Class 8 Science Chapter 4 Important Question Answers - Combustion and Flame

Q1: What is combustion, and how is it different from a simple burning process?
Ans: Combustion is a chemical process in which a substance reacts with oxygen to give off heat. The difference between combustion and simple burning lies in the presence of oxygen and the chemical reactions involved.

Q2: Can you explain the concept of ignition temperature and provide examples of substances with different ignition temperatures?
Ans: Ignition temperature is the lowest temperature at which a substance can catch fire. Examples: A matchstick has a lower ignition temperature than wood.

Q3: Why does a matchstick not catch fire on its own at room temperature but starts burning when rubbed on the side of the matchbox?
Ans: The friction between the matchstick and the matchbox generates heat, raising the matchstick's temperature above its ignition temperature, leading to combustion.

Q4: Describe the history and components of the modern safety matchstick.
Ans: Safety matchsticks consist of antimony trisulphide, potassium chlorate, powdered glass, and red phosphorus. They ignite due to friction between the matchstick and a rough surface.

Q5: What are the three essential requirements for producing fire, and how can fire be controlled by eliminating these requirements?
Ans: The essential requirements for fire are fuel, air (oxygen), and heat. Fire can be controlled by removing one or more of these requirements. For example, using a fire extinguisher cuts off the supply of air.

Q6: Explain why water is not suitable for extinguishing fires involving electrical equipment and inflammable liquids like petrol.
Ans: Water can conduct electricity and is heavier than oil, which causes it to sink below the burning oil, making it ineffective for such fires.

Q7: How does carbon dioxide (CO2) act as an effective fire extinguisher, and how is it stored and released for firefighting?
Ans: CO2, being heavier than oxygen, blankets the fire and reduces the temperature of the fuel. It is stored in high-pressure cylinders and expands upon release.

Q8: Describe what happens during spontaneous combustion and provide examples of substances that can undergo this process.
Ans: Spontaneous combustion occurs when a material bursts into flames without any apparent cause. Examples include phosphorus and some chemical reactions.

Q9: Explain the difference between rapid combustion and spontaneous combustion with suitable examples.
Ans: Rapid combustion is when a material burns rapidly, producing heat and light, such as the burning of gas. Spontaneous combustion is when a material suddenly bursts into flames without an apparent cause.

Q10: Describe the different zones of a flame and explain why goldsmiths use the outermost zone for melting gold and silver.
Ans: A flame has three zones: the luminous zone, the non-luminous zone, and the outermost zone. Goldsmiths use the outermost zone because it has the highest temperature.

Q11: What makes a good fuel, and can you provide examples of solid, liquid, and gaseous fuels?
Ans: A good fuel is readily available, cheap, burns easily in air, produces a large amount of heat, and leaves behind no undesirable substances. Examples include wood (solid), petrol (liquid), and natural gas (gaseous).

Q12: Explain the concept of fuel efficiency and how the calorific value is used to measure it.
Ans: Fuel efficiency is the ability of a fuel to produce heat energy. The calorific value measures the amount of heat energy produced by burning 1 kg of a fuel.

Q13: Discuss the harmful effects of increasing fuel consumption on the environment, including the release of carbon monoxide and carbon dioxide.
Ans: Increased fuel consumption leads to the release of harmful pollutants such as unburnt carbon particles and carbon monoxide, contributing to respiratory diseases and global warming.

Q14: Why is the use of CNG (Compressed Natural Gas) replacing diesel and petrol in automobiles, and how does it reduce harmful emissions?
Ans: CNG produces fewer harmful products in small amounts and is considered a cleaner fuel, making it a better choice for the environment.

Q15: Can you explain why water in a paper cup does not catch fire when the cup is heated, and how does heat transfer play a role in this phenomenon?
Ans: Water in the cup prevents the paper from reaching its ignition temperature. Heat is transferred from the cup to the water by conduction, preventing the paper from burning.