Page 242 - ICSE Chemistry 8

21. A compound is represented using a chemical formula which represents the number of the elements
that a molecule of a compound consists of.
3–
22. The symbol of the ion formed by sulphur is S .
23. A chemical equa on should indicate the physical states of the reactants and products.
24. A posi ve catalyst decreases the rate of a reac on.
25. Reversible reac ons proceed in both the reac ons.
26. Ammonia and hydrogen chloride combine together to form ammonium chloride which is a liquid.
27. When solid calcium carbonate is heated, carbon dioxide gas is evolved.
28. When water is added to quicklime, heat energy is absorbed.
29. Enzyme carbohydrase is specifi c to the diges on of carbohydrates.
30. Basic oxides react with acids to produce salts.
31. Potassium and sodium react with cold water so violently that hydrogen evolved could immediately
catch fi re.
32. Bosch’s process is used to produce hydrogen on a small scale.
33. Hydrogen is a neutral gas.
34. Impure hydrogen burns in air with a characteris c pop sound.
35. Dis lled water is the purest form of water.
36. A solu on that contains more dissolved substance than could normally be dissolved at a specifi c
temperature is called saturated solu on.
37. S rring increases the rate of forma on of a solu on.
38. The salts containing water of crystallisa on are called the anhydrous salts.
39. Temporary hardness of water can be easily removed by simple means such as boiling and fi ltra on.
40. Hard water is suitable for drinking and cooking purposes.
41. Carbon is a good reducing agent.
42. Graphite is a greyish-black opaque substance which is heavier than diamond.
43. Animal or bone charcoal is used to remove fl uoride from water.
44. Hard coke is a black and porous substance used in household furnaces.
45. Gas carbon is a good conductor of electricity.

C. Answer in short.
1. How are the three states of ma er diff erent?
2. What is intermolecular space?
3. Why are the intermolecular force of a rac on between the par cles in liquids less than that in
solids but more as compared to gases?
4. Why do the par cles in solids lack kine c energy?
5. How is the average kine c energy of the par cles related to the temperature of the gas?
6. Diff eren ate between periodic and non-periodic changes.
7. How are desirable changes diff erent from undesirable changes?
8. Why is breaking of a glass plate a physical change?
9. Why is dissolving a physical change?
10. Why is baking a chapa a chemical change?
11. What is a mixture? Give examples.
12. Discuss the principle of separa on of mixtures.
13. Discuss the technique of sieving.
14. Diff eren ate between sedimenta on and decanta on.

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