📌 How to Use
First read the questions carefully using the Read section. Use the Details button to understand concepts clearly. After completing reading, click Practice Quiz to test yourself.
📘 Study MCQs
Q1. What type of reaction occurs when calcium oxide and water combine to form a single product, calcium hydroxide?
• Decomposition reaction
• Displacement reaction
• Combination reaction
• Double displacement reaction
Answer: Combination reaction
A reaction in which a single product is formed from two or more reactants is known as a combination reaction.
Q2. Quick lime
• Calcium oxide
• Calcium carbonate
• Calcium hydroxide
Answer: Calcium hydroxide
Calcium oxide reacts with water to produce slaked lime, which is calcium hydroxide [Ca(OH)₂].
Q3. Quick lime is calcium oxide, with the chemical formula CaO.
• Ca(OH)₂
• CaCO₃
• CaO
• H₂O
Answer: CaO
Q4. In the reaction CaO(s) + H₂O(l) → Ca(OH)₂(aq), what is observed regarding temperature?
• Temperature decreases.
• A large amount of heat is released.
• No change in temperature.
• It becomes cold.
Answer: A large amount of heat is released.
The text states that the reaction releases a large amount of heat, which can be felt by touching the beaker.
Q5. What is the product formed when calcium hydroxide reacts with carbon dioxide in the air during whitewashing?
• Calcium oxide
• Calcium chloride
• Calcium carbonate
• Calcium sulphate
Answer: Calcium carbonate
Calcium hydroxide reacts slowly with carbon dioxide to form a thin layer of calcium carbonate (CaCO₃) on the walls.
Q6. What gives a shiny finish to walls after whitewashing?
• Calcium hydroxide
• Calcium oxide
• Calcium carbonate
• Water
Answer: Calcium carbonate
Calcium carbonate is formed after two to three days and gives a shiny finish to the walls.
Q7. What is the chemical formula of marble?
• CaO
• Ca(OH)₂
• CaCO₃
• CO₂
Answer: CaCO₃
It is interesting to note that the chemical formula of marble is also CaCO₃, the same as the layer formed in whitewashing.
Q8. Which of the following is a combination reaction?
• CaCO₃ → CaO + CO₂
• Zn + CuSO₄ → ZnSO₄ + Cu
• C + O₂ → CO₂
• NaOH + HCl → NaCl + H₂O
Answer: C + O₂ → CO₂
Burning of coal (C + O₂ → CO₂) is given as an example where two substances combine to form a single product.
Q9. In simple language, what defines a combination reaction?
• A single substance breaks down.
• Two compounds exchange ions.
• Two or more substances combine to form a single product.
• Heat is absorbed.
Answer: Two or more substances combine to form a single product.
The text defines it: “when two or more substances (elements or compounds) combine to form a single product, the reactions are called combination reactions.”
Q10. What is the product in the combination reaction: 2H₂(g) + O₂(g) → ?
• H₂O₂
• 2H₂O
• HO₂
• H₂O
Answer: 2H₂O
The formation of water from hydrogen and oxygen is a combination reaction: 2H₂(g) + O₂(g) → 2H₂O(l).
Q11. Reactions in which heat is released along with the formation of products are called what?
• Endothermic reactions
• Exothermic reactions
• Thermal reactions
• Combination reactions only
Answer: Exothermic reactions
Reactions in which heat is released along with the formation of products are called exothermic chemical reactions.
Q12. The reaction between quicklime and water to form slaked lime is an example of what kind of reaction?
• Endothermic and combination
• Exothermic and combination
• Endothermic and decomposition
• Exothermic and decomposition
Answer: Exothermic and combination
It is a combination reaction (single product formed) and it releases a large amount of heat, making it exothermic.
Q13. Which of the following is an exothermic reaction?
• Burning of natural gas
• Evaporation of water
• Melting of ice
• Photosynthesis
Answer: Burning of natural gas
Burning of natural gas (CH₄ + 2O₂ → CO₂ + 2H₂O) is given as an example of an exothermic reaction.
Q14. What process in our body is described as an exothermic process?
• Digestion
• Respiration
• Thinking
• Breathing in
Answer: Respiration
The text states: “Do you know that respiration is an exothermic process?” It provides energy.
Q15. During respiration, what does glucose combine with in our cells to provide energy?
• Nitrogen
• Carbon dioxide
• Oxygen
• Water
Answer: Oxygen
Glucose combines with oxygen in the cells of our body during respiration to provide energy.
Q16. What are rice, potatoes, and bread broken down into during digestion?
• Proteins
• Fats
• Carbohydrates
• Glucose
Answer: Glucose
Carbohydrates in these foods are broken down to form glucose.
Q17. What is the special name for the reaction where glucose provides energy by combining with oxygen?
• Digestion
• Combustion
• Respiration
• Photosynthesis
Answer: Respiration
The special name of this reaction (C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy) is respiration.
Q18. Which of the following is also an example of an exothermic reaction?
• Formation of ice
• Decomposition of vegetable matter into compost
• Dissolving salt in water
• Boiling water
Answer: Decomposition of vegetable matter into compost
The decomposition of vegetable matter into compost is given as an example of an exothermic reaction.
Q19. In the balancing steps for Fe + H₂O → Fe₃O₄ + H₂, what was done first to balance oxygen atoms?
• Put coefficient 4 as 4 H₂O
• Changed H₂O to H₂O₄
• Put coefficient 3 in front of Fe
• Added more H₂
Answer: Put coefficient 4 as 4 H₂O
To balance oxygen atoms, a coefficient 4 was placed before H₂O, making it 4H₂O.
Q20. When balancing an equation, what must we remember NOT to alter?
• The coefficients in front of formulae
• The order of substances
• The formulae of the compounds or elements
• The physical states
Answer: The formulae of the compounds or elements
To equalise atoms, we cannot alter the formulae of the compounds or elements involved. We can only change coefficients.
Q21. In the partly balanced equation Fe + 4H₂O → Fe₃O₄ + H₂, which element was balanced next?
• Oxygen
• Iron
• Hydrogen
• None
Answer: Hydrogen
After oxygen, hydrogen atoms were still not balanced. The next step was to balance hydrogen atoms.
Q22. To equalise hydrogen atoms in the partly balanced equation, what coefficient was placed before H₂?
• 2
• 3
• 4
• 1
Answer: 4
To balance 8 H atoms from 4H₂O, 4 molecules of H₂ (giving 8 H atoms) were placed on the product side: Fe + 4H₂O → Fe₃O₄ + 4H₂.
Q23. After balancing hydrogen, which element was left to be balanced in the equation 3Fe + 4H₂O → Fe₃O₄ + 4H₂?
• Oxygen
• Hydrogen
• Iron
• Water
Answer: Iron
The text states: “Examine the above equation… only one element is left to be balanced, that is iron.”
Q24. What coefficient was placed before Fe to balance the iron atoms?
• 1
• 2
• 3
• 4
Answer: 3
To equalise Fe (1 on LHS, 3 in Fe₃O₄ on RHS), three atoms of Fe were taken on the LHS: 3Fe + 4H₂O → Fe₃O₄ + 4H₂.
Q25. What is the final balanced equation for the reaction between iron and steam?
• Fe + H₂O → Fe₃O₄ + H₂
• Fe + 4H₂O → Fe₃O₄ + 4H₂
• 3Fe + 4H₂O → Fe₃O₄ + 4H₂
• 3Fe + H₂O → Fe₃O₄ + H₂
Answer: 3Fe + 4H₂O → Fe₃O₄ + 4H₂
Step VI presents the final balanced equation as 3Fe + 4H₂O → Fe₃O₄ + 4H₂.
Q26. What is the method of balancing chemical equations described in the text called?
• Algebraic method
• Oxidation number method
• Hit-and-trial method
• Inspection method
Answer: Hit-and-trial method
This method is called the hit-and-trial method as we make trials to balance the equation using the smallest whole number coefficient.
Q27. In the hit-and-trial method, what do we use to balance the equation?
• Change the subscripts
• Use fractions as coefficients
• Use the smallest whole number coefficients
• Ignore some elements
Answer: Use the smallest whole number coefficients
We make trials to balance the equation by using the smallest whole number coefficient.
Q28. What is the correct order of steps taken to balance Fe + H₂O → Fe₃O₄ + H₂?
• Balance Fe, then O, then H
• Balance H, then O, then Fe
• Balance O, then H, then Fe
• Balance H, then Fe, then O
Answer: Balance O, then H, then Fe
Steps: First oxygen was balanced (4H₂O), then hydrogen (4H₂), then iron (3Fe).
Q29. After putting the coefficient 4 for H₂O, how many oxygen atoms are on the left side?
• 1
• 4
• 2
• 8
Answer: 4
4 molecules of H₂O contain 4 oxygen atoms.
Q30. After putting the coefficient 4 for H₂, how many hydrogen atoms are on the right side?
• 2
• 4
• 6
• 8
Answer: 8
4 molecules of H₂ (H₂) contain 4 x 2 = 8 hydrogen atoms.
Q31. In the final check of the balanced equation, what must be equal on both sides?
• Number of molecules only
• Number of atoms of each element
• Total mass only
• Physical states
Answer: Number of atoms of each element
We count atoms of each element on both sides. They must be equal for the equation to be balanced.
Q32. In the respiration equation, what are the products?
• Glucose and oxygen
• Carbon dioxide and water only
• Carbon dioxide, water, and energy
• Oxygen and energy
Answer: Carbon dioxide, water, and energy
The respiration reaction is: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy.
Q33. What is the primary purpose of respiration at the cellular level?
• To absorb oxygen
• To release carbon dioxide
• To provide energy
• To produce water
Answer: To provide energy
The glucose combines with oxygen in our cells and provides energy needed to stay alive.
Q34. What type of reaction is the burning of coal (C + O₂ → CO₂)?
• Exothermic combination
• Endothermic combination
• Exothermic decomposition
• Displacement
Answer: Exothermic combination
It is a combination reaction (two reactants, one product) and combustion releases heat, making it exothermic.
Q35. In the reaction for burning natural gas, what is the coefficient of O₂?
• 1
• 2
• 3
• 4
Answer: 2
The balanced equation is CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g).
Q36. Which activity in the chapter involves a combination reaction that gives out heat?
• Activity 5.1 (Burning of Mg ribbon)
• Activity 5.3 (Zinc and acid)
• Activity 5.4 (Calcium oxide and water)
• Both Activity 5.1 and 5.4
Answer: Both Activity 5.1 and 5.4
The text asks to identify the type of reaction in Activity 5.1 where heat is given out with a single product, which is a combination reaction, similar to Activity 5.4.
Q37. What is the physical state of water (H₂O) in the respiration equation?
• (g)
• (l)
• (aq)
• (s)
Answer: (l)
In the respiration equation C₆H₁₂O₆(aq) + 6O₂(aq) → 6CO₂(aq) + 6H₂O(l) + energy, water is liquid (l).
Q38. In the partly balanced equation (Step IV), Fe + 4H₂O → Fe₃O₄ + 4H₂, which element is still unbalanced?
• Oxygen
• Hydrogen
• Iron
• All are balanced
Answer: Iron
After Step IV, hydrogen and oxygen are balanced, but iron is not (1 Fe on left, 3 Fe on right).
Q39. What is the total number of hydrogen atoms on the left in the final balanced equation 3Fe + 4H₂O → Fe₃O₄ + 4H₂?
• 4
• 6
• 8
• 10
Answer: 8
4 molecules of H₂O contain 4 x 2 = 8 hydrogen atoms.
Q40. What is the key characteristic used to identify a reaction as a combination reaction?
• Release of heat
• Formation of a single product from multiple reactants
• Formation of a precipitate
• Evolution of gas
Answer: Formation of a single product from multiple reactants
The defining feature is the formation of a single product from two or more reactants.
Q41. What solution is used for whitewashing walls?
• Calcium oxide in water
• Calcium carbonate in water
• Calcium hydroxide in water
• Calcium chloride in water
Answer: Calcium hydroxide in water
A solution of slaked lime (calcium hydroxide) produced by the reaction of CaO with water is used for whitewashing.
Q42. Why is the reaction between calcium oxide and water considered exothermic?
• It absorbs heat.
• It releases a large amount of heat.
• There is no temperature change.
• It makes the beaker cold.
Answer: It releases a large amount of heat.
The text explicitly states it releases a large amount of heat, which can be felt.
Q43. According to the text, what provides us with the energy to stay alive?
• The water we drink
• The air we breathe
• The food we eat
• The sleep we get
Answer: The food we eat
We get the energy needed to stay alive from the food we eat, through the process of digestion and respiration.
Q44. In the context of balancing, what does the term “smallest whole number coefficient” refer to?
• Using coefficients like 1, 2, 3, etc., not fractions
• Using the smallest possible subscripts
• Using decimal coefficients
• Using coefficients of 1 only
Answer: Using coefficients like 1, 2, 3, etc., not fractions
The hit-and-trial method aims to use the smallest set of whole numbers (1,2,3…) as coefficients to balance the equation.
Q45. What is the correct way to write “four water molecules” when balancing an equation?
• H₂O₄
• (H₂O)₄
• 4 H₂O
• H₄O₂
Answer: 4 H₂O
The text gives the example: to balance oxygen atoms we can put coefficient 4 as 4 H₂O and not H₂O₄ or (H₂O)₄.
Q46. After balancing, the final equation 3Fe + 4H₂O → Fe₃O₄ + 4H₂ has how many iron atoms on each side?
• 1 on left, 3 on right
• 3 on left, 3 on right
• 4 on left, 4 on right
• 3 on left, 1 on right
Answer: 3 on left, 3 on right
3Fe gives 3 Fe atoms on the left. Fe₃O₄ contains 3 Fe atoms on the right.
Q47. Which of these everyday processes is NOT mentioned as exothermic in the text?
• Burning of natural gas
• Respiration
• Decomposition of compost
• Photosynthesis
Answer: Photosynthesis
Photosynthesis is not listed as exothermic. Burning of natural gas, respiration, and decomposition into compost are listed.
Q48. What happens to the reaction mixture in an exothermic reaction?
• It cools down.
• It warms up.
• Its colour always changes.
• A gas is always evolved.
Answer: It warms up.
In exothermic reactions, heat is released, making the reaction mixture warm.
Q49. In the balancing process, what was the state of the equation after Step V?
• Unbalanced
• Partly balanced
• Fully balanced
• Incorrect
Answer: Fully balanced
After Step V (adding coefficient 3 to Fe), the equation 3Fe + 4H₂O → Fe₃O₄ + 4H₂ was fully balanced, as confirmed in Step VI.
Q50. What is the main reason for balancing a chemical equation using the hit-and-trial method?
• To make it look neat
• To obey the law of conservation of mass
• To include physical states
• To identify the type of reaction
Answer: To obey the law of conservation of mass
Balancing ensures the number of atoms of each element is equal on both sides, which obeys the law of conservation of mass. The hit-and-trial is one method to achieve this.