Class 8 Science Chapter 4 Question Answers - Combustion and Flame

Q1: State the difference between burning of a candle and burning of a fuel like coal

Ans: Candle burns with a flame but coal does not burns with a flame also coal is a carbon product and its burning is harmful for environment but candle is made from wax its burning is not as much harmful as burning of coal
Q2: Explain combustion and combustible along with examples.

Ans: The chemical process in which a substance reacts with oxygen to give off heat is called combustion. In combustion the release of heat can result in the production of light in the form of either glowing or a flame.The substance which undergoes combustion is called combustible or fuel. Fuel may be solid, liquid or gas.
Example: Burning of a coal shows the process of combustion and coal here is combustible or fuel.
Q3: Food is a fuel for the body. Justify this statement.

Ans: Food is a fuel for our body because inside our body food is broken down into simpler form by reaction with oxygen and with the release of heat and energy.
Q4: Identify the materials in which combustion can take place Wood, paper, kerosene oil, iron nails, brick, stone, charcoal.

Ans: Wood, paper, kerosene oil, charcoal

Q5: On putting glass over a lighted candle, the candle flame flickers and produce smoke, why so?

Ans: Take two lighted candle A and B, and place them on a table, now put a transparent glass over candle B and see what happens to the candle B, we will observe that candle flame flickers and produces smoke and finally it goes off, this is because on putting glass over it, the air supply was cut off and candle was unable to burn in the absence of air.
Q7: When the clothes of a person catch fire, the person is covered with a blanket to extinguish fire, explain why?

Ans: To cut off the air supply of the fire, so that the fire gets off and the person could be saved from fire.
Q8: What is forest fire?

Ans: A forest fire is a natural disaster consisting of a fire which destroys a forested area, and is dangerous for the people living in forest area as well as for the wildlife. During extreme heat of summer, at some places dry grass catches fire, the fire gets spread from grass to tree, and very soon whole forest catches fire. It is very difficult to control such fires.
Q9: What do you mean by ignition temperature? Why a matchstick dose not catch fire on its own at room temperature?

Ans: The lowest temperature at which a substance catches fire or the lowest temperature at which combustion begins and continues in a substance is called its ignition temperature.
Match Stick cannot catch fire on its own at room temperature because it can catch fire only at its ignition temperature not at room temperature.
Q10: Why does the matchstick start burning on rubbing it on the side of matchbox?

Ans: The striking surface of the matchbox contains red phosphorus and the head of the matchstick contains potassium chlorate. So when the matchstick is rubbed on the matchbox, some of the red phosphorus is converted to white phosphorus, a chemical i.e. so volatile that it ignites in air.
Q11: Why does kerosene oil catch fire faster than wood?

Ans: This is because the specific heat capacity of the wood is more than the kerosene oil. So, the wood takes time to burn but burns for longer period than the kerosene oil.
Q12: Why we should store kerosene oil with proper care?

Ans: Because kerosene oil can catch fire very easily as its ignition temperature is lower than other combustible material.
Q13: Explain why inflammable substance can easily set on fire? 

Ans: Inflammable substances have very low ignition temperature and thus they can easily catch fire with a flame, example: diesel, kerosene oil, alcohol, Liquefied Petroleum Gas (LPG) etc.
Q14: How do we control fire?
Ans: As we know there are three essential requirements for producing fire these are fuel, air and heat. Fire can be controlled by removing one or more of these requirements that is either by cutting of the air supply, or by bring down the temperature of the fuel or both.
Q15: How a fireman extinguishes fire?

Ans: Fireman through water with pressure on fire, water helps in cooling down of the combustible material so that its temperature is brought below its ignition temperature and fire does not get spread. A part from this combustible material is surrounded by water vapour which helps in cutting of the air supply and finally fire is extinguished.
Q16: Write short note on fire extinguisher.

Ans: The most commonly used fire extinguisher is water, it works when wood and paper are on fire, but if electrical equipment is on fire water may conduct electricity and may harm those trying to douse the fire, also water is not suitable for fires involving oil and petrol, since water is heavier than oil it sinks below the oil and oil keeps burning on the top. So if electrical equipment and inflammable materials are on fire Carbon dioxide is the best extinguisher. Since it is heavier than oxygen it covers the fire so that the contact between fuel and oxygen is cut off and the fire is controlled, it also bring s down the temperature of the fuel. For this purpose carbon dioxide is stored at high pressure as a liquid in cylinders.
Q17: Explain the essential requirements for producing fire.

Ans: Following are the essential requirements for producing a fire:

1. Fuel: It is the combustible material
2. Air: Air supplies oxygen which supports combustion and without which it is impossible to set on a fire.
3. Heat: Heat is important as it raises the temperature of the fuel beyond the ignition temperature.

Q18: Why water is not a good fire extinguisher in case of electrical equipment and inflammable materials?

Ans: The most commonly used fire extinguisher is water, it works when wood and paper are on fire, but if electrical equipment is on fire water may conduct electricity and may harm those trying to douse the fire, also water is not suitable for fires involving oil and petrol, since water is heavier than oil it sinks below the oil and oil keeps burning on the top. So if electrical equipment and inflammable materials are on fire Carbon dioxide is the best extinguisher. Since it is heavier than oxygen it covers the fire so that the contact between fuel and oxygen is cut off and the fire is controlled, it also bring s down the temperature of the fuel. For this purpose carbon dioxide is stored at high pressure as a liquid in cylinders.
Q19: How many types of combustion are there? Name them.

Ans: There are three types of combustion:

1. Rapid combustion: Bring a burning matchstick or gas lighter near a gas stove and turn on the knob of gas stove, we will find the gas burns rapidly and produces heat and light, such combustion is called as rapid combustion
2. Spontaneous combustion: In this type of combustion a material suddenly bursts into a flame, without the application of any apparent .e;g spontaneous combustion of coal dust.
3. Explosion: When a cracker is ignited a sudden reaction takes place with the evolution of heat, light and sound, this type of combustion is called Explosion.

Q20: What is flame?

Ans: A flame is the visible, gaseous part of a fire. The substance which vaporise during burning give flames, It is caused by a highly exothermic reaction taking place in a thin zone.

Q21: Introduce a glass plate into the luminous zone of the steady candle flame and hold it for few seconds, then remove it? What did you observe on the glass plate?

Ans: We will observe a circular blackish ring formed on the glass plate, this indicates the deposition of unburnt carbon particles present in the luminous zone of the flame.
Q22: State some of the characteristics of a good fuel.

Ans: A good fuel is one which is:

• Readily available
• Ignite easily
• Burn well, not with explosion
• Cheap
• Produces a large amount of heat
• Have low smoke and ash content
• Should be easy to store and transport

Q23: How calorific value of a fuel is related to amount of heat produced by fuel? Mention calorific value of wood, coal, petrol, CNG and Biogas.

Ans: Calorific value of the fuel is the amount of heat energy produced on complete combustion of 1 kg of fuel. It is expressed in kilojoule per kg.

Calorific value of wood is 17000-22000 kJ/kg

Calorific value of coal is 25000-33000 kJ/kg

Calorific value of CNG is 50000 kJ/kg

Calorific value of biogas is 35000-40000 kJ/kg

Q24: Why we say increasing fuel consumption has harmful effects on environment?

Ans: Increasing fuel consumption has harmful effects on environment because:

• Carbon fuels like wood, coal and petroleum releases unburnt carbon particles that are dangerous pollutants causing diseases like asthma and respiratory disorders
• Incomplete combustion of these fuel releases a very poisonous gas carbon monoxide
• Release of carbon dioxide gas by combustion of fuels is becoming one of the cause of global warming