Combustion and Flame Class 8 Science in pdf
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Combustion is the process of burning substances in the presence of air or oxygen, producing heat and light. There are different types of combustion, including rapid, spontaneous, and explosive combustion, each characterized by its reaction and conditions. Effective methods to extinguish fires involve removing the combustible substance, cooling it below its ignition temperature, or cutting off the air supply.
Combustion and Flame Class 8 Science in pdf
- 1. COMBUSTION The process of burning of substances in air or oxygen with the evolution of heat and light is known as combustion. COMBUSTIBLE SUBSTANCES Substances that burn in air or oxygen to produce heat and light are called combustible substances. INFLAMMABLE SUBSTANCES Substances that have a very low ignition temperature and hence can catch fire easily are called inflammable substances. COMBUSTION COMBUSTIBLE SUBSTANCES INFLAMMABLE SUBSTANCES IGNITION TEMPERATURE The lowest temperature at which a combustible substance starts burning in the presence of air is called its ignition temperature. Different substances have different ignition temperatures. The conditions necessary for combustion are: 1. Presence of a combustible substance 2. Presence of supporter of combustion 3. Heating the substance to its ignition temperature In the given figure the paper cup with water in it does not burn because it loses its heat to the water by conduction and hence its temperature does not rise above its ignition temperature even when held above the candle flame. All right copy reserved. No part of the material can be produced without prior permission
- 2. 1. RAPID COMBUSTION The type of combustion in which a substance burns rapidly to produce heat and light is called rapid combustion. For e.g. Burning of paper, LPG etc. 2. SPONTANEOUS COMBUSTION The type of combustion in which a material bursts into flame without the application of heat is called spontaneous combustion. For e.g. Sodium is stored in kerosene and phosphorous is stored in water to prevent spontaneous combustion as they burst into flame when they come in contact with oxygen at room temperature due to low ignition temperature. 3. EXPLOSION The combustion reaction that occurs suddenly with the evolution of a large amount of gas is called an explosion. For e.g. Firecrackers Fire can be extinguished by one of the following methods: 1. By removing the combustible substance 2. By cooling the substance below its ignition temperature 3. Cutting off the air supply There are three parts in a soda-acid type fire extinguisher: 1. Container – It is cylindrical in shape and contains sodium bicarbonate 3 NaHCO solution. 2. Bottle – It is small in size and contains concentrated sulphuric acid 2 4 . H SO 3. Knob – Bottle is attached to it. WORKING When the knob is struck, the bottle breaks and 2 4 H SO reacts with 3 NaHCO liberating large amount of 2 CO gas with great force. 2 CO forms a blanket around the fire cutting off the air supply, due to which the fire gets extinguished. CHEMICAL REACTION 3 2 4 2 4 2 2 2 2 2 Sulphuric Sodium Water Carbon Sodium Acid Sulphate Dioxide Bicarbonate NaHCO H SO Na SO H O CO Effective for fires caused by solids and liquids Effective for fires caused by liquids and faulty electric wires All right copy reserved. No part of the material can be produced without prior permission
- 3. A fuel is a substance that can be used to produce heat at a reasonable cost. Fuels contain potential energy that is locked within the chemical bonds of their constituent molecules. When they are burnt, a large part of this energy is released as heat. This forms the basis of their use as fuels. Calorific value of a fuel can be defined as the amount of heat liberated when one kilogram of the fuel is completely burnt in sufficient supply of oxygen. This S.I. unit of calorific value is joule per kilogram (J/Kg), though commonly the unit kilojoule per kilogram (kJ/Kg) is used. All right copy reserved. No part of the material can be produced without prior permission