8th

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

Ans- The list of conditions under which combustion can take place are following –

1) A combustible substance.

2) Oxygen

3) The temperature of the combustible substance should be upto ignition temperature.

Q.2- Fill in the blanks-

(a) 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 _______.

Ans- (a) pollution

(b) kerosene

(c) ignition temperature

(d) water

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

Ans- The use of CNG in automobiles has reduced pollution in our city as-

(i) CNG is a clean fuel.

(ii) It does not produce harmful gases on burning.

(iii) No residue is left behind after the burning of CNG.

Q.4- Compare LPG and wood as fuels.

Ans-

Q.5- Give reasons –

(a) Water is not used to control fire involving electrical equipment.

(b) LPG is a better domestic fuel than wood.

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

Ans- (a) Water is not used to control fire involving electrical equipment because water is a good conductor of electricity. It may harm the person extinguishing the fire. It may also spread the electric current and can cause a huge damage.

(b) LPG is a better domestic fuel than wood because –

(i) Transportation and storage of LPG is easy as compared to wood.

(ii) Like wood, LPG does not produce smoke.

(iii) The ignition temperature of LPG is very low as compare to wood.

(iv) Like wood, LPG does not leave behind ash.

(v) The calorific value of LPG is much more as compared to wood.

(vi) LPG does not cause pollution.

(c) Paper by itself catches the fire easily due to its low ignition temperature. But when paper is wrapped around an aluminium pipe, then paper transfers some of its heat to the pipe and the paper does not get its ignition temperature.

Q.6- Draw a labelled diagram of a candle flame.

Ans-

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

Ans- The unit in which the calorific value of a fuel is expressed is

Q.8- Explain how CO₂ is able to control fires.

Ans- CO₂ is heavier than oxygen. It covers the fire like a blanket and cut off the contact between the fuel and the oxygen. Hence controlled the fire.

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

Ans- The green leaves have moisture in them, so their ignition temperature is high and they do not burn easily. On the other hand, the ignition temperature of dry leaves is low. That’s why they catch the fire easily.

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

Ans- A goldsmith uses the outermost zone of a flame for melting gold and silver because this zone is the hottest part of a flame.

Q.11- In an experiment 4.5 kg of a fuel was completely burnt. The heat produced was measured to be 1,80,000 kJ. Calculate the calorific value of the heat.

Ans- Amount of combustible substance = 4.5 kg

Value of heat produced = 1,80,000 kJ

Calorific value of heat =

Heat produced/Amount of fuel

= 1,80,000/4.5

= 40,000 kJ/kg

Q.12- Can the process of rusting be called combustion. Discuss.

Ans- The process of rusting is quite different from combustion. During combustion, heat and light is produced. Both these factors are absent in rusting. Also iron is an incombustible substance and a combustible substance is required for combustion.

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 the shorter time.

Ans- The outermost part of a flame being the hottest part will heat the water quickly as compared to the yellow part. Thus, the water in Ramesh’s beaker will get heated in shorter time.

Chapter At A Glance

• The substances which burn in air are called combustible.
• Oxygen is essential for combustion.
• During the process of combustion heat and light are given out.
• Ignition temperature is the lowest temperature at which a combustible substance catches fire.
• Inflammable substances have very low ignition temperature.
• Fire can be controlled by removing one or two requirements essential for producing fire.
• Water is commonly used to control fire.
• Water cannot be used to control the fire including electrical equipments and oil.
• There are various types of combustion such as rapid combustion, spontaneous combustion, explosion etc.
• There are three different zones of a flame – dark zone, luminous zone and non-luminous zone.
• An ideal flame is readily available, readily combustible and easy to transport. It has high calorific value. It does not produce smoke and gases that pollute environment.
• Fuels differ in their efficiency and cost.
• Fuel efficiency is expressed in terms of its calorific value which is expressed in the units of kJ/kg
• Unburnt carbon particles are dangerous pollutants causing respiratory problems.
• Incomplete combustion of gases produce poisonous carbon monoxide gas.
• Increased percentage of carbon dioxide in air has been linked to global warming.
• Oxides of sulphur and nitrogen produced by the burning of coal, diesel and petrol cause acid rain which is harmful for soil, crops and buildings.

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