When we burn a
candle, paper, kerosene, coal, wood or cooking gas (LPG), oxygen is needed. The
oxygen needed for the burning of candle, paper, kerosene, coal, wood and
cooking gas comes from the air around us. Thus, for burning a substance
continuously so as to make fire, a continuous supply of fresh air is needed. If
we cut off the supply of fresh air to a burning substance, then the burning
substance will not get oxygen necessary for burning to continue and hence the
substance will stop burning. In rockets, as they go high in the atmosphere, the
availability of oxygen is considerably reduced. Therefore in rockets along with
the fuel, oxygen is also carried for combustion.
The process of
burning of a substance in the presence of oxygen and releasing a large amount
of light and heat is called burning. If the process does not emit flame
then it is called combustion.
Activity 4: Oxygen is necessary
two candles on a table. Ensure that both the candles are of same size and
height. Mark them as candle 1 and candle 2 using a chalkpiece. Light both the
candles. Now, cover candle 2 with glass tumbler as shown in the figure Observe
the happenings at both the candles.
does happen to candle 1? _________
does happen to candle 2? _________
guess why did the covered candle extinguish?
us summarize the happenings.
candle 1 continues to burn, unless it is blown – off by strong moving air or
any other external force. This is because fresh air is continuously available
to the candle for its burning process.
2 glows for a while and then gets put – off. When the burning candle is covered
with a glass tumbler, the candle can use the oxygen available in the air inside
the glass tumbler. Since only a small amount of air is present inside the glass
tumbler – only a small portion of oxygen is available for the candle to continue
glowing. When all the oxygen of the air inside the gas jar is used up, then the
burning candle gets extinguished.
repeat the candle – glowing experiment taking four containers of different
sizes. For example, you can take a 250ml conical flask, a 500ml bottle, a one –
litre jar, a two – litre jar. Cover the burning candle one by one with these
containers and find out how long it takes for the candle to extinguish in each
case. Record your observations in the following table.
you write interpretation based on your observations at the table? _________