Combustion and Flame - Exemplar Solutions 2

CBSE Class 8 Science
NCERT Exemplar Solutions
CHAPTER 6
Combustion and Flame

VERY SHORT ANSWER QUESTIONS
16.  Fill in the blanks in the following sentences.
(a) A ____________ process in which a substance reacts with __________ to give off heat is called combustion.
(b) When the clothes of a person catch __________, the person is covered with a __________ to extinguish fire.
(c) The ______ temperature at which a substance catches fire is called its  ____ temperature.
(d) The substances which have very ________________ ignition temperature and can easily catch fire with a flame are called __________ substances.
(e) The substances which vapourise during __________, give flame.
Ans. (a) Chemical, oxygen; (b) fire, blanket (c) lowest, ignition; (d) low, inflammable (e) burning.

17. Some words (underlined) in the following sentences are jumbled up. Write them in their correct form.
(a) Seldie is a combustible substance.
(b) Slags is a non-combustible material.
(c) Chittsmack does not burn by itself.
(d) Some substances on combustion produce thea and mafel.
(e) The amount of heat energy produced on complete combustion of 1 kg of a fuel is called its ficalroic value.
Ans.  (a) diesel; (b) glass; (c) matchstick; (d) heat, flame; (e) calorific

18. Two glass jars A and B are filled with carbon dioxide and oxygen gases, respectively. In each jar a lighted candle is placed simultaneously. In which jar will the candle remain lighted for a longer time and why?
Ans. The beaker, filled with oxygen will remain lighted for a longer time as oxygen is the support of combustion because combustion cannot take place in the absence of oxygen. On the other and Carbon-dioxide which is heavier than oxygen covers the fire and consequently breaks the contact between fir and the oxygen. It also cools down the fuel because  is stored under pressure in cylinder and easily expands and cools down when released.

19. Anu wants to boil water quickly in a test tube. On observing the different zones of the flame, she is not able to decide which zone of the flame will be best for boiling water quickly. Help her in this activity.
Ans. Anu should keep her test tube in the outermost zone which is the hottest part of the flame.

20. Why is the use of diesel and petrol as fuels in automobiles being replaced by Compressed Natural Gas (CNG) in big cities?
Ans. It is because CNG produces harmful products in very small amount and is a cleaner fuel.