Test: Combustion & Flame- 2 - Class 8 MCQ

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 defined by their inability to ignite or burn in the presence of air or oxygen. They do not burn in any medium due to their inherent properties.' To address the user-reported error and clarify the question, the question statement should focus on the inherent property of non-combustible substances: Question Statement: Non-combustible substances are defined by their inability to burn.

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.

Combustible substance changes into vapour at its ignition temperature and start burning to give heat and light.
At its ignition temperature, a combustible material ignites and begins to burn, but among the given options, the most appropriate process is:

2. Changes into vapour.

This is because at the ignition temperature, many combustible materials reach a point where they produce sufficient vapors that can ignite when mixed with air and exposed to a heat source.

Sodium can catch fire without any external heat supply when come in contact with oxygen. Sodium is kept under kerosene oil to prevent accidental fire and protecting the sodium metal.

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.