GSEB Class 8 Science Solutions Chapter 6 Combustion and Flame

Question 1. List conditions under which combustion can take place.
Answer: The necessary conditions for combustion to occur include:

• Presence of a combustible substance.
• Attainment of the ignition temperature.
• A proper supply of air to offer oxygen.

Exam Tip: Remember these three key requirements for combustion, as they are fundamental to understanding fire safety and processes.

Question 2. Fill in the blanks.
(a) Burning of wood and coal causes _______ of air.
(b) A liquid fuel, used in homes is _______.
(c) Fuel must be heated to its _______ before it starts burning.
(d) Fire produced by oil cannot be controlled by _______.
Answer:
(a) Burning of wood and coal causes **pollution** of air.
(b) A liquid fuel, used in homes is **kerosene**.
(c) Fuel must be heated to its **ignition temperature** before it starts burning.
(d) Fire produced by oil cannot be controlled by **water**.
In simple words: When wood and coal burn, they make the air dirty. Kerosene is a common liquid fuel for homes. Things need to get hot enough to their "ignition temperature" to start burning. Water cannot put out oil fires.

Exam Tip: For fill-in-the-blanks, recall key terms and concepts related to combustion, fuels, and fire safety. Ensure your answers are precise and accurate.

Question 3. Explain how the use of CNG in automobiles has reduced pollution in our cities.
Answer: Using CNG instead of petrol and diesel helps lower pollution in these ways:

• It generates less carbon monoxide gas.
• It generates less carbon dioxide gas.
• It generates a smaller amount of sulphur dioxide and nitrogen dioxide, which can cause acid rain.
• No residue is left behind after it burns.

Exam Tip: When discussing environmental benefits of fuels, always mention specific pollutants reduced and their associated harmful effects (e.g., acid rain) to score well.

Question 4. Compare LPG and wood as fuels.
Answer: Here is a comparison of LPG and wood as fuels:
**LPG:**

• It is a gaseous fuel.
• It does not create smoke.
• Its calorific value is higher (55000 kJ/kg).
• It is easily stored in cylinders.
• It does not cause any pollution.

• It is a solid fuel.
• It produces smoke.
• Its calorific value is lower (17000 kJ/kg).
• It needs more area to keep.
• It leads to pollution.

Exam Tip: When comparing fuels, focus on key properties like physical state, environmental impact (smoke/pollution), energy content (calorific value), and storage convenience.

Question 5. Give reasons:
(a) Water is not used to control fires involving electrical equipment.
(b) LPG is a better domestic fuel than wood.
(c) Paper by itself catches fire easily whereas a piece of paper wrapped around an aluminium pipe does not.
Answer:
(a) Water is a good conductor of electricity. If used on electrical fires, it can conduct electric current and cause a severe shock. Therefore, it is unsafe to use water for fires involving electrical equipment.
(b) LPG has a much higher calorific value, meaning it produces more heat per unit mass. Additionally, it creates no pollution, unlike wood which produces smoke and other harmful substances. These factors make LPG a superior domestic fuel.
(c) Paper has a low ignition temperature, allowing it to easily catch fire on its own. However, when wrapped around an aluminium pipe, the aluminium absorbs the heat quickly, preventing the paper from reaching its ignition temperature. Thus, the paper does not ignite in this situation.
In simple words: (a) Water can give you an electric shock if used on electrical fires. (b) LPG is better than wood because it gives more heat and doesn't pollute. (c) Paper burns fast, but if it's around an aluminium pipe, the pipe takes away the heat, so the paper won't burn.

Exam Tip: For 'Give reasons' questions, always explain the scientific principle behind the observation, such as conductivity, calorific value, or ignition temperature.

Question 6. Make a labelled diagram of a candle flame.
Answer:
In simple words: A candle flame has different colored parts. The tip (blue) is the hottest. The middle part (yellow) is moderately hot and bright. The inner part (black) near the wick is the least hot.

Exam Tip: Practice drawing the candle flame and clearly labeling each zone along with its corresponding temperature and color. This is a common question in exams.

Question 7. Name the unit in which the calorific value of a fuel is expressed.
Answer: The unit in which the calorific value of a fuel is shown is kilojoules per kilogram (kJ/kg).
In simple words: We measure how much heat a fuel gives off using a unit called kilojoules per kilogram, or kJ/kg.

Exam Tip: Remember the standard unit (kJ/kg) for calorific value as it is frequently asked in multiple-choice or short-answer questions.

Question 8. Explain: how CO2 is able to control fires.
Answer: Carbon dioxide (CO2) helps to control fires in two main ways:

• CO2 creates a blanket around the fire, which stops the supply of air, specifically oxygen, that the fire needs to burn.
• CO2 also helps to reduce the temperature of the fuel, bringing it below its ignition point.

Exam Tip: Explain both mechanisms of CO2 (cutting off oxygen and cooling) to provide a complete answer for how it controls fires.

Question 9. It is difficult to burn a heap of green leaves but dry leaves catch fire easily. Explain.
Answer: Green leaves contain some water, which makes their ignition temperature higher. Due to this, they do not catch fire easily. In contrast, dry leaves have no water, so their ignition temperature is much lower, allowing them to ignite quickly.
In simple words: Green leaves have water, so they need more heat to burn. Dry leaves have no water, making them easier to catch fire.

Exam Tip: Always link the presence of moisture to an increased ignition temperature when explaining why wet materials are harder to burn.

Question 10. Which zone of a flame does a goldsmith use for melting gold and silver and why?
Answer: A goldsmith uses the outer zone (which is the non-luminous zone) of a candle flame to melt gold and silver. This is because the outer zone is the hottest part of the flame and has the highest temperature.
In simple words: Goldsmiths use the blue, outer part of the flame because it's the hottest and best for melting metals like gold and silver.

Exam Tip: Remember that the outermost zone of a flame is the hottest due to complete combustion and good air supply; this is a key concept related to flame structure and applications.

Question 11. In an experiment 4.5 kg of a fuel was completely burnt. The heat produced was measured to be 180,000 kJ. Calculate the calorific value of the fuel.
Answer:
Total mass of fuel \( = 4.5 \) kg
Total heat produced \( = 180,000 \) kJ
Heat produced by burning 1 kg of fuel \( = \frac { 180,000 \, \text{kJ} }{ 4.5 \, \text{kg} } \)
\( = 40,000 \) kJ/kg.
So, the calorific value of the fuel is \( 40,000 \) kJ/kg.
In simple words: To find the calorific value, you divide the total heat made by how much fuel was burned. In this problem, 180,000 kJ divided by 4.5 kg gives 40,000 kJ/kg.

Exam Tip: Always state the formula for calorific value (Total Heat Produced / Mass of Fuel) and show your steps clearly. Ensure the final answer includes the correct units (kJ/kg).

Question 12. Can the process of rusting be called combustion? Discuss.
Answer: The process of rusting cannot be called combustion because no heat and light are produced during this process. Because of this reason, iron is not considered a combustible substance.
In simple words: Rusting is not combustion because it doesn't create heat or light, which are key signs of burning. Iron also isn't thought of as a burning material.

Exam Tip: Clearly define combustion by its characteristics (heat and light production) and use these criteria to explain why rusting, despite being a slow oxidation process, is not combustion.

Question 13. Abida and Ramesh were doing an experiment in which water was to be heated in a beaker. Abida kept the beaker near the wick in the yellow part of the candle flame. Ramesh kept the beaker in the outermost part of the flame. Whose water will get heated in a shorter time?
Answer: The water heated by Ramesh will get warm in a shorter time. This is because he placed his beaker near the hottest zone of the flame, which is the outermost part.
In simple words: Ramesh's water will heat up faster because he put it in the flame's hottest spot.

Exam Tip: Relate the heating speed directly to the temperature of the flame zone. Always identify the hottest part of the flame as the most efficient for heating.