Class 8 General Science Notes: - Unit-6- Chemical Reaction and Bonding: Solved Exercise Part- D (Structured Questions and Answers)

(National Book Foundation - New Book/New Syllabus Based on National Curriculum Pakistan 2023-2024 and Onwards)

(Class 8 General Science Notes for 8th Centralized Exam 2023-2024 and onward, FDE, Islamabad)

Click Here for Solved MCQs Unit-4

D. Structured questions

i). When coal burns it produces carbon dioxide and leaves ash behind. Ash produced in this reaction is lighter than coal.

a) Does this contradict the law of conservation of mass? Justify your answer.

Ans:  No, it does not contradict the law of  Conversation of mass because carbon dioxide is also produced.

b) Classify this reaction

Ans:  It is an Exothermic reaction.

c) Write a balanced chemical equation for this reaction.

Ans:  C  +  O₂   ⟶    CO₂              

    

ii). Element X is in Group I-A and element Y is in Group VII-A.

a) What type of bond between them will be formed? Explain.

Ans:  Covalent bond.

b) Draw electron-dot and electron-cross structures to show the formation of bond between them.

Ans:  

iii). Classify the following reactions as exothermic or endothermic reaction.

a) Combustion

Ans:  Exothermic reaction

b) Decomposition

Ans:  Endothermic reaction

c) Neutralization

Ans:  Endothermic reaction

d) Reaction between baking soda and vinegar

Ans:  Exothermic reaction

iv) Two chemical reactions are occurring in Beaker A and Beaker B. In beaker A, the temperature changes from 25°C to 40°C. In beaker B, temperature changes from 25°C to 20°C.

a) What are the changes in temperature in these beakers?

Ans:  Endothermic A

b) Which of these reactions is exothermic and endothermic?

Ans:  Exothermic B

25°C  -  20°C  =  5°C  

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