HOTS questions - Combustion and flame - Class 8 Science

HOTS Questions - Class 8 Science - Combustion and flame

Q1) A chemistry teacher is explaining combustion to her class. She asks her students to identify the type of combustion that occurs when a candle is lit. What would be the correct answer and why?

Answer: The correct answer would be "Rapid Combustion." When a candle is lit, the wax in the candle rapidly combusts to produce heat and light. This type of combustion is called rapid combustion.

Explanation: The combustion of wax in a candle is an example of rapid combustion. The wax contains hydrocarbons which react with oxygen to produce heat and light. The reaction is exothermic and releases a large amount of energy in the form of heat and light.

Q2) A student is cooking noodles on a gas stove. She observes that the flame is blue at the bottom and yellow at the top. Explain the reason for this observation.

Answer: The yellow flame is due to incomplete combustion while the blue flame is due to complete combustion.

Explanation: The blue flame is hotter and is produced when the gas is completely burned. The yellow flame is cooler and is produced when the gas is incompletely burned. The yellow flame is due to the presence of carbon particles, which emit yellow light when heated.

Q3) Suppose you are camping in the forest and want to start a fire to keep warm. You are provided with dry leaves, dry twigs, and dry branches. Which of these materials will catch fire easily and why?

Answer: The dry leaves will catch fire easily because they have a lower ignition temperature.

Explanation: Dry leaves have a lower ignition temperature compared to twigs and branches. This means that they require less heat to start burning. Dry twigs and branches have a higher ignition temperature and require more heat to start burning.

Q4) Suppose you have a gas stove at home. Why do you need to light a matchstick to start the stove? Why won't it light up automatically?

Answer: The gas stove needs a spark or flame to ignite the gas, which will then produce heat. The gas is not self-igniting, which is why we need to use a matchstick or lighter to ignite it.

Q5) Why is a sparkler used during Diwali and other celebrations? What type of combustion does it involve?

Answer: Sparklers use rapid combustion, which produces a shower of sparks and colors. They are used during celebrations like Diwali for entertainment and to add to the festive atmosphere.

Q6) Why do we need to turn off the gas stove immediately if the flame goes off accidentally?

Answer: If the flame goes off accidentally, gas continues to flow, which can be dangerous as it can accumulate and cause an explosion. Turning off the gas stove immediately cuts off the gas supply and minimizes the risk of an accident.

Q7) Why do cars have exhaust pipes? What is the function of the exhaust pipe?

Answer: The exhaust pipe is used to expel the gases produced during combustion in the engine. These gases can be harmful to humans and the environment, so they need to be directed away from the car and released into the atmosphere.

Q8) Why do we need to use a chimney or exhaust fan in the kitchen while cooking?

Answer: While cooking, there is a lot of smoke, fumes, and gases produced which can be harmful to our health if inhaled. The chimney or exhaust fan helps to remove the smoke, fumes, and gases from the kitchen, ensuring good air quality.

Q9) Why is it dangerous to use petrol or other flammable substances to light a fire in a fireplace or bonfire?

Answer: Petrol and other flammable substances are highly volatile and can ignite easily, causing an explosion. This can be extremely dangerous and cause injury or death. It is recommended to use kindling or paper to start a fire in a fireplace or bonfire.

Q10) Why is it important to store combustible substances like kerosene or diesel in safe containers away from heat sources?

Answer: Combustible substances like kerosene and diesel are highly flammable and can easily catch fire. Storing them in safe containers away from heat sources minimizes the risk of accidental fires and explosions.

Q11) Why do firefighters use water to extinguish fires?

Answer: Water is used to extinguish fires as it helps to cool down the burning material, which stops the combustion process. It also helps to suffocate the fire by removing oxygen from the area.

Q12) Why is it important to turn off all electrical appliances before leaving the house?

Answer: Electrical appliances consume electricity and generate heat, which can lead to short circuits and cause a fire. Turning off all electrical appliances before leaving the house reduces the risk of accidental fires and also helps to conserve energy.

Q13) Why is it important to use fire-retardant materials for curtains, upholstery, and carpets in public places like movie theaters or shopping malls?

Answer: Fire-retardant materials have chemicals that make them resistant to catching fire. Using such materials in public places minimizes the risk of accidental fires and ensures the safety of the people present in the area.

Q14) You are provided with three watch glasses containing kerosene, alcohol, and water, respectively. Suppose you bring a burning matchstick near these materials one by one, which material(s) will catch fire instantly and why?

Answer: Kerosene and alcohol will catch fire instantly. This is because kerosene and alcohol have low ignition temperatures as compared to water. Therefore, even a small spark can ignite them.

Q15) A fire broke out in the chemistry lab of your school. You are provided with a bucket of water, a sand bucket, and a fire extinguisher. Which one would you use to extinguish the fire and why?

Answer: A fire extinguisher should be used to extinguish the fire. This is because fire extinguishers are designed to extinguish different types of fires, such as electrical fires, chemical fires, etc. A bucket of water or sand may not be effective in extinguishing the fire and can even make the situation worse in case of an electrical fire.

Q16) Why is it dangerous to use water to extinguish an oil fire?

Answer: Water cannot be used to extinguish an oil fire because oil is lighter than water, and when water is poured on oil, it sinks to the bottom and the oil floats on top. The water then vaporizes into steam, which pushes the oil out of the container, and the fire spreads even more.

Q17) An experiment was conducted to test the flammability of different materials. A matchstick was brought close to paper, wood, and plastic. Which material(s) caught fire and why?

Answer: Paper and wood caught fire easily because they are made of combustible materials such as cellulose. Plastic did not catch fire as easily because it is made of non-combustible materials.

Q18) Why do firefighters wear special protective clothing when they enter a burning building?

Answer: Firefighters wear special protective clothing made of fire-resistant materials to protect themselves from heat and flames. This clothing helps them to stay safe and perform their duties effectively.

Q19) In a chemistry lab, a student was heating a test tube containing a substance over a Bunsen burner. Suddenly, a flame erupted from the test tube. What could be the reason for this?

Answer: The substance inside the test tube may have been flammable or combustible. When it was heated, it reached its ignition temperature and caught fire.

Q20) Tina was cooking in the kitchen when suddenly the cooking oil caught fire. She panicked and poured water on the fire. Was it the right thing to do? Why or why not? What should she have done to extinguish the fire?

Answer: No, it is not the right thing to do as water can spread the fire by splashing the burning oil. Tina should have turned off the heat source and covered the pan with a lid to smother the fire or used a fire extinguisher suitable for oil fires.

Q21) A group of friends were camping in the woods and decided to build a bonfire to keep warm at night. They gathered some twigs, leaves, and branches and lit the fire. However, the fire quickly spread and started burning nearby bushes and trees. What should the friends have done to prevent the fire from spreading further?

Answer: The friends should have kept a safe distance from the fire and not let it grow too big. They should have cleared the area around the fire of any flammable materials like dry leaves or grass. They could also have dug a pit or made a ring of stones around the fire to contain it.

Q22) Raj was cooking with a gas stove and smelled gas. He tried to ignite the burner but it wouldn't light up. What should he have done to avoid a gas explosion?

Answer: Raj should have turned off the gas supply immediately, opened the windows and doors to ventilate the room, and avoided using any electrical switches or appliances that could create a spark. He should have checked the gas connection for leaks before using the stove again.

Q23) Nisha was working in the chemistry lab and accidentally spilled some flammable chemicals on the floor. She cleaned it up with a cloth and threw it in the trash bin. What should Nisha have done instead?

Answer: Nisha should have immediately informed the lab instructor or the safety officer about the spill and followed the proper procedure for cleaning up the spill. She should have used an absorbent material like sand or kitty litter to soak up the spill and disposed of it in a separate waste container designated for hazardous waste.

Q24) Manu was heating oil to fry potato chips. The cooking oil all of a sudden caught fire; he poured water to extinguish the fire. Do you think this action was suitable? If yes, why? If not, why not? In such a condition what should Manu have done?

Answer: No, pouring water on a burning oil fire is not suitable because water will not extinguish the fire but will only cause the burning oil to splash, spread, and cause a more significant fire. In such a situation, Manu should have turned off the heat source to remove the fuel for the fire, covered the burning pot with a damp cloth or metal lid to smother the flames, or used a fire extinguisher designed for Class K fires (kitchen fires).

Q25) Riya was camping in the forest and wanted to start a campfire to cook her dinner. What are the steps she should take to safely start and maintain a campfire? What precautions should she take to ensure the fire does not spread to the surrounding area?

Answer: Riya should find a safe location for the campfire, away from any flammable materials such as dry leaves or grass. She should clear the area around the campfire site of any debris. She should then dig a shallow pit and line it with rocks. Riya should gather small sticks, twigs, and dry leaves to use as kindling. She should then light the kindling and add larger pieces of wood as the fire grows. To prevent the fire from spreading, Riya should keep a bucket of water and a shovel nearby at all times. She should never leave the fire unattended and should always fully extinguish it before leaving the area.

Q26) During a science experiment, Riya observed that a piece of paper caught fire when it was brought near a burning candle flame. Why did the paper catch fire? What is the minimum temperature required for ignition? How can this be related to forest fires?

Answer: The paper caught fire because it reached its ignition temperature, which is the minimum temperature required to start a fire. The ignition temperature for paper is approximately 451 degrees Fahrenheit (233 degrees Celsius). Forest fires can start when the temperature and humidity are high, and there is an ignition source, such as a lightning strike or a discarded cigarette. When the temperature and humidity are high, vegetation and debris on the forest floor can dry out, making them more susceptible to catching fire.

Q27) Ankit wanted to light a gas stove to cook dinner but found that the stove's igniter was not working. He used a matchstick to light the stove and noticed that the flame produced a blue color. What is the cause of the blue color? What precautions should be taken while handling gas stoves?

Answer: The blue color of the flame is due to complete combustion of the gas. When the gas burns completely, it produces carbon dioxide and water vapor, which do not produce a visible flame. Precautions that should be taken while handling gas stoves include ensuring that there are no gas leaks, not using the stove as a source of heat, keeping flammable materials away from the stove, and turning off the gas supply after use.

Q28) During a science exhibition, a student demonstrated the principle of fire extinguishers by using different types of extinguishers to put out small fires. Which type of fire extinguisher is suitable for which type of fire? Why is it important to use the right type of extinguisher?

Answer: There are four main types of fire extinguishers, each suitable for a different type of fire. Class A extinguishers are suitable for fires involving wood, paper, and other ordinary combustibles. Class B extinguishers are suitable for fires involving flammable liquids such as gasoline, oil, and grease. Class C extinguishers are suitable for fires involving electrical equipment. Class D extinguishers are suitable for fires involving flammable metals. It is important to use the right type of extinguisher because using the wrong type can make the fire worse or cause it to reignite.

Q29) Manish was cleaning his car's engine using a flammable cleaning spray. The engine suddenly caught fire, and he panicked. What should Manish have done to extinguish the fire? What precautions should be taken while using flammable cleaning agents?

Answer: Manish should have used a fire extinguisher to extinguish the fire. If a fire extinguisher was not available, he could have used a blanket or sand to smother the flames

Q30) Priya was cooking in the kitchen when the cooking oil suddenly caught fire. What should she do to extinguish the fire? Why is it not recommended to use water to extinguish oil fires?

Answer: Priya should use a fire extinguisher or cover the pan with a lid to cut off the oxygen supply. It is not recommended to use water as it will cause the oil to splash, spread the fire, and can cause serious burns.

Q31) In the middle of a barbecue party, a gust of wind caused the charcoal grill to tip over and spread hot coals all over the grass. What should the host do to prevent the grass from catching fire?

Answer: The host should use sand, baking soda, or a fire extinguisher to smother the coals and prevent the grass from catching fire.

Q32) While lighting the candles on her birthday cake, Sneha's hair caught fire. What should she do to extinguish the fire?

Answer: Sneha should immediately stop, drop, and roll on the floor to extinguish the fire. She should never run, as it will cause the fire to spread.

Q33) During a science experiment, a student added a piece of burning magnesium ribbon to a beaker containing oxygen gas. What happened? Why did it happen?

Answer: The magnesium ribbon burned brightly with a blinding light and formed a white powder (magnesium oxide) in the beaker. This happened because oxygen gas is a strong oxidizing agent, and it reacted with the magnesium to produce magnesium oxide.

Q34) A small fire broke out in the chemistry lab due to a chemical spill. What should be done to extinguish the fire?

Answer: A fire extinguisher labeled for use on Class B fires (flammable liquids and gases) should be used to extinguish the fire. The lab should be evacuated immediately.

Q35) While grilling chicken, Rohit noticed that the flames were getting too high. What should he do to prevent a flare-up?

Answer: Rohit should move the chicken away from the flames, adjust the grill's temperature, or reduce the amount of fat in the chicken to prevent a flare-up.

Q36) A group of friends were camping in a forest and decided to light a campfire. What precautions should they take to ensure the fire does not spread?

Answer: They should light the fire in a designated fire ring or pit, clear the area of any dry leaves or twigs, keep a bucket of water or sand nearby, and never leave the fire unattended.

Q37) A worker was welding metal pipes in a construction site when a spark ignited a nearby flammable liquid. What should be done to extinguish the fire?

Answer: A fire extinguisher labeled for use on Class B fires (flammable liquids and gases) should be used to extinguish the fire. The worker should also wear protective clothing and follow proper welding safety guidelines.

Q38) While ironing clothes, Shreya accidentally dropped the iron on the carpet, causing it to catch fire. What should she do to extinguish the fire?

Answer: Shreya should use a fire extinguisher labeled for use on Class A fires (ordinary combustibles such as paper, wood, or cloth) to extinguish the fire. If a fire extinguisher is not available, she should smother the fire with a heavy blanket or use water.

Q39) An electrical fire broke out in a warehouse. A fire engine rushed to the spot. A junior fireman rushed to the spot with a water hose to extinguish the fire. His senior stopped him and asked him not to use this method. Why do you think the senior stopped the junior from using a water hose? Which type of fire extinguisher do you think the senior would have used to put off the electrical fire?

Answer: The senior fireman stopped the junior from using a water hose because water is a conductor of electricity and can cause electrocution and severe electrical shock to the person operating the hose. The senior would have used a carbon dioxide (CO2) fire extinguisher or a dry chemical fire extinguisher suitable for Class C fires (electrical fires). These types of extinguishers do not conduct electricity and do not leave a residue that could damage the electrical equipment.

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Revision notes - CBSE Class 8 Science - Coal and petoleum

Introduction to Combustion:

Combustion is a chemical reaction that occurs between a fuel and an oxidizing agent, typically oxygen. During this reaction, the fuel reacts with oxygen to produce heat and light. This reaction is exothermic, which means that it releases energy in the form of heat and light. Combustion is essential for many processes in our daily lives, such as cooking, heating, and transportation.

Ignition Temperature:

The ignition temperature is the minimum temperature required to initiate combustion. This temperature varies for different fuels and is the lowest for highly flammable fuels like petrol or alcohol.

Controlling Fire:

Fire can be controlled by removing one or more of the three elements required for combustion: heat, fuel, and oxygen. This can be achieved by using fire extinguishers, water, or sand.

Types of Combustion:

Combustion can be classified based on the nature and intensity of the reaction. The three types of combustion are:

Rapid Combustion:

Rapid combustion occurs when a substance burns rapidly and produces heat and light. For example, burning of a matchstick or a gas stove.

Spontaneous Combustion:

Spontaneous combustion occurs when a material suddenly bursts into flames without the application of any apparent cause. For example, burning of phosphorus or camphor.

Explosion:

An explosion occurs when a large amount of gas formed in the reaction is liberated. For example, a firecracker or a missile.

Flame:

A flame is the visible, gaseous part of a fire. Flames have different zones, each with its own temperature and characteristics.

Zones of a Flame:

A flame has three different zones: the dark zone, the luminous zone, and the non-luminous zone.

Dark Zone:

The dark zone is located near the surface of a combustible substance, where there is incomplete combustion. It is the least hot zone of the flame.

Luminous Zone:

The luminous zone is the brightest zone of the flame and is due to partial combustion. It is located above the dark zone.

Non-Luminous Zone:

The non-luminous zone is the outermost part of the flame and is the hottest zone due to complete combustion. It is located above the luminous zone.

Fuel:

A fuel is any substance that undergoes combustion to obtain energy that can be used to heat or move another object. A good fuel should be readily available, cheap, burn easily at a moderate rate, have a high calorific value, and not leave behind any undesirable substances after combustion.

Types of Fuels:

Fuels can be classified into three types:

Solid Fuels:

Solid fuels are combustible substances that are solid at room temperature. Examples of solid fuels include coal, wood, charcoal, and camphor.

Liquid Fuels:

Liquid fuels are volatile liquids that produce combustible vapor. Examples of liquid fuels include petrol, kerosene, and diesel.

Gaseous Fuels:

Gaseous fuels are combustible gases or a mixture of combustible gases. Examples of gaseous fuels include CNG, LPG, propane, and biogas.

Harmful Effects of Burning Fuels:

Burning fuels can have harmful effects on the environment and human health. Fuels containing carbon, such as wood, coal, or fossil fuels, release unburnt carbon particles that cause respiratory diseases like asthma or even cancer. Incomplete combustion of carbon fuels produces carbon monoxide, which is a poisonous gas that can be fatal with prolonged exposure. Increased concentration of carbon dioxide in the air, which is a greenhouse gas,

Interesting facts related to combustion and fuels:

• The hottest flame temperature ever recorded was around 4,000 degrees Celsius, generated by the burning of thermite, a mixture of powdered aluminum and iron oxide.

• The flame of a matchstick burns at a temperature of around 600 degrees Celsius, while a candle flame can reach up to 1,400 degrees Celsius.

• The combustion of fossil fuels like coal, oil and natural gas releases about 90 billion tons of carbon dioxide into the atmosphere every year, contributing to global warming and climate change.

• Some organisms like the bombardier beetle and the pyrophorus beetle use chemical reactions in their bodies to generate heat and produce a combustion-like reaction to defend themselves from predators.

• The first fuel used by humans for cooking and heating was wood, which is still widely used in many parts of the world, especially in rural areas.

• In space, where there is no air to support combustion, fires burn differently and are much more dangerous. NASA has conducted several experiments to study the behavior of flames in microgravity conditions.

• Biofuels like ethanol, biodiesel and biogas are gaining popularity as sustainable alternatives to fossil fuels, as they are produced from renewable sources like agricultural crops, waste materials and animal byproducts.

• Some scientists are working on developing new types of fuels like hydrogen, which produces only water vapor when burned, and carbon-neutral fuels that do not contribute to climate change.