Class 8 Science Chapter 4 Question Answers - Combustion and Flame

Q1: What is combustion?
Ans: Combustion is a chemical process in which a substance reacts with oxygen to give off heat.

Q2: What does coal produce during its combustion?
Ans: Coal produces carbon dioxide, heat and light during its combustion.

Q3: What does magnesium burn to form?
Ans: Magnesium burns to form magnesium oxide and produces heat and light.

Q4: What are combustible and non-combustible substances?
Ans: Substances which undergo combustion are said to be combustible, whereas non-combustible substances are those which don’t burn.

Q5: Define ignition temperature of a fuel.
Ans: The lowest temperature at which a fuel catches fire is called its ignition temperature.

Q6: Define explosion.
Ans: A type of combustion during which a huge amount of heat and light is evolved with a boom, along with the production of gas, is known as an explosion; e.g., the exploding of fireworks, i.e., crackers, etc.

Q7: What is flame?
Ans: Flame is a region where the burning or combustion of gaseous substances takes place.

Q8: Give two examples each of solid fuels, liquid fuels, and gaseous fuels.
Ans:

• Solid fuels – Wood, cow dung, etc.
• Liquid fuels – Kerosene, petrol, etc.
• Gaseous fuels – Hydrogen, methane, etc.

Q9: Why is sodium kept immersed in kerosene?
Ans: Sodium has a very low ignition temperature, i.e., it catches fire on coming in contact with air, so it is kept in kerosene.

Q10: Define the term ‘petrochemicals’.
Ans: Petrochemicals are substances derived from petroleum and natural gas. They are used in the production of detergents, fibers, plastics, and other industrial products.

Q11: What are inflammable substances? Give examples.
Ans: The substances which have a very low ignition temperature and can easily catch fire with a flame are called inflammable substances; e.g., LPG, petrol, alcohol, etc.

Q12: Give two examples of fuels that are used to generate electricity.
Ans: Two examples of fuels that are used to generate electricity are petrochemicals and coal.

Q13: What is rapid combustion?
Ans: When a substance burns instantly and produces a huge amount of heat and light, the combustion is called rapid combustion; e.g., the instant burning of LPG in a gas stove.

Q14: What is fuel?
Ans: Anything that burns to produce energy in an efficient manner is called a fuel.

Q15: Define spontaneous combustion.
Ans: A type of combustion in which the substance suddenly catches fire without the supply of heat or friction externally is called spontaneous combustion; e.g., forest fires.

Q16: Why isn’t hydrogen gas used as a domestic or industrial fuel, although it has a very high calorific value? State three reasons for the answer.
Ans: Although hydrogen gas has a very high calorific value, it is not used as a domestic or industrial fuel due to the following reasons:

• It is expensive.
• It burns with an explosion.
• It is extremely inflammable, so it is risky to store and transport hydrogen.

Q17: Why does a piece of paper burn with yellow flame? Give a reason.
Ans: The inadequate supply of oxygen during the combustion of the piece of paper produces solid carbon particles that rise up in the flame. They become hot and glow to give off yellowish light. This makes the piece of paper burn with a yellowish flame.

Q18: Explain complete combustion.
Ans: This type of combustion involves complete burning of the combustible substance. No residue is left behind. Ash or smoke is not given off during or after this type of combustion. Mostly gases, such as hy¬drocarbons go through this form of combustion. On combustion, hydrocarbon produces carbon dioxide, water and heat.

Q19: Explain how water gets boiled in paper cup without burning it.
Ans: When we heat the paper cup containing water, the heat given to it is rapidly transferred to water from the paper cup. The temperature of water goes on increasing until it attains its boiling point, and starts boiling. As, during this process, the heat is continuously being transferred to water; the paper cup does not attain its ignition temperature. Hence, it does not burn.

Q20: It is observed at petrol pumps and airports, that hydrocarbon fire extinguishers are used, instead of soda-acid fire extinguisher. Give reasons why.
Ans: At petrol pumps and airports, there is more probability of fire break out due to oil. In such situations, soda-acid fire extinguisher does not work as it contains water or uses water to take off fire by cooling down the place. Water being heavier than oil sinks to bottom and hence, fire does not get controlled. In such a case, hydrocarbon fire extinguisher is very useful, as it contains turkey red oil, which causes the foaming of carbon dioxide gas under pressure. The foam covers the surface of the burning substances and dispels the supply of air to control fire.