8th Class Science Combustion and Flame Flame

      Flame

Flame can be defined as the glowing body of mixed gases undergoing the process of combustion. Flames generally consist of a mixture of oxygen (or, air) and another gas, usually such combustible substances as hydrogen, carbon monoxide, or hydrocarbon.

A typical flame is that of a burning candle. When the candle is lighted, the heat of the match melts the wax, which is carried up the wick and then vaporized by the heat. The vaporized wax is then broken down by the heat and, finally, combines with the oxygen of the surrounding air, producing a flame and generating heat and light. The candle flame consists of three zones that are easily distinguished. The innermost zone, a nonluminous zone, is composed of a gas-air mixture at a comparatively low temperature. In the second, or luminous, zone, hydrogen and carbon monoxide are produced by decomposition and begin to react with oxygen to form water and carbon dioxide, respectively. In this zone the temperature of the flame—about 590° to 680° C (about 1090° to 1250° F)—is great enough to dissociate the gases in the flame and produce free particles of carbon, which are heated to incandescence and then consumed. The incandescent carbon produces the characteristic yellow light of this portion of the flame. Outside the luminous zone is a third, invisible zone in which the remaining carbon monoxide and hydrogen are finally consumed.

All combustible substances require a definite proportion of oxygen for complete burning. In the burning of a candle, or of solids such as wood or coal, this oxygen is supplied by the surrounding atmosphere. In blowpipes and various types of gas burners, air or pure oxygen is mixed with the gas at the base of the burner so that the carbon is consumed almost instantaneously at the mouth of the burner. For this reason such flames are nonluminous. They also occupy a smaller volume and are proportionately hotter than a simple candle flame.

        Which one of the following is a solid fuel?

(a) Patrol

(b) CNG

(c) Coal

(d) LPG

(e) None of these

Answer: (c)

          The outermost zone of a flame is:

(a) Yellow

(b) Black

(c) Green

(d) Blue

(e) None of these

Answer: (d)

• Combustion is a process of rapid oxidation with simultaneous evolution of heat and usually light.
• Fuel is a substance that reacts chemically with another to produce energy.
• The sensation of warmth or coldness of a substance on contact is determined by the property known as temperature.
• A flame consists of three zone.
• All combustible substances require a definite proportion of oxygen for complete burning.