Combustion and Flame - Exemplar Solutions 1

CBSE Class 8 Science
NCERT Exemplar Solutions
CHAPTER 6
Combustion and Flame
MULTIPLE CHOICE QUESTIONS
1. A substance which reacts with oxygen giving heat is called a combustible substance. Which, one of the following is a combustible substance?
(a) iron nail
(b) glass
(c) stone piece
(d) wood
Ans. (d) Wood is a combustible substance.
2. Which one of the following has the highest calorific value?
(a) kerosene
(b) biogas
(c) LPG
(d) petrol
Ans. (c) LPG has the highest calorific value. If is nearly 55000 KJ/Kg.
3. Magnesium ribbon on burning in air produces
(a) magnesium oxide, water and light
(b) magnesium oxide and heat
(c) magnesium oxide, heat and light
(d) magnesium oxide, water and heat
Ans. (c) magnesium burns in air to form magnesium oxide, heat and light
2Mg +O2 --------------> 2MgO
4. Which of the following is not a combustible substance?
(a) camphor
(b) glass
(c) straw
(d) alcohol
Ans. (b) Glass is not a combustible substance , combustible substance catches fire easily in presence of air.
5. The substance that does not burn with flame is
(a) LPG
(b) camphor
(c) dry grass
(d) charcoal
Ans. (d) Charcoal does not give flame on burning.
6. On placing an inverted tumbler over a burning candle, the flame extinguishes after some time. This is because of non-availability of
(a) oxygen
(b) water vapours
(c) carbon dioxide
(d) wax
Ans. (a) Oxygen is a supporter of combustion. On inverting the tumbler, the availability of oxygen is restricted and hence the candle blow off.
7. If a person’s clothes catch fire, the best way to extinguish the fire is to:
(a) throw water on the clothes.
(b) use fire extinguisher.
(c) cover the person with a woolen blanket.
(d) cover the person with a polythene sheet.
Ans. (c) If a person’s clothes catch fire, the best way to extinguish the fire is to cover the person with a woolen blanket , woolen blanket won't allow oxygen to pass through it .
8. The substance expected to have the highest ignition temperature out of the following is
(a) kerosene
(b) petrol
(c) coal
(d) alcohol
Ans. (c) Ignition temperature is high and hence ignites after some time on application of heat.
9. Choose the correct statement about inflammable substances from the following.
They have:
(a) low ignition temperature and cannot catch fire easily.
(b) high ignition temperature and can catch fire easily.
(c) low ignition temperature and can catch fire easily.
(d) high ignition temperature and cannot catch fire easily.
Ans. (c) The substance which have very low ignition temperature and can easily catch fire with a flame are called inflammable substances.
10. Choose the incorrect statement from the following. Forest fires are usually due to:
(a) carelessness of humans
(b) heat of sun
(c) cutting of trees
(d) lightning strike
Ans. (c) cutting of trees
11. The calorific value of a fuel is expressed in a unit called
(a) kilojoule per litre
(b) kilogram per mililitre
(c) kilojoule per gram
(d) kilojoule per kilogram
Ans. (d) Calorific value of fuel expressed in unit called kilojoule per kg (kJ/Kg)
12. In villages, people use wood as fuel because:
(a) it is considered to be an ideal fuel.
(b) of its easy availability and low cost.
(c) it is environment friendly.
(d) it catches fire easily.
Ans. (b) Wood is in high demand to be used as fuel because of its casy availability and low cost.
13. Which among the following is considered as the cleanest fuel?
(a) cow dung cake
(b) petrol
(c) kerosene
(d) hydrogen gas
Ans. (d) hydrogen gas
14. Choose the incorrect statement from the following.
A good fuel is one which:
(a) is readily available.
(b) produces a large amount of heat.
(c) leaves behind many undesirable substances.
(d) burns easily in air at a moderate rate.
Ans. (c) leaves behind many undesirable substances.
 Good fuel have following qualities. 
  • Readily available
  • Inexpensive
  • Burns easily in air at a judicious rate.
  • Liberates large amount of heat.
  • Should not leave behind any undesirable residues.
  • Easy enough to store, transport, and handle.
  • It should have high calorific value.
  • It should have ignition temperature above the room, temperature.
  • It should produce less hazardous products on burning.
15. Shyam was cooking potato curry on a chulha. To his surprise, he observed that the copper vessel was getting blackened from outside. It may be due to:
(a) proper combustion of fuel.
(b) improper cooking of potato curry.
(c) improper combustion of the fuel.
(d) burning of copper vessel.
Ans. (c) improper combustion of the fuel produces unburnt carbon particles that gets on the surface of the utensils as black layer