CBSE Class 8 Science Chapter 4 Combustion And Flame LAQs

CBSE Class 8 Science Chapter 4 Combustion And Flame Long Answer Questions

Question 1. In an experiment, 4.5 kg of a fuel was completely burnt. The heat produced was measured to be 180000 kJ. Calculate the calorific value of the fuel.
Answer: The amount of heat energy produced on the complete combustion of 1 kg of a fuel is called its calorific value.

Given, total mass of fuel = 4.5 kg

Total heat produced = 180000 kJ

Now, the heat produced in burning 1 kg of fuel, i.e., calorific value

⇒ \(\begin{aligned}
& =\frac{\text { Total heat produced }}{\text { Total mass of fuel }}=\frac{180000}{4.5} \\
& =\frac{180000 \times 10}{45}=\frac{1800000}{45} \\
& =\frac{360000}{9}=40000 \mathrm{~kJ} / \mathrm{kg} \quad \text { (divid }
\end{aligned}\)

Hence, calorific value of fuel =40000 kJ/kg

Question 2. Can the process of rusting be called combustion? Discuss.
Answer: The process of rusting cannot be called combustion because, in this process, no heat and light are produced. For this reason, iron is not considered a combustible substance.

Question 3. 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 heated by Ramesh will get heated in a shorter time because he kept his beaker near the hottest (non-luminous) zone of the flame. 

Read And Learn More CBSE Class 8 Science Long Answer Questions

Question 4. Sparkling rods light up when inserted into a flame. The hotter the flame, the quicker the rod lights up. Tito’s picture shows four different zones of a flame.

1. In which zone should a sparkling rod be inserted to light it the fastest?

1. Zone 1
2. Zone 2
3. Zone 3
4. Zone 4

Answer: 1. Zone 1

2. Why does the flame of a candle go off when all the wax melts?

Answer: The flame of the candle goes off because no more wax is available on the candle, which can burn and give flame.

Which zone has unburnt carbon particles?

1. Zone 1
2. Zone 2
3. Zone 3
4. Zone 4

Answer: 3. Zone 3

Question 5. Observe the given picture and answer the following questions

1. Which is the hottest part of the flame in the given figure of a candle?
Answer: A or the outermost part of the flame.

2. Why does the middle zone of the candle flame glow with yellow colour?
Answer: The Middle zone of the candle flame contains unburnt carbon particles, which give the flame a yellow colour.

3. State any two conditions under which combustion takes place.
Answer: The Presence of a supporter of combustion and the presence of a combustible substance are two required conditions for combustion.

Question 6.

(1) In an experiment, 6.5 kg of a fuel was completely burnt. The heat produced was measured to be 1,90,000 KJ. Calculate the calorific value of the fuel in KJ per kg.

Answer: \(\begin{aligned}
& \text { Calorific value }=\text { Energy } / \text { Mass } \\
& =\frac{190000}{65}=29230 \mathrm{~kJ} / \mathrm{kg}
\end{aligned}\)

(2) CNG is a better fuel when compared to petrol or diesel. Regarding the data in the table, evaluate this statement

Answer: CNG produces less carbon monoxide than diesel and produces less greenhouse gas than diesel. Also, CNG has a higher energy density than diesel.

Question 7. The picture shows four types of fire extinguishers. Each extinguisher is suited for specific sources of fire.

1. A tailor has money for any two of the fire extinguishers. Which pair of fire extinguishers should be bought for his tailoring shop?

1. P and Q
2. Q and S
3. P and R
4. S and R

Answer: 3. P and R

2. Fire extinguisher K is suitable for kitchen fires.

1. Which fire extinguisher can perform most of the work of K?

1. P
2. Q
3. R
4. S

Answer: 2. Q

2. How does dumping sand on a fire extinguish the flame?

Answer: Sand absorbs heat and cuts off the oxygen supply, hence, it helps in extinguishing the flame.

Question 9. Although wood has a very high calorific value, we still discourage its use as a fuel. Explain.
Answer: Burning of wood has several disadvantages despite having a high calorific value. These are as follows-

Burning wood produces a lot of smoke, which causes respiratory diseases like asthma.

The cutting down of trees to obtain as a wood fuel leads to deforestation which is very harmful to the environment.

Trees provide us many useful substances. To obtain fuel wood, when trees are cut down, then all useful substances which can be obtained from trees are lost.

Question 10. A forest fire produces a lot of air pollution. Write a brief about the reasons for forest fires.
The reasons for forest fires are as follows

• At high temperatures, sometimes dry grass catches fire, which spreads throughout the forest.
• A campfire may also be a reason for fire.
• Lightning.
• The use of fires by villagers to ward off wild animals.
• Fire was lit intentionally by people living around forests for recreation.
• The friction of bamboo due to high wind velocity and rolling stones.

Question 11. Manu was heating oil to fry potato chips. The cooking oil all of a sudden caught fire, he poured water to extinguish the fire. Do you think, this action was suitable? If yes, why? If not, why not? In such a condition what should Manu have done?
Answer: Pouring water to extinguish the fire due to oil is not a suitable action. It is because oil is lighter than water. So, water will settle down below the oil. In such a condition, sand or soil should be used.

They cut off the supply of air to the fire. Thus, fire can be controlled. But sand or soil will not be available at that place.

So, Manu should have switched off the flame ofthe burner and put a lid on the frying pan. By doing this, the contact between fuel and oxygen is cut, and the flame could go off.