Form 1:
Air and Combustion

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To find the composition of air supporting combustion using a candle stick

Procedure

Measure the length of an empty gas jar M1.

Place a candle stick on a Petri dish.

Float it on water in basin/trough.

Cover it with the gas jar.

Mark the level of the water in the gas jar M2.

Remove the gas jar.

Light the candle sick.

Carefully cover it with the gas jar.

Observe for two minutes. Mark the new level of the water M3.

Set up of apparatus

Sample observations

Candle continues to burn then extinguished/goes off

Level of water in the gas jar rises after igniting the candle

Length of empty gas jar = M1= 14cm

Length of gas jar without water before igniting candle = M2= 10 cm

Length of gas jar with water before igniting candle = M1 - M2= 14- 10 = 4 cm

Length of gas jar with water after igniting candle = M3 = 8 cm

Length of gas jar without water after igniting candle = M1 - M3 = 10 -8 = 2 cm

Explanation

Candle burns in air. In a closed system (vessel), the candle continues to burn using the part of air that support burning/combustion.

This is called the active part of air.

The candle goes off/extinguished when all the active part of air is used up. The level of the water rises to occupy the space /volume occupied by the used active part of air.

The experiment is better when very dilute sodium/potassium hydroxide is used instead of water. Dilute Potassium/ sodium hydroxide absorb Carbon (IV) oxide gas that comes out from burning/combustion of candle stick.

From the experiment above the % composition of the:

(i) Active part of air can be calculated:

M2 - M3/M2 x 100% => 10- 8/10 x 100% = 20%

(ii) Inactive part of air can be calculated:

100% -20% = 80% // M3 => 8/10 x 100% = 80%

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