Activity Based Science Learning >Hot air is lighter


You must have, heard that the hot air being lighter moves up. It is easy to see the smoke and steam rising up, but how to see the rising hot air. In this experiment you will not exactly see the hot air rising up but what you will see will be very close to make you believe that hot air rises up.
Required Material

A cardboard box, Candle, match box, thin transparent sheet, cello tape, dhoop stick (agar-batti). 

Construction Procedure:

Take a cardboard box 1 foot long, 6 inches wide and of 6 inches height. Place it on a table. Make a door on one of its vertical surfaces containing the width so that from time to time it may be opened and then be properly closed as well. But after the door is closed air should not enter the box. Make the box air tight from all other sides. If you so desire, you may cut a window on the opposite vertical surface and paste well a transparent sheet on it. Now on the top surface of the box, cut two circular windows a small distance apart. Fold the plastic sheet cylindrically and using cello tape make two chimneys. The diameter of these chimneys must be the same as that of the circular windows. Now, fix these chimneys on the windows and paste them well. We will call it smoke chamber. 


1.         Light the candle and on opening the door made on the vertical wall of the box, place is just below one of the chimneys. The flame of the candle must not be very close to the chimney; otherwise the plastic may melt spoiling the chimney.

2.         Close the door properly, Wait nearly for a minute.

3.         Burn an agar-batti. Where is its smoke going? Surely it will go up as smoke always goes in the upward direction.

4.         Now take it inside the chimney which has candle below it. The smoke from the agar-batti now rises up even more rapidly as compared to the previous case.

5.         Now, take the agar-batti inside the other chimney. This time the smoke goes down the chimney. Think, why the smoke in one chimney goes up while in the other chimney it goes down? 


Due to the burning candle, the air above it gets heated. The density of hot air is less than that of cold air and so it gets lighter and rises up. Thus, hot air continuously comes out of the chimneys above the candle. As a result the pressure of the air inside starts falling below the atmospheric pressure, and so the fresh air start rushing into the box through the other chimney. This fresh air keeps the candle burning continuously. In this manner, a convection current is set up which keeps on circulating continuous in both the chimneys i.e. hot air keeps moving out through the chimney and cold air keeps entering the box through the other chimneys In order to check this we use the smoke from the agar-batti. The hot air keeps moving out of the chimney above the candle. Therefore, the smoke of the agar-batti placed inside this chimney rises up along with the hot air. On the other hand, as cold air goes down through the other chimney the smoke of the agar-batti also moves down.  

Do and think a little more:

What will happen if the door of the box is left open? Why the flame of the candle always points up, and why is it broad at the centre and thin or tapered at the two ends? Think of some other experiment which can demonstrate that the hot air is lighter.



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