Goa Board Class 8 Solutions for Science – Combustion and Flame (English Medium)
Page No. 75:
List conditions under which combustion can take place.
Combustion is a process of reaction of a substance with oxygen. There are certain conditions required for combustion to take place. They are:
- Presence of a fuel
- Air (or oxygen)
- Ignition temperature (minimum temperature at which a substance catches fire)
Concept insight: below ignition temperature combustion cannot takes place
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 ________.
(c) ignition temperature
Explain how the use of CNG in automobiles has reduced pollution in our cities.
As the amount of unburnt carbon particles produced is very less in comparison to petrol and amount of harmful gases produced is also very less ,so CNG is comparatively a cleaner fuel than petrol.
Compare LPG and Wood as fuels.
(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 aluminum pipe does not.
(a) Water is a good conductor of electricity. If it is used for controlling a fire involving electrical equipments, then the person dousing the fire might get an electric shock. Also, water can damage electrical equipments.
(b) LPG is a better domestic fuel because being a gaseous fuel it does not produce smoke and un-burnt carbon particles, which cause respiratory problems.
(c) A piece of paper wrapped around aluminium pipe does not catch fire easily. This is because aluminium, being a metal, is a good conductor of heat. Therefore, heat is transferred from the paper to the metal and the paper does not attain its ignition temperature.
Make a labelled diagram of candle flame
Name the unit in which the calorific value of a fuel is expressed.
The calorific value of a fuel is expressed in kilojoules per kilogram (kJ/kg).
Explain how CO2 is able to control fires.
CO2 is a non-combustible gas and it is also non supporter of combustion. It extinguishes fire in two ways:
- Since it is heavier than oxygen, it covers the fire like a blanket and cuts off the contact between oxygen and fuel.
- In cylinders, CO2 is kept in the liquid form. When released, it expands enormously and cools down. This brings down the temperature of the fuel, which helps in controlling the fire.
It is difficult to burn a heap of green leaves but dry leaves catch fire easily. Explain.
Green leaves have a lot of moisture in them. This moisture does not allow them to catch fire easily. However, dry leaves have no moisture in them. Therefore, they catch fire easily.
Which zone of a flame does a goldsmith use for melting gold and silver and why?
Goldsmiths use the outermost part/zone of the flame to melt gold and silver. This is because the outermost zone of the flame undergoes complete combustion and is the hottest part of the flame.
Can the process of rusting be called combustion? Discuss.
Yes, rusting is a kind of slow combustion because in this iron combines with oxygen giving out heat.
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?
The water in the Ramesh’s beaker will heat up in a shorter time. This is because the outermost zone of a flame is the hottest zone, while the yellow zone (in which Abida had kept the beaker) is less hot.