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Q1. What is the unit of calorific value of a fuel?

Solution

The calorific value of a fuel is expressed in a unit called kilojoule per kg (kJ/kg).

Q2. Why is hydrogen not used as a fuel extensively?

Solution

Hydrogen has the highest calorific value so it can be considered as the best fuel but it is highly inflammable so it is diificult to store, transport and handle so it is used as a fuel only where it is absolutely necessary.

Q3. Out of cow dung, coal and LPG as fuel. Which fuel would you prefer to boil a given quantity of water? Give reason for your answer.

Solution

LPG will be preferred out of all to boil a given quantity of water. This is because it has the highest calorific value in all and does not cause pollution on burning.

Q4. Where should we place a toothpick in a candle flame to burn it the fastest?

Solution

The toothpick should be placed in the outermost zone (blue) of the flame to burn it the fastest.

Q5. How can burning of fuels lead to air pollution?

Solution

Incomplete combustion of fuels release many poisonous gases like carbon monoxide in the environment. These poisonous gases create air pollution.

Q6. Can rusting of iron be considered as an example of combustion? Explain the answer with reasons.

Solution

Yes, rusting of iron is an example of slow combustion.Combustion is a chemical process in which a substance reacts with oxygen to give off heat and other products. During rusting, iron reacts with oxygen and water to form iron oxide (rust). Hence, it is a combustion reaction in which very little heat is liberated.

Q7. (a) What happens when coal is burnt in an insufficient supply of air in a room with closed doors and windows? (b) The use of petrol and diesel as fuels in automobiles is being replaced by CNG. Why?

Solution

(a) When coal is burnt in an insufficient supply of air in a room with closed door and windows, then a very poisonous gas called carbon monoxide is released. Excessive inhaling of carbon monoxide can kill a person. (b) Diesel produces extremely suffocating and corrosive sulphur dioxide gas. Petrol produces oxides of nitrogen. Oxides of nitrogen and sulphur dissolve in rain water and form acids. Such rain is called acid rain. It is harmful for buildings, crops and soil hence their use as fuels in automobiles is being replaced by CNG (Compressed Natural Gas), because CNG produces much less harmful products and smoke.

Q8. Give two examples of inflammable substances.

Solution

Petrol and LPG.

Q9. Why is it dangerous to burn coal in closed room?

Solution

A closed room has limited supply of oxygen. Incomplete combustion of coal gives carbon monoxide gas. It is a very poisonous gas.

Q10. How does carbon dioxide help in putting off fire?

Solution

Carbondioxide covers the surface of burning substance and does not allow the oxygen to reach the substance. In absence of oxygen the fire gets put off.

Q11. Which fuel has highest calorific value?

Solution

Hydrogen gas

Q12. (a) What are the different types of fuels? Give two examples for each. (b) How is the efficiency of a fuel expressed?

Solution

(a) There are three types of fuels: 1. Solid fuels: Wood and Coal. 2. Liquid fuels: Kerosene and Diesel. 3. Gaseous fuels: Natural gas and Biogas. (b) The efficiency of a fuel is expressed in terms of its calorific value. The amount of heat produced by the complete burning of 1 kilogram of a fuel is called its calorific value.

Q13. At airports and petrol pumps hydrocarbon fire extinguishers are used, not the sods-acid fire extinguisher. Why?

Solution

At airports or petrol pumps the probability of breaking fire from oils are more. In case of fires due to oils, soda- acid fire extinguisher is not successful, because it uses water to cool the combustible substance. But water is heavier than oil and settles down below the oil particles and oil keeps on burning on the top. Hence, fire cannot be controlled. In such case hydrocarbon fire extinguisher is used which has turkey red oil, which causes the foaming of carbondioxide under pressure. The foam covers the surface of burning substance and cuts off the supply of air to control fire.

Q14. Which zone of candle flame is dark and why?

Solution

The innermost zone of flame is black in colour. This zone has unburnt wax vapours as sufficient oxygen is not available for combustion.

Q15. (i) Write two harmful effects of acid rain. (ii) Which industry is the main source of gaseous pollutants like sulphur dioxide and nitrogen dioxide?

Solution

(i) (a) It corrodes the buildings. (b) It is harmful to soil and crops. (ii)Petroleum refineries.

Q16. What causes global warming?

Solution

The combustion of fuels release carbon dioxide in the environment. The increased concentration of carbon dioxide in the air causes global warming.

Q17. Draw a labeled diagram of a candle flame.

Solution

Different zones of candle flame

Q18. (a) What substances are used to extinguish fire? (b) What should we use to extinguish fire in case of electric short circuit? (c) What cannot be used to control fire due to oils?

Solution

(a) Water, sand or soil and fire extinguishers are the substances used to extinguish fire. (b) Sand or soil should be used to extinguish fire in case of electric short circuit. (c) Water and soda-acid fire extinguishers cannot be used to control fire due to oils.

Q19. (a) When fuels burn, the type of flame produced depends on the proportion of oxygen which is available for burning of fuel. What are these different types of flame? Give examples. (b) Describe an activity to show that the middle zone of candle flame consists of unburnt carbon particles.

Solution

(a) The different types of flame produced depending on the proportion of oxygen available for burning of fuel are: i. When the oxygen supply is sufficient, then the fuels burn completely producing mainly a blue flame. This blue flame does not produce much light, so it said to be a non-luminous flame. Example: In kitchen gas stoves, LPG burns with a blue flame. ii. When the oxygen supply is insufficient, then the fuels burn incompletely producing mainly a yellow flame. This yellow flame produces light, so it said to be a luminous flame. Example: When kerosene is burned in a lamp, it burns with a yellow, luminous flame. (b) Activity: Light a candle. Hold a clean glass plate with the help of a pair of tongs and introduce it in the middle zone of the candle flame. Hold the glass plate in this position for about 10 seconds. We will find that a blackish ring is formed on the glass plate. This blackish ring is produced due to deposition of unburnt carbon particles present in the luminous zone of the candle flame. This activity shows that the partial combustion of wax vapours in the middle zone produces unburnt carbon particles.

Q20. Write a short note on acid rain.

Solution

Combustion of coal and diesel produce sulphur dioxide gas which is suffocating and corrosive in nature. Petrol engines also release oxides of nitrogen. These oxides of nitrogen and sulphur dissolve in rain water and form acids. Such rain is called acid rain. It is very harmful for crops, buildings and soil.

Q21. Give the full forms of LPG and CNG.

Solution

LPG- Liquid Petroleum Gas CNG – Compressed Natural Gas

Q22. Why LPG burns with a blue flame?

Solution

The flame of the gas appears blue because complete combustion produces enough energy to excite and ionize the gas molecules in the flame.

Q23. Mention any two ways in which carbon dioxide can be supplied for extinguishing fires?

Solution

Carbon dioxide can be supplied: 1. As compressed gas in cylinders. When released from the cylinder, CO₂ expands enormously in volume and cools down the fire. 2. By releasing a lot of dry powder of chemicals like sodium bicarbonate (baking soda) or potassium bicarbonate near the fire, these give off CO₂ gas which extinguishes the fire.

Q24. How can increase in temperature caused due to burning of fuels be harmful to earth?

Solution

Increase in temperature of the atmosphere of earth may result in the melting of glaciers which leads to a rise in sea level, causing flood in coastal areas.

Q25. Name the type of combustion in the following cases: Rusting Burning of LPG Match sticks catching fire Digestion of food

Solution

Rusting - Slow Combustion Burning of LPG - Rapid Combustion Match sticks catching fire - Rapid Combustion Digestion of food - Slow Combustion

Q26. It is advised to handle LPG with care. Why?

Solution

LPG is a combustible fuel with low ignition temperature. It catches fire easily. Therefore it is advised to handle LPG with care.

Q27. What is ignition temperature?

Solution

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

Q28. Mention the role of a fire extinguisher.

Solution

The role of a fire extinguisher is to cut off the supply of air, or to bring down the temperature of the fuel or both.

Q29. How can burning carbon fuels cause respiratory diseases such as asthma?

Solution

Carbon fuels like coal, petroleum and wood release unburnt carbon particles. These fine particles are dangerous pollutants causing respiratory diseases such as asthma.

Q30. What are the conditions necessary for combustion to take place?

Solution

There are three conditions necessary for combustion to take place: 1. Presence of a combustible substance. 2. Presence of a supporter of combustion like oxygen (air). 3. Attainment of ignition temperature. It is necessary to heat a combustible substance to its ignition temperature so that it may undergo combustion.

Q31. You are asked to choose a good fuel. Write four characteristics you will keep in view?

Solution

Characteristics of a good fuel are:

Readily available

Cheap

High calorific value

It does not leave behind any harmful substances

Q32. Why is CNG said to be a clean fuel?

Solution

CNG produces very less harmful products on burning. Thus CNG is a cleaner fuel

Q33. Why carbon fuels like wood, coal, petroleum etc are not considered to be a clean fuel?

Solution

Carbon fuels like wood, coal, petroleum release unburnt carbon particles on burning which are responsible for many diseases. Thus they are not considered as a cleaner fuel.

Q34. How is efficiency of fuel expressed?

Solution

Fuel efficiency is expressed in terms of its calorific value which is expressed in units of kilojoule per kg.

Q35. Name the poisonous gas released from the incomplete combustion of fuels.

Solution

Carbon monoxide.

Q36. Arrange the following in increasing order of calorific value: coal, petrol, LPG, CNG, Hydrogen gas

Solution

The increasing order of calorific value of the following fuels is: Coal < petrol < CNG < LPG < Hydrogen gas

Q37. (a) Why soda-acid fire extinguisher is not successful in controlling fire caused due to burning oils? (b) How does a soda-acid fire extinguisher work?

Solution

(a) Soda-acid fire extinguisher is not successful in controlling fire due to oils because it uses water to cool the combustible substance. But water is heavier than oil and settles down below the oil particles and oil keeps on burning on the top. Hence, fire cannot be controlled. (b) Soda-fire extinguisher is based on cooling the temperature of the burning object. This type of fire extinguisher contains sulphuric acid and sodium bicarbonate. When the extinguisher becomes functional, the sulphuric acid reacts with sodium bicarbonate to produce carbon dioxide. Carbon dioxide when released cuts off the supply of oxygen and water is released to bring down the ignition temperature of the burning object. In this way, fire is controlled.

Q38. Which gas reacts with combustible substance during the combustion process?

Solution

Oxygen gas present in air reacts with combustible substance during the combustion process.

Q39. (i) For centuries wood was used as domestic and industrial fuel. But now it has been replaced by other fuels. Why? (ii) What is responsible for destroying the beauty of the Taj?

Solution

(i) Burning of wood gives a lot of smoke which is very harmful for human beings and it causes respiratory problems also hence it has been replaced by other pollution free fuel. (ii) Sulphur dioxide and nitrogen dioxide are emitted by burning of fuels in Mathura oil refinery and other rubber processing, automobile chemical industries working in and around Agra. When these oxides of sulphur and nitrogen mix with rain water then acid rain is caused. This acid rain corrodes the marble of Taj. Due to this, colour of white marble has started to fade.

Q40. What should be done to control fire due to oils?

Solution

In case of fire due to oils, water is not useful as it is heavier than oil and settles down below oil particles. In such cases, sand or soil should be used. They cut off the supply of air to the fire and it is put off.

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