CBSE Guess > Papers > Question Papers > Class X > 2009 > Science > Science
Q. 7. Describe an activity to observe what happens when quick lime is added to water taken in a beaker. State one important observation and name the type of reaction taking place. 2
Ans. Activity:
(i) Small amount of water is taken in a beaker.
(ii) A little amount of quicklime is added slowly to the beaker.
(iii) When we touch the beaker carefully, the beaker feels to be quite hot. Therefore the chemical reaction between quicklime and water to form slaked lime is characterised by a change in temperature i.e. rise in temperature.
Thus the reaction between quicklime and water is an exothermic reaction (i.e. heat producing 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.
Q. 9. What is dispersion of light? Name the (i) component of white light that deviates the least and (ii) the component that deviates the most while passing through a glass prism. 2
Ans. The splitting up of white light into seven colours on passing through a transparent medium like a glass prism is called dispersion of light. When white light passes through the glass prism:
(i) The red component of the white light is deviated least.
(ii) The violet component of the white light is deviated the most.
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