CBSE Class 8 Science: Combustion and Flames (NCERT)

What is Combustion?

Combustion is a chemical process in which a substance reacts with oxygen to give off heat and sometimes light (either as a glow or a flame). The substance that undergoes combustion is called a combustible substance, also known as fuel. Fuels can exist in solid (wood, coal), liquid (kerosene, petrol), or gaseous (LPG, CNG) states. On the other hand, substances like glass, stones, and iron nails do not burn in the presence of air and are called non-combustible substances. For combustion to occur, a chemical reaction must take place where the chemical bonds of the fuel break and combine with oxygen, releasing a significant amount of thermal energy. This is why combustion is classified as an exothermic chemical reaction.

Three Crucial Conditions for Combustion

1. Presence of a Combustible Substance (Fuel) — There must be a material that can burn. Without a fuel like wood, paper, LPG, or petrol, no combustion can take place.
2. Supply of Oxygen (Air) — Oxygen acts as the supporter of combustion. A continuous supply of fresh air is necessary to sustain the chemical reaction.
3. Attaining the Ignition Temperature — The lowest temperature at which a substance catches fire is called its ignition temperature. A fuel will not burn unless it is heated to this minimum temperature. Substances with very low ignition temperatures that catch fire easily are called inflammable substances (e.g., LPG, alcohol, petrol).

Types of Combustion Explained

Rapid Combustion

A type of combustion in which a substance burns rapidly and produces heat and light in a short time, such as burning LPG on a kitchen stove.

Spontaneous Combustion

A type of combustion in which a material suddenly bursts into flames without the application of any apparent external heat source, like phosphorus catching fire at room temperature.

Explosion

A sudden reaction where a large amount of gas is evolved along with the release of heat, light, and sound, usually triggered by pressure or ignition, like firecrackers.

Exam Tip: Mastering the Zones of a Candle Flame

This is a highly tested concept in CBSE Class 8 exams! Draw the diagram of a candle flame and label its three distinct zones:

1. Outer Zone (Blue): This is the zone of complete combustion. It has the maximum supply of oxygen, making it the hottest part. It is non-luminous.
2. Middle Zone (Yellow): This is the zone of partial (incomplete) combustion. Unburnt carbon particles glow yellow, making it luminous. It is moderately hot.
3. Inner Zone (Black): This consists of unburnt wax vapors. Since no oxygen reaches this zone, no combustion occurs here. It is the least hot part.

Common mistake: Do not confuse the brightness of the middle zone with its temperature! The outer blue zone is actually hotter than the bright yellow middle zone.

Practice Questions with Solutions

• Q: Explain why water is not used to control fires involving electrical equipment. A: Step 1: Understand the physical property of tap water. Water is a good conductor of electricity. Step 2: Identify the danger of using water on electrical fires. If water is poured onto electrical equipment, it can conduct electricity to the person trying to extinguish the fire, leading to a fatal electric shock. Step 3: Recognize the potential damage to the equipment. Water can also permanently damage sensitive electrical devices. Final answer: Water conducts electricity, which poses a severe risk of electric shock to the operator, and it can ruin the electrical equipment. Therefore, non-conducting extinguishing agents like carbon dioxide ($CO_2$) are used instead.
• Q: Compare LPG and wood as fuels in terms of efficiency and environmental impact. A: Step 1: Analyze calorific value. LPG has a high calorific value (approx 55,000 kJ/kg) compared to wood (approx 17,000 to 22,000 kJ/kg), meaning LPG produces more heat per unit mass. Step 2: Analyze smoke and residues. Wood leaves behind ash and produces harmful smoke (carbon monoxide and fine particles) causing respiratory diseases, whereas LPG burns cleanly without leaving ash or smoke. Step 3: Analyze storage and transport. LPG is easily stored in cylinders and transported via pipes, whereas wood is bulky and requires cutting down trees, leading to deforestation. Final answer: LPG is a highly efficient and cleaner fuel than wood because it has a much higher calorific value, burns cleanly without producing smoke or ash, and does not cause deforestation.
• Q: Why does paper wrapped around an aluminum pipe not burn easily when heated? A: Step 1: Recall the definition of ignition temperature. A substance catches fire only when it reaches its ignition temperature. Step 2: Analyze the role of the aluminum pipe. Aluminum is a metal and an excellent conductor of heat. Step 3: Trace the flow of heat energy. When the paper wrapped on the aluminum pipe is heated, the heat is rapidly conducted away from the paper to the aluminum pipe. Final answer: The heat supplied to the paper is quickly absorbed and conducted away by the aluminum pipe. As a result, the paper does not reach its ignition temperature and does not catch fire easily.
• Q: What is acid rain and how is it related to the burning of fossil fuels? A: Step 1: Identify the gases released by burning fossil fuels. Combustion of coal and diesel releases sulfur dioxide ($SO_2$) and nitrogen oxides ($NO_x$) into the air. Step 2: Describe the reaction in the atmosphere. These gases dissolve in rainwater to form sulfurous, sulfuric, and nitric acids. Step 3: State the consequences. The acidic rainwater falls back to earth as acid rain, which damages crops, buildings, and soil fertility. Final answer: Burning fossil fuels releases harmful gases like sulfur dioxide and nitrogen oxides, which dissolve in rainwater to form acids. This acidic rain damages crops, soil, and historical monuments like the Taj Mahal.

Frequently Asked Questions