We use different kinds of fuel for various purposes at home, in industry and for running automobiles. Can you name a few fuels used in our homes? Name a few fuels used in trade and industry. What fuels are used for running automobiles? Your list will contain fuels like cowdung, wood, coal, charcoal, petrol, diesel, compressed natural gas (CNG), etc.
Recall the activity of burning of
magnesium ribbon performed in Class
VII. We learnt that magnesium burns to
form magnesium oxide and produces
heat and light (Fig. 6.1).
We can perform a similar activity with a piece of charcoal. Hold the piece with a pair of tongs and bring it near the flame of a candle or a Bunsen burner. What do you observe?
You must have seen or heard of fire breaking out in homes, shops and factories. If you have seen such an accident, write a short description in your note book. Also, share the experience with your classmates.
Bring a burning matchstick or a gas
lighter near a gas stove in the kitchen.
Turn on the knob of the gas stove. What
do you observe?
We find that the gas burns rapidly and produces heat and light. Such combustion is known as rapid combustion.
Observe an LPG flame. Can you tell the
colour of the flame. What is the colour
of a candle flame?
Recall your experience of burning a magnesium ribbon in Class VII. If you do not have experience of burning the remaining items in Table 6.2 you can do that now.
The substances which vapourise during burning, give flames. For example, kerosene oil and molten wax rise through the wick and are vapourised during burning and form flames. Charcoal, on the other hand, does not vapourise and so does not produce a flame. In Activity 6.6, could the vapours of wax coming out of the glass tube be the cause of the flame produced?
Recall that the sources of heat energy for domestic and industrial purposes are mainly wood, charcoal, petrol, kerosene, etc. These substances are called fuels. A good fuel is one which is readily available. It is cheap. It burns easily in air at a moderate rate. It produces a large amount of heat. It does not leave behind any undersirable substances.
Suppose you were asked to boil a given quantity of water using cow dung, coal and LPG as fuel. Which fuel would you prefer? Give your reason. You may take the help of your parents. Do these three fuels produce the same amount of heat? The amount of heat energy produced on complete combustion of 1 kg of a fuel is called its calorific value. The calorific value of a fuel is expressed in a unit called kilojoule per kg (kJ/kg). Calorific values of some fuels are given in Table 6.4.