What are the Conditions Necessary for Combustion?

We have seen coal burning in the air which produces carbon dioxide and water. When we burn a candle in the air, CO2, with heat and light is produced. All the examples are chemical processes in which a new substance is formed with the evolution of heat. A chemical process in which a substance reacts with vapooxygen to give off heat is called combustion. The substance that undergoes combustion is said to be combustible or fuel which may be solid, liquid or gas. Sometimes, light as a glow or a flame is also given off during combustion.

Types of Combustion

1. Rapid Combustion: Combustion in which gas burns rapidly and produces heat and light is called rapid combustion. For example- Phosphorus which burn in air at room temperature.
2. Spontaneous Combustion: The type of combustion in which a material suddenly bursts into flames, without the application of any apparent cause is called spontaneous combustion.
3. Explosion: A sudden reaction that takes place with the evolution of heat, light and sound is called an explosion. A large amount of gas formed in the reaction is liberated. For example- The burning of firecrackers.

Conditions necessary for Combustion

The following conditions are necessary for combustion to take place-

1. Presence of a combustible substance: Combustion is only possible if the substance is combustible, i.e. the substances which can burn. Examples-wood, paper, kerosene oil, straw, match stick etc. If there is no combustible substance, then combustion can't take place.
2. Presence of air: Adequate supply of air is needed (e.g., oxygen) for combustion.
3. Attainment of ignition temperature: A substance starts to burn only after it has attained a certain minimum temperature. The temperature at which a substance burns in the presence of air is called its ignition temperature. A substance cannot catch fire if its temperature is lower than its ignition temperature.

The above three conditions are also called fire triangles. Substances like alcohol, petrol, LPG, and nylon fibers catch fire very easily. This is because these substances have low ignition temperatures. Such substances are called inflammable substances. Special care needs to be taken while storing or transporting inflammable substances.

Experiment to prove that Air is necessary for Combustion

In this experiment, we will fix a lighted candle on a table. Put a glass over the candle and rest it on a few wooden blocks in such a way that air can enter the chimney. Now remove the blocks and let the chimney rest on the table. Again observe the flame. Finally, put a glass plate over the chimney. Watch the flame again. Now we will see what happened in all three cases-

1. In the first case, the candle burns without any problem and flame do not flicker off and the candle remains unaffected. as air flows steadily in the chimney without any interruption from the space available at the bottom because of wooden blocks.
2. In the second case, the candle burns for a few seconds only and the flame of the candle flicker off. The candle does not burn unaffected and flickers off producing smoke as air is not coming from the bottom of the chimney.
3. In the third case, the flame goes off due to the unavailability of air and smoke is produced. Oxygen present in air supports combustion. Without oxygen, burning is not possible.

Ignition Temperature

The ignition temperature of a material is the temperature at which it ignites. For combustion to begin, the temperature at which the fuel vaporizes on its surface must be reached. If a fuel is in the form of vapour, it ignites instantly (gas). Cooking gas ignites very instantly.

Because spirit and gasoline vapourize at room temperature, they catch fire quickly. Camphor, on the other hand, ignites quickly. At normal temperature, cloth and paper ignite. Because kerosene vaporizes at temperatures above 150°C, it takes a long time to catch fire. 

When a piece of fabric or paper is drenched with kerosene and set on fire, the paper instantly takes fire. In turn, this warms up kerosene, elevating its surface temperature to 150°C or more, igniting the kerosene beneath it. The same may be said for vegetable oils. The wick of a vegetable oil lamp takes a long time to heat up before catching fire. The oil underneath the wick is vaporized by the flame on the wick's tip, which keeps the flame burning.

Combustion in Everyday life

The applications of combustion and flame phenomena may be divided into five categories.

• Heating Devices: Combustion of gases, wood, coal, and liquid fuels is used to heat equipment for vapour generation (steam, etc.) in metallurgy and industry in general.
• Explosives: Explosive combustion and detonation are frequently employed in a variety of applications where mechanical action or explosion are the ultimate aims. The theory of explosive combustion and detonation is used in practical applications.
• Engines: Various engines, gas turbines, turbojets, and ramjets are among them. The Otto engine works by compressing a mixture in a cylinder with a piston. The mixture is ignited with a spark just before the piston reaches the top, and the flame propagates at a normal velocity into the unburned mixture, raising the pressure and pushing the piston.
• Rocket Propulsion: The results of gaseous, liquid, or solid propellant combustion in rockets are expelled at high velocity from the combustion chamber through the (de Laval) nozzle. To estimate the thrust necessary, a thorough understanding of the kinetics of chemical reactions in the nozzle is required. The thrust diminishes as the combustion products' mean molecular weight increases. As a result, low molecular weight mixtures with high heat of combustion are utilised in rockets.
• Chemical Reactions: To create chemical reactions, flames are utilised in a variety of methods. One example is the bead test in analytical chemistry. The reducing power of a flame with inadequate oxygen is only used in a few circumstances. Some flames create economically valuable soot, and the production of coke and charcoal is reliant on precise combustion and flame management.

Sample Problems

Problem 1: What is combustion?

Solution: 

A chemical process in which a substance reacts with oxygen to give off heat is called combustion. The substance that undergoes combustion is said to be combustible or fuel which may be solid, liquid or gas. Sometimes, light as glow or a flame is also given off during combustion.

Problem 2: What are the types of Combustion? 

Solution:

The types of combustion are-

1. Rapid Combustion: Combustion in which gas burns rapidly and produces heat and light is called rapid combustion. For example- Phosphorus which burn in air at room temperature.
2. Spontaneous Combustion: The type of combustion in which a material suddenly bursts into flames, without the application of any apparent cause is called spontaneous combustion.
3. Explosion: A sudden reaction that takes place with the evolution of heat, light and sound is called explosion. A large amount of gas formed in the reaction is liberated. For example- Burning of firecrackers.

Problem 3: What are the conditions necessary for Combustion? 

Solution:

The following conditions are necessary for combustion to take place-

1. Presence of a combustible substance: Combustion is only possible if the substance is combustible, i.e. the substances which can burn. Examples-wood, paper, kerosene oil, straw, match stick etc. If there is no combustible substance, then combustion can't take place.
2. Presence of a supporter of combustion: Adequate supply of a supporter of combustion (e.g., oxygen) is essential for combustion.
3. Attainment of ignition temperature: A substance starts to burn only after it has attained a certain minimum temperature. The temperature at which a substance burns in the presence of air is called its ignition temperature. A substance cannot catch fire if its temperature is lower than its ignition temperature.

Problem 4: Explain the working of a matchstick and fire extinguisher. 

Solution: 

The main component of the bulb of a matchstick is red phosphorus which turns into white phosphorus on heating. White phosphorus spontaneously ignites which increases the temperature of the wooden stem to the ignition point and the matchstick starts burning.

Fire can be controlled by removing any or some factors of combustion, i.e. fuel, oxygen (air) and ignition temperature.  The role of the fire extinguishers is to cut off the supply of oxygen or bring down the temperature of the fuel or both.

Problem 5: Place a piece of burning wood or charcoal on an iron plate. Cover it with a tumbler and observe what happens. Does charcoal stop burning after some time? Can you think of the reason why it stops burning? 

Solution:

Yes after sometime charcoal stops burning as burning charcoal or wood is covered with a tumbler, charcoal will burn until oxygen is present in the jar. During burning, carbon dioxide is also produced by charcoal. After sometime oxygen present in jar get exhausted due to burning and simultaneously charcoal is covered with carbon dioxide, thus, charcoal stop burning.

Problem 6: Make two paper cups by folding a sheet of paper. Pour some water in one of the cups and heat both of them with a candle. What do you observe? 

Solution:

Paper cup with water does not burn and paper cup which does not contain water starts burning quickly. Paper cup which contains water do not burn till water is vaporized as heat introduced to paper cup containing water, goes to water. Water keeps the ignition temperature of the paper cup low. Thus, this is the reason paper cup containing water does not burn. Paper cup without water reached its ignition temperature required to burn paper quickly and starts burning.