Class 8 Science Chapter 4 HOTS Questions - Combustion and Flame

Q1: Three candles X, Y, and Z are fixed on a table. A glass chimney is placed over candle X, and is rested on wooden blocks in a manner which allows the entrance of oxygen. To candle Y, a glass chimney is placed in such a way that it is simply rested on the table. For candle Z, a similar arrangement is made along with the placement of a glass plate over the chimney. Candle _____i____ bums freely. The flame of candle _____ii_____ will flicker off giving smoke, and that of candle _____iii_____  will simply go off.
(a) i-X, ii-Y, iii-Z
(b) i-Z, ii-Y, iii-X
(c) i-Y, ii-X, iii-Z
(d) i-Z, ii-X, iii-Y
Ans: a
Sol: Candle X burns freely as air can enter the chimney from below. However, the chimney is placed over candle Y in such a way that it is rested on the table. This means that air does not enter the chimney from below. Thus, the flame of candle Y flickers and produces smoke. On the other hard, the flame of candle Z goes off as air is not available for burning. The correct answer is A.

Q2: The ignition temperature of sodium is very _____i____. To prevent its _____ii_______ combustion, it is stored under _______iii______,. The information in which alternative completes the given statements?
(A)  i-low, ii-rapid, iii-Kerosene
(B)  i-low, ii-spontaneous, iii-Kerosene
(C)  i-high, ii-rapid, iii- water
(D)  i-high, ii-spantaneous, iii-water
Ans: b
Sol: The minimum temperature at which a substance catches fire is called ignition temperature. The ignition temperature of sodium is very low. Hence, it catches fire easily at room temperature. It reacts vigorously with oxygen at room temperature. To prevent its spontaneous combustion, it is stored under kerosene. The correct answer is B.

Q3: The gas that is used in fire extinguishers is
(a)  Nitrogen                            
(b)  Hydrogen
(c)  Sulphur dioxide              
(d)  Carbon dioxide
Ans: d
Sol: Carbon dioxide is an excellent fire extinguisher. It is a non-combustible gas and heavier than air. Hence, carbon dioxide is used in fire extinguishers. The correct answer is D.

Q4: Ordinarily, kerosene oil bums in air with a yellow flame. However, in stoves, it bums with a blue flame. Which of the following statements explains the above observation? 
(a)  Complete combustion of kerosene takes place in stoves. 
(b)  The ignition temperature of kerosene is attained easily in stoves. 
(c)  Stoves are made of iron and kerosene bums with a blue flame in the presence of iron.
(d) None
Ans: a
Sol: Ordinarily, kerosene oil burns in air with a yellow flame producing a lot of smoke. However, the stoves are made in such a manner that combustion of kerosene takes place in presence of large amount of oxygen. Thus, complete combustion of kerosene takes place and blue flame is obtained as a result. The correct answer is A.

Q5: Consider the given diagram X and Y in the given figure represent
(a)  Sulphur oxides and nitrogen oxides
(b)  Silicon oxides and nitrogen oxides
(c)  Sulphur oxides and chlorine gas
(d)  Silicon oxides and chlorine gas
Ans: a
Sol: Smoke from industries has large quantities of nitrogen oxides and sulphur oxides. These oxides combine with water vapour present in atmosphere to form acid droplets that remain suspended in the atmosphere. When it rains, these droplets mix with rain water and cause acid rains. The correct answer is A.

Q6: Incomplete combustion of fossil fuels releases a gas 'X?. This gas is poisonous in nature and it bums with a blue flame. Which gas does X represent?
(a)  Ozone
(b)  Chlorine
(c)  Carbon dioxide
(d)  Carbon monoxide
Ans: d
Sol: Incomplete combustion of fossil fuels leads to the formation of carbon monoxide. Carbon monoxide can be poisonous if inhaled in large quantities. It burns with a blue flame. The correct answer is D.

Q7: Harish performed two different experiments I and II. In experiment I, he provides a substance 'X? with some heat. He found out that a sudden reaction takes place to form heat, light, and sound. In experiment II, he provides heat to a substance 'Y?. He found out that when Y bums, only heat and light are produced. Experiment I represents   an   example of __i__, and experiment II represents an example of __ii___. The information in which alternative completes the given statement?
(a)  i-Explosion, ii-Rapid combustion
(b)  i-Explosion, ii-Sportaneous combustion
(c)  i-Spontaneous combustion, ii-Explosion
(d)  i-Rapid combustion, ii-Spontaneous combustion
Ans: a
Sol: A substance when burns to produce huge amount of heat, light, and sound represents an example of explosion. On the other hand, when a substance burns rapidly to produce huge amount of heat and light represents an example of rapid combustion. Hence, experiment I represents an explosion, whereas experiment II represents a rapid combustion. The correct answer is A.

Q8: Sakshi took a piece of four substances W, X, Y, and Z. She tried to bum each of them by bringing a lighted matchstick near them. She observed that Y and Z bum readily, whereas W bums readily only after sustained heating. However, X does not bum at all. Among the given substances taken by Sakshi, which substance has the highest ignition temperature?
(a)  W
(b)  X
(c)  Y
(d)  Z
Ans: b
Sol: The ignition temperature of a substance is the lowest temperature at which a substance catches fire. On bringing a lighted matchstick near the substances W, X, Y, and Z, the ignition temperature of X is not reached. However, the ignition temperature of W, Y, and Z are reached. Hence, while X does not burn, W, Y, and Z burn readily. Thus, the ignition temperature of X would be the highest among the substances taken by Sakshi. The correct answer is B.

Q9: Geeta took four combustible substances I, II, II, and IV. She found out that the ignition temperatures of I, n, III, and IV is233^∘C, 473^∘C, 210^∘C,and 170^∘C, respectively. She heats all the substances to 190^∘C,. Among the given substances, which substance will catch fire when heat is supplied to it?
(a)  I
(b)  II
(c)  III
(d)  IV
Ans: d
Sol: Different substances catch fire at different temperatures. The ignition temperature of a substance is the lowest temperature at which a substance catches fire or starts burning. A substance cannot catch fire as longs as its temperature is lower than its ignition temperature. When substances I, II, III, and IV are heated to190^∘Cby Geeta, only the ignition temperature of substance IV is reached. Thus, only IV will catch fire. The correct answer is D.

Q10: If a goldsmith wants to make jewellery of gold and silver, then he has to melt them. To do this, he will use the ___i____ zone of the flame as this zone is the ____ii____ zone of the flame. The information in which alternative completes the given statement?
(a)  i-Innermost, ii-Hottest
(b)  i-Innermost, ii-Moderately hot
(c)  i-Outermost, ii-Moderately hot
(d)  i-Outermost, ii-Hottest
Ans: d
Sol: The flame of a candle contains three different zones: outermost zone, middle zone, and innermost zone. The outermost zone of the flame is the hottest part of the flame, and the innermost zone of the flame is the least hot part. The middle zone is moderately hot. As gold and silver melts at a very high temperature, a goldsmith has to blow the outermost zone, which is the hottest part of the flame, to melt gold and silver. The correct answer is D.

Q11: A certain gas is used by living organisms for respiration, and this is used in the process of combustion. In the process of combustion, the gas reacts with combustible substances to produce heat. The mentioned gas is
(a) Oxygen
(b) Nitrogen
(c) Carbon dioxide
(d) Carbon monoxide
Ans: a
Sol: Oxygen gas is used by human beings and other living organisms for respiration. Combustion is a chemical process in which a combustible substance reacts with oxygen. Heat and sometimes light energy is released during combustion. Hence, the gas mentioned is oxygen. The correct answer is A.

Q12: Sujit was performing an experiment in the laboratory. He collected four substances I, II, III, and IV and kept all of them in contact with air at room temperature. He observed that substance III suddenly bursts into flames. Then, he heated I, II, and IV and found out that while I and II catch fire, substance IV does not. However, substance IV catches fire on prolonged heating. Which substance undergoes spontaneous combustion in Sujit's experiment?
(a) I
(b) II
(c) III
(d) IV
Ans: c
Sol: Spontaneous combustion is the one in which a substance starts burning without any heat provided to it. In the given experiment, substance III suddenly bursts into flames when kept in contact with air at room temperature. Hence, it represents spontaneous combustion. The correct answer is C.

Q13: Ravi takes three substances X, Y, and Z. On heating these substances he observes that they bum at different temperatures. He finds out that X catches fire on little heating, while Y and Z do not catch fire. On further heating of Y and Z, he observes that Y catches fire, while Z does not. The correct arrangement of X, Y, and Z in order of their increasing ignition temperatures is
(a)  X<Y<Z
(b)  Z<Y<X
(c)  Z<X<Y
(d)  Y<X<Z
Ans: a
Sol: The ignition temperature of a substance is the minimum temperature at which a substance catches fire or starts burning. While X burns on little heating, Y and Z do not burn. This shows that the ignition temperature of X is lower than that of Y and Z. On further heating of Y and Z, it was found that Y had caught fire at a temperature that was lower than Z (as the latter does not start burning). Thus, it can be concluded that the ignition temperature of Y is lower than that of Z. Hence, the correct arrangement of the substances (X, Y, and Z) in order of their increasing ignition temperatures is X<Y<Z. The correct answer is A.

Q14: Why is it never advised to sleep in a room with burning coal fire?
(a)  Incomplete combustion of coal produces carbon monoxide, which is poisonous
(b)  Incomplete combustion of coal produces carbon dioxide, which is poisonous
(c)  Burning of coal produces oxides of nitrogen, the fumes of which are very toxic
(d)  Burning of coal produces oxides of nitrogen that combine with moisture present in air to form acids
Ans: a
Sol: Incomplete combustion of fuels such as coal produces carbon monoxide (CO) gas. Carbon monoxide is a poisonous gas and can even result in death, if inhaled in large quantities. Hence, it is dangerous to burn coal in a closed room while sleeping. The correct answer is A.

Q15: Global warming is a result of ____i_____ percentage of ____ii_____ in air. The information in which alternative completes the given statement?
(a)  i-An increased, ii-Carbon dioxide
(b)  i-A decreased, ii-Carbon dioxide
(c)  i-An increased, ii-nitrogen
(d)  i-A decreased, ii-nitrogen
Ans: a
Sol: Fuels, on combustion, release carbon dioxide(CO₂)in the environment. As a large amount of fuel is burnt daily to meet our energy needs, the concentration of carbon dioxide in the atmosphere is increasing everyday. This increase in the concentration of carbon dioxide in the atmosphere causes a rise in the temperature of the Earth. This phenomenon is called global warming. The correct answer is A.