Here are important questions from NCERT Class 8 Science Chapter 6 Combustion and Flame.

1. Name a few fuels that we use in our everyday life.

Cow dung, wood, coal, charcoal, petrol, diesel, CNG etc.

2. What is the main difference between the burning of a candle and coal?

When a candle burns, we can see the flame. Coal burns without a flame.

3. What happens when magnesium is burned?

When magnesium is burned, it produces magnesium oxide, heat and light.

4. What happens when coal burns?

When coal burns, it produces carbon dioxide, heat and light.

5. What is combustion?

Combustion is the chemical process in which a substance reacts with oxygen to give off heat.

6. Any substance that undergoes combustion is ……………………………..

combustible

7. The substance that undergoes combustion can also be called ……………………….

fuel

8. Combustion cannot take place without ………………………………

air

9. In the sun, heat and light are produced by …………………………….

nuclear reactions

10. What is ignition temperature?

The lowest temperature at which a substance catches fire is its ignition temperature.

11. Ancient Egyptians used small pieces of pinewood dipped in ………………………… as matches.

sulphur

12. When a person’s clothes catch fire, why are they covered with a blanket?

For combustion air is necessary. By covering a person whose clothes have caught fire with a blanket, we can make air unavailable and thus extinguish the fire.

13. The head of a matchstick contains ………………………… and ……………………….

antimony trisulphide and potassium chloride

14. How does a match stick produce fire?

The head of a matchstick contains antimony trisulphide and potassium chlorate. The rubbing surface has powdered glass and a little red phosphorous. When the matchstick is struck against the rubbing surface, some red phosphorous gets converted into white phosphorous. This reacts with potassium chlorate to produce enough heat to ignite antimony trisulphide and start the combustion.

15. Why doesn’t a paper cup containing water burn when it is heated using a candle?

The heat supplied to the paper cup is transferred to the water it contains by conduction. Due to the presence of water, the ignition temperature of the paper does not reach. Hence, the paper does not burn.

16. What are inflammable substances?

Substances which have very low ignition temperature and which can easily catch fire with a flame are called inflammable substances.

17. Give some examples of highly inflammable substances.

Petrol, alcohol, LPG

18. What is the full form of LPG?

Liquified Petroleum Gas

19. How does pouring water cause a fire to extinguish?

Water cools the burning material so that its temperature is brought below its ignition temperature. Water vapours also surround the combustible material and thus cut off the supply of air. This extinguishes the fire.

20. What are the three essential requirements for producing fire?

The three essential requirements are: fuel, air and heat. Fuel is the substance that gives off heat when it reacts with air. A source of heat is required to raise the temperature of the fuel beyond its ignition temperature.

21. What is the most common fire extinguisher?

Water

22. Why isn’t water used to extinguish electrical equipment on fire?

Water may conduct electricity and harm those trying to extinguish the fire. Hence, water is not used to extinguish fire caused by a short circuit.

23. Why can’t water be used to extinguish fire involving oil and petrol?

Water is heavier than oil. Hence, if water is poured on burning oil, it will sink below the oil and oil will keep burning on the top.

24. What is the best extinguisher for fire involving electrical equipment and inflammable materials?

Carbon dioxide

25. What makes carbon dioxide a good fire extinguisher?

Fuel can burn only in the presence of oxygen. Since carbon dioxide is heavier than oxygen, it covers the fire like the blanket. Since the contact between the fuel and oxygen is cut off, the fire is controlled. Another advantage of using carbon dioxide as a fire extinguisher is that it does not harm the electrical equipment.

26. How is carbon dioxide stored in a fire extinguisher?

Carbon dioxide is stored at high pressure as a liquid in cylinders. When it is released from the cylinder, it expands enormously and cools down. Thus, it not only forms a blanket around the fire, but also brings down the temperature of the fuel.

27. When released near fire, sodium bicarbonate or potassium bicarbonate gives off …………………………..

carbon dioxide

28. Name a substance that burns in air at room temperature.

Phosphorus

29. What is spontaneous combustion?

The type of combustion in which a material suddenly bursts into flames without the application of any apparent cause is called spontaneous combustion.

30. Why do some substances burn with a flame?

Substances which vapourise during burning give flames. For example, when kerosene oil is burned, it rises through the wick and is vapourized and form flames.

31. Why doesn’t charcoal give flames?

Charcoal does not vapourize during burning and hence it does not give flames.

32. What are fuels?

Fuels are the sources of heat energy for domestic and industrial purposes.

33. What are the characteristics of a good fuel?

A good fuel should be cheap and readily available. It should burn easily in the air at a moderate rate and produce a lot of heat. At the same time, it should not leave behind any undesirable substances.

34. What is the calorific value of fuel?

The amount of heat energy produced on complete combustion of 1 kg of fuel is called its calorific value.

35. What is the unit of the calorific value of fuel?

Kilojoule per kg (kJ/kg)

36. How does the burning of fuel affect the environment?

When carbon fuels like wood, coal or petrol are burned, they release unburned carbon particles into the atmosphere. These fine particles cause various respiratory ailments.

Incomplete combustion of these fuels also leads to the generation of poisonous carbon monoxide gas. Combustion of most fuels also leads to the production of carbon dioxide, a gas responsible for increasing the rate of global warming.

Burning of coal and diesel releases sulphur dioxide, which is a suffocating and corrosive gas.

37. Why is it dangerous to burn coal in a closed room?

Incomplete combustion of coal leads to the production of carbon monoxide which is a very poisonous gas. Hence, it is dangerous to burn coal in a closed room.

38. What is acid rain?

Burning of coal and diesel leads to the production of sulphur dioxide gas. Petrol engines produce gaseous oxides of nitrogen. Oxides of nitrogen and sulphur dissolve in rain water and form acids. Such rain is called acid rain. Acid rain causes considerable damage to crops, buildings and soil.

39. Why are petrol and diesel being replaced by CNG?

CNG is a cleaner fuel. While its combustion does emit some harmful products, it produces only a small amount of them. Hence, CNG is replacing petrol and diesel.

True or False

1. A combustible substance cannot catch fire as long as its temperature is lower than its ignition temperature – true

2. Kerosene catches fire at room temperature – false

3. Calorific value of methane is higher than that of diesel – true

4. Cow dung cake has a higher calorific value than wood – false

5. CNG has a higher calorific value than LPG – false