Page 237 - ICSE Chemistry 8

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19. The radical ammonium is represented as AM .
20. Oxygen atom combines with another oxygen atom by sharing two electrons to form oxygen molecule.
21. An element is represented by a chemical symbol.
22. The valency of an element is always a whole number that varies from 1 to 6.
23. A molecule of sodium chloride is formed by combining the atoms of the elements sodium and
chlorine.
24. It is not important for an equa on to indicate the informa on about pure state and solu on forms
of reactants and products.
25. Decomposi on of water into hydrogen and oxygen needs electricity to take place.
26. Chemical reac ons are the processes in which new substances with new proper es are formed.
27. When carbon and sulphur are heated to form carbon disulphide, heat energy is evolved.
28. Photosynthesis in green plants takes place in the presence of chlorophyll that acts as a catalyst.
29. The decomposi on brought about by light is called chemical decomposi on.
30. Copper oxide reacts with hydrogen to form copper by the removal of oxygen from copper.
31. Hydrogen is the simplest and the lightest element.
32. The metals below hydrogen in the ac vity series displace hydrogen from water and dilute acids.
33. In the laboratory, hydrogen is prepared by the ac on of dilute sulphuric acid or dilute hydrochloric
acid on zinc granules.
34. Hydrogen is a highly infl ammable gas and burns with a pop sound.
35. Pure hydrogen burns with a green fl ame in air.
36. The addi on of chlorine kills bacteria and other harmful substances present in water.
37. Muddy water can be cleaned by adding a crystal of alum.
38. Pure water is a good conductor of heat and electricity.
39. The par cles of a colloid show Tyndall eff ect.
40. Permanent hardness of water can be removed by adding chemicals such as washing soda.
41. Carbon is soluble in all solvents.
42. Allotropes have the same chemical proper es but diff erent physical proper es.
43. Graphite is the purest form of carbon.
44. Buckminsterfullerene is used in fl at-screen televisions.
45. Ac vated charcoal is prepared by destruc ve dis lla on of cane sugar or glucose.
C. Answer in short.

1. What does the kine c molecular theory of ma er state?
2. Why do liquids tend to evaporate when le exposed to air?
3. On what factors does the change of state of ma er depend?
4. What causes the gas to spread easily and faster when kept in open as compared to solids and liquids?
5. What does the law of conserva on of mass state?
6. Give some examples of changes that take place in our surroundings.
7. Diff eren ate between reversible and irreversible changes.
8. How are natural changes diff erent from man-made changes?
9. Why is mel ng of ice a physical change?
10. Why is burning of paper a chemical change?
11. Diff eren ate between elements and compounds.
12. What are the characteris cs of pure substances?
13. How is a suspension diff erent from emulsion?
14. Why the separa on of mixture becomes essen al?

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