Class 8 Science Chapter 4 HOTS Questions - Combustion and Flame

Multiple Choice Questions (MCQs)

Q1. What compound is commonly found in the head of a matchstick?

a) Antimony disulfide

b) Potassium chlorate and antimony trisulfide

c) White phosphorus and potassium chlorate

d) Red phosphorus and potassium chlorate

Ans: b) Potassium chlorate and antimony trisulfide

The head of a matchstick contains:

• Potassium chlorate
• Antimony trisulfide

These compounds are crucial for the ignition process, allowing the match to light when struck against a rough surface.

Q2. Which of the following does not burn with a flame?

a) Candle

b) Coal

c) Kerosene oil

d) Molten wax

Ans: d) Molten wax

Molten wax does not burn with a flame. Instead, it melts and can only ignite when it turns into vapour.

Q3. A chemical process in which a substance reacts with oxygen to release heat is called _______.

a) Photosynthesis

b) Respiration

c) Combustion

d) Transpiration

Ans: c) Combustion

Combustion is a chemical process where a substance reacts with oxygen, producing heat and light. Key points include:

• It involves a combustible material, which can be solid, liquid, or gas.
• During combustion, heat is released, and sometimes light appears as a flame or glow.
• Common examples of combustible substances include magnesium and charcoal.
• Air is essential for combustion; without it, burning cannot occur.

Q4. In the Sun, heat and light are produced by ___________________.

a) Combustion

b) Explosion

c) Nuclear reaction

d) Conduction

Ans: c) Nuclear reaction

The Sun produces heat and light through nuclear reactions, specifically through a process called nuclear fusion.

Q5. Magnesium burns in air to produce ________.

a) Mg(OH)₂

b) MgO

c) MgO₂

d) MgCO₃

Ans: b) MgO

When magnesium burns in air, it reacts with oxygen to produce:

• Magnesium oxide (MgO)

This reaction releases both heat and light.

Q6. Inflammable substances have:

a) High ignition temperature

b) Low ignition temperature

c) No ignition temperature

d) High boiling point

Ans: b) Low ignition temperature

Inflammable substances ignite easily due to their low ignition temperature. This means:

• They can catch fire with minimal heat.
• Examples include petrol, alcohol, and LPG.

Q7. Choose the best fire extinguisher.

a) Oxygen

b) Water

c) Carbon dioxide

d) Blanket

Ans: c) Carbon dioxide

Carbon dioxide is an effective fire extinguisher because it:

• Displaces oxygen, which is necessary for fire.
• Suffocates the flames, preventing them from spreading.
• Is safe to use on electrical equipment without causing damage.

Overall, carbon dioxide is a reliable choice for extinguishing various types of fires.

Q8. The hottest part of a candle flame is:

a) Luminous zone

b) Dark zone

c) Hot zone

d) Non-luminous zone

Ans: c) Hot zone

• The hottest part of a candle flame is the hot zone.
• This zone is where combustion occurs most efficiently.
• It is located just above the wick and is crucial for producing heat and light.

Q9. The color of the middle zone of the candle flame is:

a) Orange

b) Black

c) Yellow

d) Blue

Ans: c) Yellow

The middle zone of a candle flame is typically yellow because of incomplete combustion. This zone is where some wax vapours burn, producing a bright yellow light.

• The flame has three zones:
  • Outer zone: Blue, indicating complete combustion.
  • Middle zone: Yellow, indicating partial combustion.
  • Innermost zone: Black, where unburnt wax vapours are present.
• The yellow colour is due to the presence of unburnt carbon particles.

Q10. Complete combustion of wax vapors occurs in ____________.

a) Middle zone

b) Luminous zone

c) Non-luminous zone

d) Dark zone

Ans: c) Non-luminous zone

Non-luminous zone Complete combustion of wax vapours occurs in the non-luminous zone, where there is enough oxygen for the reaction. Key points include:

• This zone is the hottest part of the flame.
• It ensures efficient burning of wax vapours.
• Inadequate oxygen in other zones leads to incomplete combustion.

Q11. The calorific value of wood compared to LPG is:

a) Less

b) More

c) Same

d) No calorific value

Ans: a) Less

• Wood has a lower calorific value compared to LPG.
• This means wood is less efficient as a fuel.
• In practical terms, LPG produces more heat per kilogram than wood.

Q12. The unit of calorific value is:

a) kJ/kg

b) kg/kJ

c) kB/kJ

d) g/kJ

Ans: a) kJ/kg

• The calorific value of fuels is usually measured in kilojoules per kilogram (kJ/kg).
• This unit indicates the amount of heat energy produced when 1 kg of fuel is completely burned.

Q13. Global warming is a result of:

a) Increased O₂

b) Increased CO₂

c) Increased water vapor

d) Increased CO

Ans: b) Increased CO₂

Global warming is mainly caused by the increased levels of carbon dioxide (CO₂) in the atmosphere. Key points include:

• CO₂ is released from burning fossil fuels.
• Higher CO₂ levels trap heat in the atmosphere.
• This leads to a rise in global temperatures.

The consequences of global warming include:

• Melting of polar ice caps.
• Rising sea levels, causing coastal flooding.
• Long-term submersion of low-lying areas.

Q14. A very poisonous gas is:

a) Carbon dioxide

b) Nitrogen

c) Carbon monoxide

d) Oxygen

Ans: c) Carbon monoxide

Carbon monoxide is a highly toxic gas that poses serious health risks when inhaled. Key points include:

• It is produced during the incomplete combustion of fuels.
• Exposure can lead to severe health issues, including death.
• It is particularly dangerous in enclosed spaces, such as rooms with burning coal.

Always ensure proper ventilation when using fuels to prevent carbon monoxide accumulation.

Q15. A cleaner fuel for automobiles is:

a) Petrol

b) Diesel

c) CNG

d) LPG

Ans: c) CNG

Compressed Natural Gas (CNG) is regarded as a cleaner fuel for automobiles because it:

• Produces lower emissions compared to petrol and diesel.
• Helps reduce air pollution in urban areas.
• Is less harmful to the environment.

Overall, CNG is a more sustainable choice for vehicle fuel.

Q16. This cannot be used for fires involving electrical equipment:

a) Carbon dioxide

b) Oxygen

c) Water

d) Fire extinguisher

Ans: c) Water

• Water should not be used on electrical fires because:
• It can conduct electricity, increasing the risk of shock.
• It may cause further damage to the electrical equipment.
• Alternative extinguishers, like carbon dioxide, are safer for such fires.

Q17. A liquid fuel used in homes is:

a) Water

b) LPG

c) CNG

d) Cooking oil

Ans: b) LPG

LPG (Liquefied Petroleum Gas) is a widely used liquid fuel in homes, particularly for cooking. Key points include:

• It is a cleaner alternative to traditional fuels like wood.
• LPG burns efficiently, producing a high amount of heat.
• It is stored under pressure in cylinders, making it easy to transport.
• Using LPG helps reduce indoor air pollution compared to solid fuels.

Q18. Carbon monoxide is produced by:

a) Incomplete combustion of fuels

b) Explosion of fuels

c) Complete combustion of fuels

d) Sleeping in a closed room

Ans: a) Incomplete combustion of fuels

Carbon monoxide is produced during the incomplete combustion of fuels. This occurs when there is not enough oxygen available for the fuel to burn completely. Key points include:

• It is a poisonous gas that can be very dangerous.
• Burning fuels like coal in a closed space can lead to deadly levels of carbon monoxide.
• Proper ventilation is essential to prevent carbon monoxide buildup.

Q19. Sources of heat energy for domestic and industrial purposes include:

a) Matchstick

b) Fuel

c) Petrol

d) Heater

Ans: b) Fuel

Fuel is a key source of heat energy used in both domestic and industrial settings. Here are some important points:

• It is widely available and often affordable.
• It burns easily in air at a moderate rate.
• It generates a significant amount of heat.
• Good fuels leave minimal undesirable residues.

While no fuel is considered ideal, the best options meet most of these criteria.

Q20. For melting gold and silver, a goldsmith uses:

a) Innermost zone of a flame

b) The least hot region

c) Non-luminous zone

d) Dark zone

Ans: a) Innermost zone of a flame

The innermost zone of a flame is the hottest part, making it ideal for melting gold and silver. Key points include:

• It reaches the highest temperature needed for melting.
• Goldsmiths use this zone to ensure efficient melting.
• This zone is characterised by a lack of unburnt materials.

Q21. The zone of partial combustion is __________.

a) Luminous zone

b) Innermost zone

c) Dark zone

d) Non-luminous zone

Ans: a) Luminous zone

• The luminous zone is the area where partial combustion occurs.
• This zone produces light due to the presence of unburnt carbon particles.

Q22. The use of diesel and petrol as fuel in automobiles is being replaced by:

a) LPG

b) CNG

c) Coal

d) Bitumen

Ans: b) CNG

CNG (Compressed Natural Gas) is increasingly being used as a cleaner alternative to diesel and petrol in vehicles. Here are some key points about CNG:

• CNG produces fewer harmful emissions compared to traditional fuels.
• It is a more environmentally friendly option, helping to reduce air pollution.
• The adoption of CNG supports efforts to combat global warming.

Overall, CNG is a significant step towards cleaner fuel options in the automotive industry.

Q23. When electrical equipment is on fire, the best extinguisher to use is:

a) Water

b) Sunlight

c) CO₂

d) O₂

Ans: c) CO₂

• CO₂ is the best extinguisher for electrical fires.
• It does not conduct electricity, making it safe to use.
• CO₂ suffocates the fire by cutting off its oxygen supply.
• It is also less likely to damage electrical equipment.

Q24. What is the telephone number of the fire service in Saudi Arabia?

a) 997

b) 998

c) 999

d) 993

Ans: b) 998

The emergency number for the fire service in Saudi Arabia is 998.

Q25. When compared to wood, the ignition temperature of kerosene oil is:

a) Higher

b) Lower

c) The same

d) No ignition temperature

Ans: a) Higher

• Kerosene oil has a higher ignition temperature than wood.
• This means it is less flammable under similar conditions.