CBSE Class 8 Science Combustion and Flame VBQs

Question. Which of these best explains why a chullah fire (or a camp fire) burns brighter when it is fanned?

(a) the air tries to put out the fire, so the fire burns brighter
(b) the amount of fuel available to the fire increases
(c) the amount of oxygen available to burn increases
(d) it removes the burnt ashes allowing the wood to burn

Answer : C

Question. Which of these is an ill-effect of a depleting ozone layer?
(a) infection of lungs
(b) skin cancer
(c) retarded growth
(d) forest fires

Answer : B

Question. Water was poured over a large oil fire to extinguish it. What would happen and why?
(a) The fire will be put off since water extinguishes fires
(b) The fire will spread since oil floats on water
(c) The fire will be put off since water dissolves oil.
(d) The fire will spread since water contains hydrogen that is a combustible gas

Answer : B

Question. One of the effects of depleting green vegetation is reduction in the amount of oxygen in the earth's atmosphere. This effect will be similar to shifting from
(a) a mountain top to sea level
(b) sea level to a mountain top
(c) sea level to below sea level
(d) the poles to Sahara desert.

Answer : B

Question. In a movie, a spaceship explodes and continues to burn as it moves through space. This can NOT happen because
(a) space does not have oxygen.
(b) gases expand in a vacuum.
(c) spaceships are made of metals that cannot explode.
(d) would have extinguished the fire.

Answer : A

Question. Which of these provides the energy for the motion of a petrol car?
(a) Petrol
(b) Its battery
(c) Its engine
(d) Petrol and its battery

Answer : A

Question. Calorie is a unit of energy. Some food items mention on their label a certain number of calories.  What is the reason for this?
(a) Food is used to measure energy.
(b) The calorie figure indicates how much energy the food would give when eaten.
(c) The calorie figure indicates how much energy was used to make the food item.
(d) Food items having more than a certain number of calories are banned.

Answer : B

Question. Kiran's mother placed a candle in a bowl as shown and lit it. A little while later, Kiran opened a bottle of Pepsi and poured it quickly into the bowl. What will happen?

(a) The candle flame will burn brighter.
(b) The candle flame will be put out.
(c) The candle and flame will be completely unaffected.
(d) The candle will break.

Answer : B

Question. Students of a class were asked to select a source of energy that is ENVIRONMENT FRIENDLY. Which among these should they select?
(a) Petrol
(b) Solar energy
(c) Cooking gas
(d) Coal

Answer : B

Question. Which type of pollution is caused by bursting a lot of crackers during Diwali?
(a) Water pollution
(b) Air and sound pollution
(c) Light and sound pollution
(d) Only sound pollution

Answer : B

Question. When a lighted candle placed partly in a  trough of water is covered by an inverted  glass, water rushes up as shown and the  flame extinguishes. The above means that:

(a) We knew that the statement in the box was true even before doing the experiment.
(b) We knew that the statement in the box was false even before doing the experiment.
(c) We believe that the statement in the box explains the observation but need to check it.
(d) Our experiment has shown us that the statement in the box is the correct explanation.

Answer : C

Question. A fine copper gauze is taken and placed about half a centimeter above a Bunsen Burner. When the gas is switched on and lighted above the gauze, the flame burns only above the copper gauze as shown in the figure. This demonstrates that:

(a) Copper is a very good conductor of heat.
(b) Copper is a very poor conductor of heat.
(c) The wire gauze blocks the flow of oxygen.
(d) The wire gauze block the flow of the gas.

Answer : A

Question. When a lighted candle placed partly in a  trough of water is covered by an inverted  glass, water rushes up as shown and the  flame extinguishes. If the above hypothesis is true, what will happen after the candle blows out in this new experimental set-up

(a) The mouse will die or show signs of suffocation.
(b) There will be no change in the mouse's behaviour.
(c) The glass will break and the mouse will escape.
(d) The mouse will become visibly more active.

Answer : A

Question. Which chemical change takes place when vinegar is added to baking soda?
(a) A gas is released.
(b) A blue powder is formed.
(c) The combination burns.
(d) The colour of the solution changes

Answer : A
 

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.