8th Science, Exercise, 13. Chemical Change and Chemical Bond.

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1. Generally a ... bond is formed when atoms of two elements having similar properties combine.

2. Calcium carbonate + Hydrochloric acid → ... + Carbon dioxide + water

3. Oxygen is a ................. in respiration.

4. Due to the ... force of attraction between opposite charges, the oppositely charged ions get attracted to each other, and a chemical bond is formed between them.

5. Define a covalent bond.

6. ... gases do not form any chemical bonds, as their electron octet/duplet is complete, while the atoms with incomplete electron octets/duplets form chemical bonds.

7. Explain by writing a word equation. Respiration is a chemical change.

8.  Carbon dioxide + Water → Green plant → ...+ Oxygen

9. What are the methods of classification of changes?

10. The common substance that burns in all these fuels is '... .'

11. Explain by writing a word equation.  Bubbles are seen on adding lemon juice to baking soda.

12. An atom uses its ... electrons during the formation of a chemical bond.

13. Write the electronic configuration of sodium and chlorine.

14. Draw the formation of the ionic bond of NaCl.

15. The shared ... becomes a common property of both the atoms, and thereby the electron octet/duplet of both the atoms becomes complete.

16. Define "ionic bond."

17. The number of chemical bonds formed by one atom is its ... .

18. New substances formed as a result of the reaction are called ... .

19. What is the difference between physical and chemical change?

20. CO₂ + H₂O ---Sunlight---Green Plant → ... + O₂

21.  C₆H₁₂O₆ + O₂  -------respiration-->  ... + H₂O

22. Draw the formation of an ionic bond in an MgCl₂ molecule.

23. Explain by writing a word equation. Hard water gets softened on mixing with a solution of washing soda.

24. Draw the formation of a covalent bond in an H₂ molecule.

25. What are the two main methods of completing the electron octet pair?

26. Draw the formation of a covalent bond in an H₂O molecule

27. Classify the following changes into physical and chemical changes.

Ripening of mango, melting of ice, boiling of water, dissolution of salt in water, Ripening

of banana, fragrance on ripening fruit, darkening of a cut potato, bursting of an inflated

balloon, sound of a bursting firecracker, foul smell from spoiled food.

28. Carbon dioxide + Calcium hydroxide ----->  Calcium ... + Water

29. Carbon + Oxygen → ...

30. ... water contains the chloride and sulphate salts of calcium and magnesium in a dissolved state.

 31. Calcium chloride + Sodium carbonate →  ... + Sodium chloride         

32. How does the colourless limewater turn milky?

33. Write word and chemical equations for the chemical change taking place in magnesium salts during the softening of hard water.

35. Write down the difference between physical change and chemical change. 

36. Write the difference between an ionic bond and a covalent bond.

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काही वेळेस कावळा मुद्दामहून मुंग्यांचे वारूळ उध्वस्त करतो जेणेकरून त्याच्यावर मुंग्या आक्रमण करतील. मुंग्या जेव्हा कावळ्यावर हल्ला करतात, तेव्हा त्या कावळ्याच्या शरीरावर फॉर्मिक ॲसिड नावाचा घटक सोडतात. 

हेच फॉर्मिक ॲसिड कावळ्यासाठी एक औषध म्हणून काम करते. त्यामुळे कावळ्याच्या पंखात परजीवी फंगसपासून जीव वाचवण्यासाठी कावळे मुंग्यांचे वारूळ उद्ध्वस्त करताना दिसतात. कावळे मुंग्यांना हल्ला करू देतात. मुंग्या चावा घेत असताना कावळे मस्तपैकी आपले पंख पसरून निवांत बसून राहतात.

मुंग्यांमध्ये असलेले फॉर्मिक ॲसिड हे अँटीफंगल आणि अँटीबॅक्टेरियल असते. कावळ्याच्या पंखात परजीवी फंगस असतात. हेच फंगस आणि बॅक्टेरिया नष्ट व्हावेत म्हणून कावळे मुंग्यांना चावा घेऊ देतात.

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