Important Questions: Combustion And Flame - Class 8 MCQ

- The outer zone of a candle flame is the hottest part.
- This zone has a blue hue due to complete combustion.
- Oxygen is plentiful here, allowing the fuel to burn efficiently.
- The temperature in the outer zone can reach up to 1400°C.
- In contrast, the inner zone is cooler because it lacks sufficient oxygen, resulting in incomplete combustion.
- The outer zone's efficiency and high temperature make it the hottest part of the flame.

Explanation:
- The innermost part of a flame, known as the blue part or the core, actually has the least temperature compared to the outer parts of the flame.
- This might seem counterintuitive since we usually associate blue flames with high heat, but in reality, the outer parts of the flame are hotter as they contain more oxygen and burn more efficiently.
- Understanding this concept helps in applications like cooking and welding where different parts of the flame are used for specific purposes.

The statement that oxygen is required for the burning of any substance is true. Oxygen acts as an oxidizing agent, allowing substances to undergo combustion and release energy in the form of heat and light.

The substances which have very low ignition temperature and can easily catch fire with a flame are called inflammable substances. Examples of inflammable substances are petrol, alcohol, Liquified Petroleum Gas (LPG) etc.

The ignition temperature refers to the minimum temperature at which a substance or material can ignite and sustain combustion. It is an important parameter to understand the fire hazards associated with different substances.
Answer: A - The lowest temperature to catch fire.

Coal is a natural fossil fuel that can burn with both a flame and glow. The flame is produced due to the volatile matter present in coal, which vaporizes and ignites, while the glow is due to the combustion of the solid carbon content. Therefore, coal can exhibit both flame and glow when it burns.

• The substance which undergoes combustion is said to be combustible. It is called fuel.
• If a magnesium ribbon is heated,it starts burning.
• When a magnesium ribbon burns, it combines with the oxygen of air to form magnesium oxide, and liberates heat and light.

Products of combustion depend upon the nature of the combustible substance. Most of the fuel that we use are hydrocarbons.

Compounds of hydrogen and carbon are called hydrocarbons.

For e.g. LPG (main constituent is butane or C₄H₁₀), natural gas (methane or CH₄), petrol etc.

When such fuels burn, the main products are carbon dioxide and water. 

For e.g. Methane + oxygen  Carbon dioxide + Water.

- Compressed Natural Gas (CNG) is the closest to an ideal fuel among the options.
- It is relatively cheap and readily available.
- CNG is easily combustible and has a high calorific value.
- Transportation is straightforward since it can be piped directly to homes and businesses.
- It produces fewer pollutants, emitting lower levels of harmful gases compared to petrol, kerosene, and coal.
- CNG results in minimal environmental pollution, making it a cleaner energy source.