1. List conditions under which combustion can take place.
Ans:
Combustion is the process of burning a substance in the presence of oxygen.
The conditions required for combustion to take place are:
Presence of air or oxygen: Oxygen is essential to support combustion.
Presence of fuel: A combustible substance (fuel) is necessary to burn.
Attainment of ignition temperature: The substance must be heated to its ignition temperature, which is the minimum temperature required for it to catch fire.
(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) The fire produced by oil cannot be controlled by___________ .
Ans:
(a) pollution
(b) kerosene.
(c) ignition temperature
(d) water.
3. Explain how the use of CNG in automobiles has reduced pollution in our cities.
Soln:
CNG (Compressed Natural Gas) has helped reduce pollution in cities in several ways:
CNG is a cleaner fuel compared to diesel and petrol.
It produces fewer harmful emissions such as carbon monoxide and particulate matter.
CNG combustion results in lower levels of unburnt carbon particles, which are responsible for respiratory diseases.
As an alternative to diesel, petrol, and LPG, CNG reduces the overall emission of pollutants, helping improve air quality in urban areas.
4. Compare LPG and wood as fuels.
Soln:
Wood:
Wood is a traditional fuel used for both domestic and industrial purposes.
It produces a lot of smoke, which contributes to air pollution and respiratory diseases.
Overuse of wood leads to deforestation.
The calorific value of wood is between 17,000 and 22,000 kJ/kg.
Wood is used in furnaces, stoves, or fireplaces indoors, and for campfires outdoors.
LPG (Liquefied Petroleum Gas):
LPG is a cleaner alternative to wood.
It does not produce smoke or pollutants.
LPG is more fuel-efficient than wood.
The calorific value of LPG is much higher, at 55,000 kJ/kg.
Due to its cleanliness and higher efficiency, LPG is preferred for domestic use over wood.
5. Give reasons.
(a) Water is not used to control fires involving electrical equipment.
Soln:
Water is a good conductor of electricity, and if it is used on electrical fires, it can spread the electricity further, increasing the risk of electric shock. Therefore, water should not be used to extinguish electrical fires.
(b) LPG is a better domestic fuel than wood.
Soln:
LPG is a cleaner fuel compared to wood. It does not release smoke or other harmful pollutants, whereas wood burns with a lot of smoke, which contributes to air pollution and respiratory diseases. Additionally, LPG has a higher calorific value, making it more fuel-efficient.
(c) Paper by itself catches fire easily, whereas a piece of paper wrapped around an aluminium pipe does not.
Soln:
Paper catches fire easily because it has a low ignition temperature. However, when wrapped around an aluminium pipe, the heat from the fire is transferred to the aluminium, which is a good conductor of heat. This transfer of heat increases the ignition temperature of the paper, preventing it from catching fire easily.
6. Make a labelled diagram of a candle flame.
7. Name the unit in which the calorific value of a fuel is expressed.
Soln:
The calorific value of a fuel is expressed in kJ/kg, where kJ stands for kilo joules and kg stands for kilogram.
8. Explain how CO2 is able to control fires.
Soln:
CO2 controls fires in two ways:
(i) CO2 is heavier than oxygen, so it forms a blanket over the fire, cutting off the supply of oxygen to the fire. Without oxygen, the combustion process cannot continue, extinguishing the fire.
(ii) When CO2 is released from a cylinder, it is in liquid form. As it expands, it rapidly cools the surrounding area, lowering the temperature of the fuel below its ignition point, which also helps in controlling the fire.
9.It is difficult to burn a heap of green leaves, but dry leaves catch fire easily. Explain.
Soln:
Green leaves contain a significant amount of moisture, which increases their ignition temperature. As a result, they are harder to ignite. On the other hand, dry leaves have very little moisture, which lowers their ignition temperature, making them easier to catch fire.
10.Which zone of a flame does a goldsmith use for melting gold and silver and why?
Soln:
A goldsmith uses the non-luminous flame, which is the outermost part of the flame. This part is preferred because it undergoes complete combustion and is the hottest part of the flame, providing the high temperatures required for melting metals like gold and silver.
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.
12. Can the process of rusting be called combustion? Discuss.
Soln:
No, the process of rusting cannot be called combustion. While both are exothermic processes (releasing heat), rusting is a slow oxidation reaction of iron with oxygen in the air, resulting in the formation of iron oxide (rust). Combustion, on the other hand, is a rapid reaction between a substance and oxygen that releases energy in the form of heat and often light. Rusting occurs gradually and does not produce significant light, unlike combustion.
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 13. water will get heated in a shorter time?
Soln:
Ramesh’s water will get heated in a shorter time. This is because the outermost part of the flame is the hottest (non-luminous flame), which heats the beaker more efficiently. In contrast, Abida placed the beaker near the yellow (luminous) part of the flame, which is comparatively cooler and not as effective in heating the water.