CBSE Guess > Papers > Question Papers > Class X > 2009 > Science > Science
Q. 6. Write an expression for the heat produced H in time t in a heater of resistance R which is carrying a current I.
Ans. Heat produced, H = I2 x R x t joulesWhere
Q. 7. Describe an activity to demonstrate the change that takes place when white silver chloride is kept in sunlight. State the type of chemical reaction which takes place.
Activity :
(i) About 2 g of silver chloride is taken in a china dish. It is white in colour.
(ii) This china dish is placed in sunlight for some time.
(iii) It will be observed that white silver chloride turns grey due to formation of silver metal. This is a decomposition reaction.
Q. 8. State two main properties of elements on which Mendeleev’s periodic classification was based. Why could no fixed position be assigned to hydrogen in his periodic table? 2
Ans. The two main properties of elements on which Mendeleev’s periodic classification was based:
(i) on the basis of the reaction of elements with hydrogen.
(ii) on the basis of the reaction of elements with oxygen.
No fixed position was given to hydrogen in Mendeleev’s periodic table because electronic configuration of hydrogen resembles that of alkali metals. So like alkali metals, hydrogen also combines with halogens,
oxygen and sulphur to form compounds having similar formulae.
| Compounds of H | Compounds of Na |
| HCI | NaCI |
| H2O | Na2O |
| H2S | Na2S |
On the other hand, just like halogens, hydrogen also exists as diatomic molecules and it combines with metals and non-metals to form covalent compounds.
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