4.4. Flame

Observe an LPG flame. Can you tell the colour of the flame? What is the colour of a candle flame?

When we observe an LPG flame, it typically has a blue color, which indicates a complete combustion process. In contrast, a candle flame usually has a yellow or orange hue due to incomplete combustion, which produces soot and other particles. This difference in color arises from the varying temperatures and conditions under which these flames burn.

Think of a barbeque grill where a clean, blue flame indicates that the gas is burning efficiently, while a flickering yellow flame shows that something might be obstructing the air supply or that the fuel isn't burning completely, much like how a poorly maintained car engine can run inefficiently.

Recall your experience of burning a magnesium ribbon in Class VII. If you do not have experience of burning the remaining items in Table 4.2 you can do that now.

Different materials behave differently when burned. Some materials, like magnesium and kerosene, readily produce a flame upon burning. In contrast, materials like charcoal do not produce a flame because they do not vaporize effectively. This concept can be tied back to how substances interact with oxygen during combustion.

Imagine using different types of fireworks. Some explode with bright, loud bursts (like magnesium) while others burn quietly and steadily (like charcoal) without much visible action. The intensity of their combustion changes based on how they vaporize and mix with air.

The substances which vaporise during burning give flames. For example, kerosene oil and molten wax rise through the wick and are vaporised during burning and form flames.

For a flame to form, the material being burned must change from a liquid to a gas (vaporize). When wax or kerosene is heated, it turns into vapor and burns in the presence of oxygen, creating a visible flame. In contrast, solid materials like charcoal do not vaporize and thus do not produce a flame.

Consider a popcorn kernel. When it heats up, the moisture inside can create steam, and once it builds up enough pressure, the kernel pops. Similarly, in a flame, the vaporized substances are what 'burst' into light and heat.

Notice that the wax near the heated wick melts quickly. A circular blackish ring is formed on the glass plate/slide. It indicates the deposition of unburnt carbon particles present in the luminous zone of the flame.

When a candle burns, the wax melts and travels up the wick, but not all the material burns completely. The luminous zone of the candle flame contains unburnt carbon particles that deposit on surfaces, which we can see as soot. This highlights the inefficiency in combustion, especially in flames that are not well-ventilated.

Similar to how a fireplace can leave soot on the walls if not maintained well, a candle can also create this residue when it doesn't burn cleanly. This is why chimneys are important in homes with wood-burning stoves—to prevent soot buildup.