CBSE Class 8 Science Combustion and Flame VBQs

Fill in the blanks.

(a) Burning of wood and coal causes Pollution of air.

(b) A liquid fuel, used in homes is Kerosene.

(c) Fuel must be heated to its Ignition Temperature before it starts burning.

(d) Fire produced by oil cannot be controlled by Water.

Give reasons.

(a) Water is not used to control fires involving electrical equipment.
Answer : Because water is a good conductor of electricity, so it may result in electric shock to the user while controlling fire involving electrical equipments with water.

(b) LPG is a better domestic fuel than wood.
Answer : Because LPG neither produces pollution nor leave residue, on burning as compared to wood. LPG has much higher calorific value than wood.

(c) Paper by itself catches fire easily whereas a piece of paper wrapped around an aluminium pipe does not.
Answer : This is because aluminium, being a metal, is a good conductor of heat. Therefore, heat is transferred from the paper to the metal and the paper does not attain its ignition temperature.

Question. List conditions under which combustion can take place.
Answer : There are certain conditions required for combustion to take place. They are:
(i) Presence of a fuel,
(ii) Air (or oxygen),
(iii) Temperature greater than ignition temperature.

Question. Compare LPG and wood as fuels
Answer :

Question. Make a labelled diagram of a candle flame.
Answer :

Question. Explain how CO₂ is able to control fires.
Answer : CO₂ being heavier than oxygen, covers the fire like blanket and also brings down the temperature of fuel. Since the contact between the fuel and oxygen is cut off, the fire comes under control.

Question. Explain how the use of CNG in automobiles has reduced pollution in our cities.
Answer : As the amount of unburnt carbon particles produced is very less in comparison to petrol and amount of harmful gases produced is also very less ,so CNG is comparatively a cleaner fuel than petrol.

Question. It is difficult to burn a heap of green leaves but dry leaves catch fire easily. Explain.
Answer : Green leaves have a lot of moisture in them. This moisture does not allow them to catch fire easily.
However, dry leaves have no moisture in them. Therefore, they catch fire easily.

Question. 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 : Calorific value of fuel is = 180000 kf / 4.5 kg = 40,000 kj/kg

Question. Can the process of rusting be called combustion? Discuss.
Answer : Combustion is a chemical process in which a substance reacts with oxygen and gives out energy during the process in the form of either heat or light or both. Rusting of iron is an exothermic process as heat is released during rusting. Hence, it is a kind of slow combustion 

Question. Name the unit in which the calorific value of a fuel is expressed.
Answer : Calorific value is defined as the energy contained in the fuel. It is expressed in the form kilojoule per kilogram (kJ/kg).

Question. 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 water in the Ramesh's beaker will heat up in a shorter time. This is because the outermost zone of a flame is the hottest zone, while the yellow zone (in which Abida had kept the beaker) is less hot.

Question. Which zone of a flame does a goldsmith use for melting gold and silver and why?
Answer : Goldsmiths use the outermost part/zone of the flame to melt gold and silver. This is because the outermost zone of the flame undergoes complete combustion and is the hottest part of the flame.