Tuesday, 8 November 2011

Class 8 - Science - Combustion and Flame (Q & A)

COMBUSTION AND FLAME (Questions and Answers)

Q1. What is the difference between the burning of a candle and the burning of a fuel like coal?

Q2. Why a matchstick does not catch fire on its own at room temperature?
Answer: A substance does not catch fire until its temperature is raised to its ignition temperature. A matchstick, a piece of wood or a can full of kerosene do not catch fire on their own because the ignition temperature of these substance are higher that the room temperature (25 °C).

(Note: When we strike the match stick against the match box, because of friction its temperature rises to ignition temp. and it ignites.)

Q3. Akansha has made two paper cups by folding a sheet of paper. She poured 50 ml of water in one of the cups and heated both the cups separately with a candle.
  a. What happens to the empty paper cup when it is heated?
  b. What happens to the paper cup with water?
  c. Does water in this cup becomes hot?

Q4. List out the names of some inflammable substances?
Answer: Petrol, Diesel, Alcohol, Kerosene

Q5. Why carbon-di-oxide is considered as the best fire extinguisher?Answer: Fires involving electrical equipment and inflammable materials like petrol, carbon dioxide (CO2) is considered as the best extinguisher. CO2, is heavier than oxygen, it covers the fire like a blanket. This cuts off the contact between the fuel and oxygen. Hence, the fire is extinguished.
CO2 is bad conductor of electricity, it does not harm the electrical equipment.

It can be stored at high pressure as a liquid in cylinders. When release from the cylinder, it converts to gas form and expands. Because of which it lowers the temperature , helping in controlling fire.

Q6. In what form is LPG stored in cylinder? Name the chemical formula of LPG.
Answer: In gas form. LPG stands for Liquified Petroleum Gas. Its chemical name is Iso-Butane.

Q7. Why is water not used to control fires involving electrical equipments?
Answer: Water is a good conductor of electricity.

Q8. Why LPG a better domestic fuel than wood?

Q9. What is the unit in which the calorific value of a fuel is expressed?

Q10. It is difficult to burn a heap of green leaves but dry leaves catch easily. Why?

Q11. Can the process of rusting be called combustion?

Answer: No. Rusting and combustion are two separate chemical phenomenon. Rusting is the process of eating away of surface of metals by the action various chemical compounds.

Q12. Why it is dangerous to burn coal in a closed room?
Answer: Burning of coal in closed room (lack of oxygen) produces Carbon Mono-oxide (CO) in the absense of oxygen .
Carbon Mono Oxide is poisonous to humans and it does not have any smell. It is also called as silent killer gas. The fumes/smoke because of burning can cause lung damage or even death due to smoke inhalation.

Q13: Name types of combustion.
Answer: Rapid combustion, Spontaneous Combustion and Explosion.

Q14: Name the different techniques used in extinguishing  fire.
Answer: In general any fire extinguisher uses any or all of the following methods:
1. Removing the combustible substance.
2. Cooling the burning substance to below its ignition temperature.
3. Cutting off the supply of air/oxygen.

Q15: Can we consider food as fuel?
Answer: Yes.