Combustion and Flame NCERT Solution for Class 8

NCERT Solutions for Class 8th Science Chapter 7 Combustion and Flame based on the latest syllabus of CBSE and JKBOSE NCERT Books

Exercises

1. List conditions under which combustion can take place.

• Presence of a combustible substance
• Availability of oxygen (air)
• Ignition temperature to initiate the combustion

2. Fill in the blanks:

(a) Burning of wood and coal causes _______ of air.

• Pollution

(b) A liquid fuel, used in homes is _______.

• Kerosene

(c) Fuel must be heated to its _______ before it starts burning.

• Ignition temperature

(d) Fire produced by oil cannot be controlled by _______.

• Water

3. Explain how the use of CNG in automobiles has reduced pollution in our cities.

• CNG (Compressed Natural Gas) burns more efficiently than petrol or diesel and produces fewer harmful emissions like carbon monoxide, nitrogen oxides, and particulate matter. This reduces air pollution significantly.

4. Compare LPG and wood as fuels.

• LPG (Liquefied Petroleum Gas):
  • Burns cleanly with no smoke.
  • High calorific value.
  • Easy to store and transport in cylinders.
• Wood:
  • Burns with smoke and produces pollutants.
  • Lower calorific value.
  • Requires more storage space and is less convenient to handle.

5. Give reasons:

(a) Water is not used to control fires involving electrical equipment.

• Water conducts electricity, which can cause electrocution and spread the fire.

(b) LPG is a better domestic fuel than wood.

• LPG burns more efficiently, produces no smoke, has a higher calorific value, and is easier to store and use.

(c) Paper by itself catches fire easily whereas a piece of paper wrapped around an aluminum pipe does not.

• The aluminum pipe conducts heat away from the paper, preventing it from reaching its ignition temperature.

6. Make a labeled diagram of a candle flame and name different zones of a flame.

• Diagram:
  • Outer zone: Blue, complete combustion.
  • Middle zone: Yellow, partial combustion.
  • Inner zone: Dark, no combustion.

7. Name the unit in which the calorific value of a fuel is expressed.

• Kilojoules per kilogram (kJ/kg)

8. Explain how CO₂ is able to control fires.

• CO₂ is heavier than air and blankets the fire, cutting off the supply of oxygen. It also cools the fuel below its ignition temperature.

9. It is difficult to burn a heap of green leaves but dry leaves catch fire easily. Explain.

• Green leaves contain moisture, which absorbs heat and prevents the leaves from reaching their ignition temperature. Dry leaves lack moisture and easily reach their ignition temperature.

10. Which zone of a flame does a goldsmith use for melting gold and silver and why?

• The outer zone of a flame, because it is the hottest part and ensures complete combustion.

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.

• Calorific value = (Total heat produced) / (Mass of fuel)
  • Calorific value = 180,000 kJ / 4.5 kg = 40,000 kJ/kg

12. Can the process of rusting be called combustion? Discuss.

• No, rusting is a slow oxidation process that occurs at room temperature and does not produce heat or light, whereas combustion is a rapid oxidation process that releases heat and light.

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. Abida kept the beaker in the middle part of the flame. Whose water will get heated in a shorter time?

• Ramesh’s water will get heated in a shorter time because the outermost part of the flame is the hottest.

Extended Learning — Activities and Projects

1. Survey the availability of various fuels in your locality. Find out their cost per kg and prepare a tabular chart showing how many kJ of various fuels you can get for every rupee.
   • Conduct a survey and gather information about different fuels like LPG, kerosene, petrol, diesel, and coal.
   • Create a chart comparing their costs and calorific values.
2. Find out the number, type, and location of fire extinguishers available in your school, nearby shops, and factories. Write a brief report about the preparedness of these establishments to fight fire.
   • Visit the mentioned locations and note the details of fire extinguishers.
   • Prepare a report on their readiness and effectiveness in case of a fire.
3. Survey 100 houses in your area. Find the percentage of households using LPG, kerosene, wood, and cattle dung as fuel.
   • Conduct a survey in the neighborhood.
   • Calculate and present the usage percentages for different fuels.
4. Talk to people who use LPG at home. Find out what precautions they take in using LPG.
   • Interview LPG users and list safety measures they follow, such as checking for leaks, regular maintenance, and proper storage of cylinders.
5. Make a model of a fire extinguisher. Place a short candle and a slightly taller candle in a small dish filled with baking soda. Place the dish at the bottom of a large bowl. Light both the candles. Then pour vinegar into the dish of baking soda. Take care. Do not pour vinegar on the candles. Observe the following reaction. What happens to the candles? Why? In what order?
   • The reaction between vinegar and baking soda produces CO₂, which will extinguish the candles by cutting off the oxygen supply.
   • The shorter candle will go out first, followed by the taller candle.