NCERT Solutions For Class 8 Science Chapter 4 Combustion And Flame SAQs
Question 1. Why is a person covered with a blanket when the clothes of that person catch fire?
Answer:
When the clothes of a person catch fire, the person is covered with a blanket to extinguish the fire. This is because the supply of air to the burning clothes is cut off by the blanket, due to which the clothes stop burning and the fire gets extinguished.
NCERT Solutions for Combustion and Flame SAQs Class 8
Question 2. How does a matchstick burn?
Answer:
The head of the safety match contains antimony trisulfide and potassium chlorate. The rubbing surface has powdered glass and a little red phosphorus. When the match is struck against the rubbing surface, some red phosphorus gets converted into white phosphorus. This immediately reacts with potassium chlorate in the matchstick head to produce enough heat to ignite antimony trisulfide and start combustion.
NCERT Solutions for Class 8 Science Combustion and Flame SAQs
Question 3. Why can a matchstick burn a splinter of wood directly, but cannot burn a log of wood directly? How can we burn a log of wood?
Answer:
A matchstick can burn a splinter of wood directly because it has a low ignition temperature, whereas a matchstick cannot burn a log of wood directly as it has a high ignition temperature.
To burn a log of wood, a small fire is first started by burning straw (or dry grass), or paper by a matchstick, then the log of wood is placed over this fire. This fire heats the log of wood to its ignition temperature due to which it starts burning.
Question 4. A matchstick does not catch fire on its own. But when it is rubbed, it starts burning. Why?
Answer:
A matchstick does not catch fire on its own at room temperature because the ignition temperature of a matchstick is much higher than the room temperature. The matchstick starts burning on rubbed on the rough side provided on the matchbox.
It is because the heat produced by friction heats the chemical at the head of the matchstick to its ignition temperature and makes it burn.
NCERT Solutions for Combustion and Flame Chapter 4 SAQs
Question 5. The calorific values of petrol and CNG are 45000 kJ/kg and 50000 kJ/kg, respectively. If you have a vehicle that can run on petrol as well as CNG, which fuel would you prefer and why?
Answer:
We prefer CNG (Compressed Natural Gas) because the calorific value of CNG is higher than that of petrol. Therefore, CNG will be more economical. CNG will produce a larger amount of heat energy than petrol. At the same time, it produces the least air pollutants.
NCERT Class 8 Science Chapter 4 Combustion and Flame SAQs
Question 6. Cracker on Ignition produces sound. Why?
Answer:
When a cracker is ignited, a sudden reaction takes place with the evolution of heat, light, and sound, and a large amount ot gas is liberated. Such a reaction is called an explosion. So, the cracker on ignition produces sound because of the sudden formation of a large amount of gas due to the chemical reaction.