Combustion and Flame - Exemplar Solutions 3

CBSE Class 8 Science
NCERT Exemplar Solutions
CHAPTER 6
Combustion and Flame
SHORT ANSWER QUESTIONS
21. Boojho wants to separate the following materials as combustible and non-combustible. Can you help him?
Charcoal, chalk, stone, iron rod, copper coin, straw, cardboard, glass, paper, candle, wood.
Ans.  (a) Combustible — charcoal, straw, card board, paper, candle wood,
(b) Non-combustible — chalk, stone, iron rod, copper coin, glass.
22. Indicate whether the following statements are True or False. Also, write the false statements in their correct form.
(a) Air is necessary for combustion.
(b) Magnesium is a non-combustible metal.
(c) Carbon dioxide is an excellent fire extinguisher.
(d) Calorific value of wood is higher than that of coal.
Ans. (a), (c) are true statements.
(b) Magnesium is a combustible metal
(d) Calorific value of coal is higher than that of wood.
23. Match the items of Column A with the items of Column B.
Ans.  (a)–(iv), (b)–(vi), (c)–(v), (d)–(iii), (e)–(i), (f)–(ii)
24. Match the following for the flame of a candle.
Ans.  (a)–(iii)–(x), (b)–(ii)–(z), (c)–(i)–(y) 
25. If you hold a piece of iron wire with a pair of tongs inside a candle flame or a Bunsen burner flame, what will you observe? Will it produce a flame?
Ans. Iron wire will become red hot and glow. It will not produce a flame. 
26. Fill in the blanks using the words given in the box.
ignition, petrol, combustion, calorific value, combustible, inflammable
(a) A chemical process in which a substance reacts with oxygen to give off heat is called ____.
(b) Wood, paper, CNG are __________ substances.
(c) The lowest temperature at which a substance catches fire is called its __________ temperature.
(d) Ignition temperature of __________ is lower than that of wood.
(e) The substances which have very low __________ temperature and can easily catch fire with a flame are called __________ substances.
(f) The amount of heat energy produced on complete combustion of 1kg of a fuel is called its __________.
Ans. (a) combustion, (b) combustible, (c) ignition, (d) petrol, (e) ignition, inflammable, (f) calorific value.
27. People usually keep Angethi/burning coal in their closed rooms during winter season. Why is it advised to keep the door open?
Ans. Due to insufficient availability of oxygen in the closed room carbon monoxide gas is produced which can kill persons sleeping in that room.
28. Write True/False against the following statements and also correct the false statement.
(a) A physical process in which a substance reacts with oxygen to give off heat is called combustion.
(b) Water is the best extinguisher for fires involving electrical equipment.
(c) Alcohol, CNG and LPG are inflammable substances.
(d) Increased concentration of nitrogen in air is believed to cause global warming.
(e) Greater the calorific value, better is the fuel.
(f) Middle zone is the hottest zone of a flame.
(g) The substances which vapourise during burning, give flame.
Ans. (a) False – A chemical process in which a substance reacts with oxygen to give off heat is called combustion.
(b) False – Carbon dioxide is the best extinguisher for fires involving electrical equipment.
(c) True.
(d) False – Increased concentration of carbon dioxide in air is believed to cause global warming.
(e) True.
(f) False – outer zone is the hottest zone of a flame.
(g) True.
29. Cracker on ignition produces sound. Why?
Ans. Cracker bursts with the liberation of great amount of heat, light, gases and sound. This is known as explosion.
30. What do you understand by fuel efficiency?
Ans. The amount of heat energy produced on complete combustion of 1 kg of fuel is known as calorific value of the fuel expressed in a unit called kilojoule per kg (kJ/kg)