Combustion & Flame- 2 - UPSC Sci Tech Free MCQ Test with solutions

A combustible substance is one that can easily catch fire and burn when exposed to heat.

1. Wax: It is a combustible substance because it can burn when ignited, which is why it is used in candles.

2. Sand: It is non-combustible, meaning it cannot catch fire or burn under normal conditions.

3. Water: It is also non-combustible, and in fact, water is commonly used to extinguish fires.

4. Glass: It is non-combustible and does not burn; instead, it melts when subjected to very high temperatures.

Therefore, wax is the correct answer as it can burn, making it a combustible substance.

Water extinguishers work mainly by cooling (removing heat), but a dense spray of water droplets also helps to cut off oxygen.
Water cools the combustible material so that its temperature is brought below its ignition temperature. This prevents the fire from spreading. Water vapours also surround the combustible material, helping in cutting off the supply of air so that the fire is exhausted. But water cannot be used for fires involving electrical equipment and inflammable substances.

Fire can be extinguished by cutting the supply of oxygen and removing combustible substances from surrounding or lowering the temperature below the ignition temperature.

Non-combustible substances are materials that cannot ignite or burn under normal conditions, regardless of the medium (e.g., air, oxygen, methane, hydrogen, or water). Examples include sand, glass, and water.

• Option D (air or any other medium): This is correct, as non-combustible substances lack the chemical properties to undergo combustion in air (which contains oxygen, necessary for burning) or any other medium.

Oxygen gas supports the combustion of combustible substances including coal. Combustion of coal release carbon dioxide gas as well as solid residue.

B: Hottest zone: The outermost zone of a candle flame is exposed to the maximum amount of oxygen, leading to complete combustion of the fuel. This results in a blue flame, indicating the highest temperature. The products formed during complete combustion of methane (the main component of CNG and biogas) are carbon dioxide and water. This zone is crucial for efficient energy release.

Incomplete combustion of fuels release harmful gases like carbon monoxide. Carbon monoxide binds with haemoglobin and reduces the oxygen carrying capacity of blood.

Calorific value provides the amount of energy that is released when a fuel or material undergoes the process of combustion.

It is usually a measure of energy.

The SI unit (international system of units) of calorific value is Joule/Kg or Kilo Joule/Kg.

Those materials that combine with oxygen quickly burns faster. Oxygen help in combustion of substance by oxidising the carbon substance into carbon dioxide gas.

The middle zone of candle flame is known as zone of partial combustion. This zone is generally yellow in colour and has less heat.

Burning of a matchstick is an example of rapid combustion. Matchstick contains phosphorus that catches fire due to heat produce by friction.

Fire caused by burning of liquid fuels can be controlled by throwing soil and sand over it to avoid the contact of oxygen that supports the combustion.

Spontaneous combustion occurs itself with external heat. Spontaneous combustion occurs in those substances that have very low ignition temperature, lower than room temperature.

Burning of liquefied petroleum gas (LPG) is an example of rapid combustion. Rapid combustion starts very fast and lot of heat and light energy is produced.

Phosphorus is generally found as a waxy, yellow, transparent solid. When exposed to air, it spontaneously ignites and is oxidized rapidly to phosphorus pentoxide. Such heat is produced by this reaction that the element bursts into a yellow flame and produces a dense white smoke.

When a glass plate or slide is held into the luminous part of candle flame we see a circular blackish ring that represents unburnt carbon particles.

Different zone is formed in the flame having different colour. The hottest part of the flame is blue in colour present on the upper part of flame.

• Every combustible material needs to be heated up to a certain minimum temperature so that it can release enough vapours.
• These vapours mix with oxygen in the air.
• When this temperature is reached, the energy is sufficient for the chemical reaction of combustion to begin.
• Therefore, the substance starts burning at its ignition temperature.

Why not the other options?

• Extinguishes → opposite of burning, so incorrect.
• Changes into vapour → vapour formation is a step in the process, but combustion (burning) is the actual result.
• Evolves oxygen → substances don’t give oxygen, they need it.
• Melts → only a physical change, not combustion.

Conclusion:
At its ignition temperature, a combustible substance starts to burn

Sodium is a highly reactive metal that undergoes spontaneous combustion when exposed to air, particularly in the presence of oxygen and often triggered by moisture. Spontaneous combustion occurs because sodium has a very low ignition temperature, allowing it to ignite without an external heat source.

• Option B (external heat supply): Correct, as sodium’s low ignition temperature means it can ignite spontaneously upon exposure to air, without needing an external heat source like a flame or spark.

Fire caused by an electric short circuit should not be extinguished by using water because water is good conductor of electricity and may cause electric shock.