NCERT Solutions for Class 8 Science Chapter 6 Combustion and Flame

Q 1. List conditions under which combustion can take place.

Answer:

The conditions required for the combustion to take place are:

(a) Presence of oxygen

(b) Presence of inflammable substance (fuel)

(c) Maintain Ignition Temperature (the minimum temperature at which substance catches fire).

Q 2. Fill in the blanks.
(a) Burning of wood and coal causes_________ of air.
(b) A liquid fuel, used in homes is_________ .
(c) Fuel must be heated to its___________ ___________ before it starts burning.
(d) Fire produced by oil cannot be controlled by__________ .

Answer:

(a) Burning of wood and coal causes pollution of air.
(b) A liquid fuel, used in homes is LPG(Liquified Petroleum Gas).
(c) Fuel must be heated to its ignition temperature before it starts burning.
(d) The fire produced by oil cannot be controlled by water.

Q 3. Explain how the use of CNG in automobiles has reduced pollution in our cities.

Answer:

Fuels like petroleum and coal release toxic gases like carbon monoxide, carbon dioxide, and sulphur dioxide during combustion. These harmful gases cause pollution and respiratory problems.

Whereas CNG (Compressed Natural gas) is a clean fuel and produces these harmful gases in very small quantity.

(CNG is methane stored at high pressure!)

Q 4. Compare LPG and wood as fuels.

Answer:

Q 5(a). Give reasons.

Water is not used to control fires involving electrical equipment.
Answer:

Water is not used to control fires involving electrical equipment because water being a good conductor of electricity may lead the electric current to spread causing electric shocks or short-circuits

Q 5(b). Give reasons.

LPG is a better domestic fuel than wood.
Answer:

Wood produces lots of smoke and unburnt carbon particles, which cause respiratory problems. But LPG does not produce these. Thats why LPG is a better domestic fuel than wood.

Q 5(c) . Give reasons.

Paper by itself catches fire easily whereas a piece of paper wrapped around an aluminium pipe does not.

Answer:

Paper has a low ignition temperature and hence catches fire easily. But when it is wrapped around an aluminium pipe, heat is transferred to the metal and hence the ignition temperature of the paper is not reached. That is why paper by itself catches fire easily whereas a piece of paper wrapped around an aluminium pipe does not.

Q 6. Make a labelled diagram of a candle flame.

Answer:

Following is the labelled diagram of a candle flame:

Q 7. Name the unit in which the calorific value of a fuel is expressed.

Answer:

The calorific value of a fuel is expressed in kilojoules per kilogram (kJ/kg).

Q 8. Explain how is able to control fires.

Answer:

is heavier than . Therefore, it acts as a blanket over the substance and cuts off the oxygen supply required for combustion. This way is able to control fires.

Q 9. It is difficult to burn a heap of green leaves but dry leaves catch fire easily. Explain.
Answer:

It is difficult to burn a heap of green leaves but dry leaves catch fire easily because green leaves have moisture present in them. Hence the heat gets absorbed by water to provide energy to evaporate. But, the dry leaves have no moisture and hence the heat is not lost. That is why it catches fire easily.

Q 10. Which zone of a flame does a goldsmith use for melting gold and silver and why?

Answer:
The goldsmith uses the outermost non-luminous (blue) zone for melting gold and silver. This is because it is the hottest part of the flame and hence melts the metals easily.

Q 11. In an experiment 4.5 kg of a fuel was completely burnt. The heat produced was measured to be 180,000 kJ.Calculate the calorific value of the fuel.

Answer:

The calorific value of the fuel is the amount of heat produced by the complete combustion of 1 kg of fuel.

Given, Heat produced by 4.5 kg of fuel = 180000 kJ

Heat produced by 1 kg of fuel = 180,000 kJ/4.5 Kg = 40,000 kJ

The calorific value of the fuel is 40,000 kJ/kg.

Q 13. Abida and Ramesh were doing an experiment in which water was to be heated in a beaker. Abida kept the beaker near the wick in the yellow part of the candle flame. Ramesh kept the beaker in the outermost part of the flame. Whose water will get heated in a shorter time?

Answer:

The outermost part of the flame is the hottest region of the flame. Hence, water kept near it will get heated faster i.e in a shorter time.
Therefore, the water in Ramesh's beaker will get heated in a shorter time.