Short Questions Answer
1. What is a chemical reaction?
Answer: A process in which new substances with new properties are formed by the breaking and making of chemical bonds is called a chemical reaction.
2. What are reactants and products?
Answer: Substances that undergo a chemical change are called reactants, and the new substances formed are called products.
3. Why should a magnesium ribbon be cleaned before burning?
Answer: It is cleaned to remove the protective layer of magnesium oxide so that it can burn easily.
4. What is a balanced chemical equation?
Answer: A chemical equation in which the number of atoms of each element on both sides is equal is called a balanced equation.
5. Write the balanced equation for the burning of magnesium in air.
Answer: 2Mg + O₂ → 2MgO
6. What type of reaction is CaO + H₂O → Ca(OH)₂?
Answer: It is a combination reaction.
7. What is an exothermic reaction?
Answer: A reaction in which heat is released is called an exothermic reaction.
8. Define decomposition reaction.
Answer: A reaction in which a single compound breaks down into two or more simpler substances is called decomposition reaction.
9. What is corrosion?
Answer: The slow destruction of metals by chemical action of air, moisture, or acids on their surface is called corrosion.
10. What is rancidity?
Answer: When fats and oils are oxidised, they become rancid and develop an unpleasant smell and taste.
Long Questions Answer
1. What is the law of conservation of mass? How does it relate to a balanced chemical equation?
Answer: The law states that mass can neither be created nor destroyed in a chemical reaction.
Therefore, the total mass of reactants equals the total mass of products.
A chemical equation must be balanced to follow this law by ensuring equal atoms of each element on both sides.
2. Explain the types of chemical reactions with examples.
Answer:
- Combination Reaction: Two or more substances combine to form one product.
Example: CaO + H₂O → Ca(OH)₂ - Decomposition Reaction: One compound breaks into simpler products.
Example: 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂ - Displacement Reaction: One element replaces another in a compound.
Example: Fe + CuSO₄ → FeSO₄ + Cu - Double Displacement Reaction: Exchange of ions between reactants.
Example: Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl - Redox Reaction: Oxidation and reduction occur together.
Example: CuO + H₂ → Cu + H₂O
3. What are exothermic and endothermic reactions? Give examples.
Answer:
- Exothermic: Heat is released. Example: CH₄ + 2O₂ → CO₂ + 2H₂O + energy
- Endothermic: Heat is absorbed. Example: CaCO₃ → CaO + CO₂
4. Describe the process of rusting and its prevention.
Answer: Rusting is the corrosion of iron when it reacts with oxygen and moisture to form iron oxide (rust).
It can be prevented by:
- Painting
- Galvanisation
- Oiling and greasing
- Electroplating
5. What is a redox reaction? Explain with examples.
Answer: A reaction in which oxidation and reduction take place simultaneously is called a redox reaction.
Example:
CuO + H₂ → Cu + H₂O
- CuO is reduced to Cu (loss of oxygen).
- H₂ is oxidised to H₂O (gain of oxygen).
6. Write steps to balance a chemical equation using the hit-and-trial method.
Answer: Write the unbalanced equation.
- List the number of atoms of each element on both sides.
- Balance one element at a time using suitable coefficients.
- Repeat until all elements are balanced.
- Verify the total number of atoms.
Example:
Fe + H₂O → Fe₃O₄ + H₂
Balanced form: 3Fe + 4H₂O → Fe₃O₄ + 4H₂
7. What are oxidation and reduction in terms of oxygen and hydrogen?
Answer:
- Oxidation: Gain of oxygen or loss of hydrogen.
Example: 2Cu + O₂ → 2CuO - Reduction: Loss of oxygen or gain of hydrogen.
Example: CuO + H₂ → Cu + H₂O
8. Explain decomposition reactions and their types with examples.
Answer:
- Thermal decomposition: CaCO₃ → CaO + CO₂
- Electrolytic decomposition: 2H₂O → 2H₂ + O₂
- Photolytic decomposition: 2AgCl → 2Ag + Cl₂ (by sunlight)
9. What are precipitation reactions? Explain with an example.
Answer: A precipitation reaction is one in which an insoluble solid (precipitate) forms when two solutions react.
Example:
Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq)
Here, BaSO₄ is the white precipitate.
10. What are the effects of oxidation in everyday life?
Answer:
Corrosion: Metals react with oxygen and moisture forming oxides (e.g., rusting of iron).
Rancidity: Fats and oils oxidise, causing food to spoil.
Prevention:
- Use of antioxidants.
- Storage in airtight containers.
- Flushing with nitrogen gas (used in chips packets).
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