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

Q1: Which of the following are products of combustion?

A) Carbon dioxide

B) Nitrogen

C) Water

D) Energy

Ans: The correct option is A) Carbon dioxide

Combustion, also a term for burning, is a high-temperature exothermic chemical reaction between a fuel (carbon) and an oxidant, usually, atmospheric oxygen, that produces carbon dioxide and other compounds, often a gaseous mixture termed smoke. 

An example of combustion is given below
CH₄ (Methane) +2 O₂⟶ CO₂ +2 H₂O + Heat

Q2: State the conditions under which combustion occurs. Why is it more difficult to burn some combustible substances than others?

Ans: Combustion of fuel requires:

• The presence of any flammable substance, such as coal, natural gas, or wood.
• The presence of air provides oxygen.
• Heat is available to raise the fuel temperature above its particular ignition point.

Because these different substances catch fire at different temperatures, certain flammable substances are much more difficult to burn than others. Because some of these compounds have a lower ignition temperature, they easily catch fire. The temperature at which a substance ignites is critical in assessing its combustibility.

Q3: Explain how CO2 can control fires.

Ans: CO₂ can control fires in following ways:-

• Oxygen is a supporter of combustion.
• Carbon dioxide is heavier than oxygen and forms a blanket around the fire.
• Thus, cutting off the supply of oxygen to the fire.
• It also reduces the temperature of the fuel.

CO₂ is a non-combustible gas. It is also a non-supporter of combustion. CO₂ extinguishes fire in two ways: Since it is heavier than oxygen, it forms and covers the fire like a blanket form and cuts off the contact between oxygen and fuel. In cylinders, CO₂ is mostly kept in liquid form. When released, it expands enormously and cools down. This brings down the fuel’s temperature, which helps control the fire.

Q4: Which flame zone does a goldsmith use to melt gold and silver, and why?

Ans: The goldsmith uses the outermost flame zone for melting gold and silver because it is considered the hottest portion of the flame and, hence, undergoes complete uniform combustion.

• The outermost zone of a flame is blue: It is a non-luminous zone.
• In this zone, the fuel’s complete combustion occurs because there is plenty of air surrounding it.
• The outermost part has the highest temperature in the flame.
• It is the hottest part of the flame at that time.
• It is quite thin and sleek as compared to the middle zone.

Q5: Give Reason: Paper by itself catches fire easily, whereas a piece of paper wrapped around an aluminium pipe does not.

Ans: The ignition temperature is the minimum temperature required for a substance to combust, and a substance won’t ignite until it reaches this critical point. Paper has a relatively low ignition temperature, making it susceptible to catching fire quickly. However, when paper is wrapped around an aluminum pipe, its ignition temperature effectively increases, preventing easy combustion. This is due to aluminum’s properties as a metal, which make it an excellent conductor of heat. Aluminum absorbs the heat energy that would otherwise cause the paper to burn, thus when paper is wrapped around aluminum, it is less likely to catch fire.

Q6: What are the main three essential requirements to produce fire? How a fire extinguisher is useful for controlling fire.

Ans:  Three essential requirements to produce gas are:

• Oxygen.
• Combustible substance or fuel.
• Heat to acquire ignition temperature.

Fire extinguisher:

• A fire extinguisher is used to control small fires.
• A fire extinguisher contains chemicals such as Sulfuric acid (H2SO4) and Sodium bicarbonate (NaHCO3)

Working of Fire extinguisher:

• When a fire extinguisher is operated by pressing the knob, the sulphuric acid reacts with sodium bicarbonate solution, producing carbon dioxide gas (CO2).
• The carbon dioxide is non-combustible. The fire is controlled when the contact between the fuel and oxygen is cut off.

The fire extinguishers are very useful for controlling the fire in the following ways:

• Fire extinguishers extinguish the fire by turning  down the supply of air, bringing down or lowering the temperature of the fuel, or both.
• Water is the most commonly used fire extinguisher. But water works only when things like paper and wood are on fire. If an electrical equipment is on fire, water can conduct electricity and harm those things trying to douse the fire. Water is also not very suitable for fires involving oil and petrol. Water is heavier than oil. Hence, it sinks below the oil, and the oil keeps burning on top.
• For fires that are involved in electrical equipment and combustible materials like petrol or diesel, carbon dioxide (CO2) is the best extinguisher. CO2, being more heavier than oxygen, covers the fire like that of a blanket. Since the contact established between the fuel and oxygen is snapped  fire can be   controlled. The additional advantage of CO2 is that, in most of the cases, it does not harm electrical equipments.

Q7: Choose the right statement about inflammable substances from the following options given below. They have

A) low ignition temperature and cannot catch fire easily

B) high ignition temperature and can catch fire easily

C) low ignition temperature and can catch fire easily.

D) high ignition temperature and cannot catch fire easily.

Ans: The correct option is C) low ignition temperature and can catch fire easily.

Combustible substances burn in the presence of air to produce heat and light. Example: petrol, coal

Inflammable substances are highly combustible, i.e. they catch fire easily as they have low ignition temperatures.

The lowest temperature when a substance will catch fire and undergoes combustion, releasing heat and light, is called the ignition temperature.

Therefore, both conditions should be fulfilled for a substance to be inflammable.

Q8: People usually keep Angethi or burning coal in their closed rooms during winter. Why is it advised to keep the door open?

Ans: Burning coal, such as in an Angethi, releases hazardous chemicals like carbon dioxide and carbon monoxide, which can deplete oxygen levels and cause respiratory issues, especially in enclosed spaces. Keeping the door open ensures proper air and oxygen circulation, mitigating these risks. Angethi generates heat with glowing coals but minimal flames. Moreover, coal-fired power stations significantly pollute the air by emitting sulphur dioxide, contributing to acid rain, and releasing toxic gases and metals like mercury, further underscoring the need for adequate ventilation when burning coal. 

Q9: Although wood has a very high calorific value, we still discourage its use as a fuel. Explain.

Ans:  The following reasons can be considered to discourage the use of wood as a fuel :-

• It has a high ignition temperature, so the combustion process of wood would be a bit tough.
• The ignition temperature of a substance is said to be the minimum temperature at which the substance catches fire for spontaneous combustion.
• Burning wood produces many harmful gases such as Carbon monoxide (CO) and carbon dioxide (CO₂). Excess of both can kill a person, and it also causes  global warming.
• To use wood as fuel, we would have to cut trees.
• Trees are essential for any living being.

Q10: Forest fire produces a lot of air pollution. Write a brief about the reasons for forest fires.

Ans: Forest is a large geographical area dominated by trees, animals of various species, aquatic biomes and microorganisms. Forest fire is synonymous with disaster. Even before human involvement, natural, low-intensity wildfires occurred every few years to burn fuel, plant debris, and dead trees. The reasons for forest fires are listed below :

• Dry grass in the forest, when it gets very dry, high temperature helps it to burn, which causes a forest fire.
• Lightning from the sky could  also be a reason.
• Sometimes when villagers use fire to ward off animals, it burns the dry grass and results in forest fires.
• Friction between bamboo produces heat and burns  bamboo, resulting in  forest fire.

Q11: What do you understand by fuel efficiency?

Ans: Fuel efficiency is amount of heat that is produced on the complete combustion for 1 kg of a fuel, and is called its calorific value. It is usually expressed in terms of the calorific value, which is expressed in units of kilojoule per kg. The calorific value of fuel is kilojoule per kg (kJ) that is expressed in a unit. Example : The calorific value of coal is 25000-33000 kJ/kg.

Q12: You are provided with three watch glasses containing each milk, petrol and mustard oil. Suppose you bring burning candles near these materials; which material will instantly catch fire and why?

Ans: Petrol will catch fire instantly because it is highly flammable. Mustard oil and milk have very high ignition temperatures. So they will not catch fire instantly.

• Petrol is highly volatile, i.e. it forms vapour at low temperatures. Hence, it is more flammable.
• The higher the chaining in chemical composition, the lower the volatility, hence the flammability.
• Kerosene is a petroleum product, so kerosene  ignition temperature is low compared to other given substances.
• The ignition temperature is also known as the ignition point. The ignition temperature of fuel can be termed as the minimum temperature to which fuel should be heated so that it can catch fire and start burning.