Products Of Combustion

Experiment 5

Hold a saucer in a candle flame. Note the black deposit that forms on it. What element is present in the candle? What is smoke? Do we get the maximum heat from fuel when smoke is given off during combustion ?

Identification Test For Carbon Dioxide

Put some filtered lime-water in a glass beaker or tumbler. Breathe through a glass tube into the limewater, which will become cloudy from the carbon dioxide present in the breath exhaled from the lungs. This test is used to identify carbon dioxide, which always causes limewater to become cloudy. An excess of C02 causes the cloudiness to disappear.

Experiment 6

Hold a bell jar over a burning candle for a short time, collecting any gases which may be given off during combustion. Invert the bell jar, pour into it some filtered limewater, cover the jar, and shake well. Does the limewater become cloudy? What gas was given off during the burning of the candle?

Experiment 7

Cover a burning candle with a dry bell jar. Note the formation of water on the sides of the jar. Is the water a product of combustion ? Write the equation for the forming of H20.

Summary of products of combustion:

C + 0 = CO, carbon monoxide, a poisonous gas given off in combustion when the supply of oxygen is deficient. The blue flame on the surface of a coal fire is burning CO. All stoves should have perfect chimney connections to carry off this poisonous gas.

C + O2 = CO2, carbon dioxide.

H2 + 0 = H20, water.

Unconsumed carbon = smoke.

Mineral matter = ash.

Flame

When two substances unite chemically, and both are gases or vapors at the temperature of combustion, the act of union is accompanied by a flame. When one of the substances remains solid at the temperature of combustion, heat and light are given off, but there is no flame. Many substances which are solids or liquids at ordinary temperatures vaporize slowly when at the temperature of burning and hence burn with a flame. The light given by the flame is caused by the glowing, or incandescence, of the solid carbon particles. At a higher temperature the carbon is quickly and completely burned and gives no light, but burns with a blue flame.

When gas is used as a fuel it is mixed with air before burning, in order that it may be diluted, when there will be a more perfect combustion. A gas used as fuel should burn with a blue flame; a yellow or a smoky flame indicates that the carbon is not being completely consumed and there is an attendant loss of heat.

Note

Students examine carefully a Bunsen or other gas burner, increasing and decreasing the supply of oxygen by opening and closing the mixer. Note the varying color of the flame and the seeming increase or loss of heat.

Kindling Point

Every combustible solid must be raised to a certain temperature, or kindling point, before it will unite rapidly enough with oxygen to produce light. This temperature varies with different substances, but is always the same for the same substance. The kindling point of phosphorus is very low. Ordinary parlor matches are tipped with phosphorus, potassium chlorate, and glue. The heat produced by the friction of striking the match is sufficient to raise the phosphorus to its burning point. The heat produced by the burning of the phosphorus raises the wood of the match to its kindling point, and the match ignites.