of Conservation of Mass, which states that mass can neither be created nor destroyed in a chemical reaction. For board exams, marks are often deducted for unbalanced equations, as they are chemically incorrect. Balancing demonstrates a correct understanding of stoichiometry.
5. Can a combination reaction also be a redox reaction? Justify this with an important example.
Yes, a combination reaction can definitely be a redox reaction. This happens when the elements combining change their oxidation states. A classic example frequently asked is the burning of magnesium ribbon in the air:
Equation: 2Mg (s) + O₂ (g) → 2MgO (s)
Justification: Here, two substances (Mg and O₂) combine to form a single substance (MgO), making it a combination reaction. Simultaneously, Magnesium (Mg) is oxidised (gains oxygen) to form MgO, and Oxygen (O₂) is reduced, making it a redox reaction.
6. From an exam perspective, what is the key difference between corrosion and rancidity?
The key difference to highlight in an exam is the substance being oxidised.
Corrosion is the slow degradation of metals, such as iron or copper, due to their reaction with atmospheric oxygen, moisture, or chemicals. The rusting of iron (formation of hydrated ferric oxide) is a prime example.
Rancidity is the aerial oxidation of fats and oils present in food materials, leading to an unpleasant smell and taste. For instance, potato chips tasting stale after being left open.
7. A shiny brown-coloured element 'X' on heating in the air becomes black in colour. What is element 'X' and the black-coloured compound formed? Write the chemical equation for this 3-mark question.
This is a classic activity-based question.
The shiny brown-coloured element 'X' is Copper (Cu).
When heated in the air, copper reacts with oxygen to form a black-coloured compound, which is Copper(II) Oxide (CuO).
The balanced chemical equation for the reaction is:
2Cu (s) + O₂ (g) → 2CuO (s)
8. Why do all decomposition reactions require an input of energy?
Decomposition reactions involve breaking down a single compound into two or more simpler substances. Breaking the existing chemical bonds within the compound requires an external supply of energy. This energy overcomes the forces holding the atoms together. Depending on the source, this energy can be supplied in the form of:
Heat (Thermal decomposition)
Light (Photolytic decomposition)
Electricity (Electrolytic decomposition)
9. Why are antioxidants, like nitrogen gas, often added to packaged food items like chips?
This is an important application-based question related to rancidity. Fats and oils in food items like chips get easily oxidised when exposed to atmospheric oxygen, making them rancid (bad smell and taste). To prevent this, manufacturers flush the bags with an antioxidant like nitrogen gas. Nitrogen is an inert gas that displaces oxygen from the package, creating an unreactive atmosphere. This slows down the oxidation process, prevents rancidity, and keeps the food fresh for a longer period.
10. What type of questions can be expected from the decomposition of lead nitrate for the board exam?
The thermal decomposition of lead nitrate is a very important reaction. For the exam, you can expect questions focusing on:
Observations: The question might ask what is observed when lead nitrate crystals are heated. The answer involves the emission of brown fumes and the formation of a yellow residue.
Identifying Products: You must identify the products: the brown fumes are Nitrogen dioxide (NO₂), the yellow residue is Lead(II) oxide (PbO), and colourless Oxygen (O₂) gas is also evolved.
Balanced Equation: You will almost certainly be asked to write the balanced chemical equation:
2Pb(NO₃)₂ (s) → 2PbO (s) + 4NO₂ (g) + O₂ (g)
