combustion and flame full chapter notes please urgent
Answers
Combustion: It is a chemical reaction in which heat is released by a material when it reacts with oxygen.
Fuel or Combustible Substance:Any material that undergoes combustion is called a combustible substance. It is also called as fuel. Some examples of fuels are petrol, diesel, etc. The fuel may be in solid, liquid or gas state. Sometimes, light is also given off during combustion, either as a flame or as a glow.
Ignition Temperature:
(i) It is the minimum temperature at which any material catches fire.
(ii) If the temperature of combustible substance is lower than the ignition temperature then the substance will not burn.
Example: (i) Cooking oil catching fire when a frying pan is kept for long on a burning stove.
(ii) Kerosene oil and wood do not catch fire on their own at room temperature. But, if kerosene oil is heated a little, it will catch fire. But if wood is heated a little, it would still not catch fire.
Inflammable Substances:
Those materials which have low ignition temperature and catch fire easily are termed as inflammable substances. Example includes petrol, LPG, etc.
Matchsticks:
Since ages, matchsticks are in use. Long ago, Egyptians used small pieces of pinewood dipped in sulphur as matches. These days matchsticks are lot safer.
Modern matchsticks are made up with mixture of antimony trisulphide and potassium chlorate with some glue and starch applied on the head of the match. The rubbing surface has powdered glass and some red phosphorous. On striking match against rough surface, red phosphorous gets converted into white phosphorous and it reacts with potassium chlorate to ignite antimony trisulphate and so the combustion takes place.
Matchstick
Things necessary for combustion to take place:
(i) Fuel or Combustible substance.
(ii) Air (With presence of Oxygen in it).
(iii) Temperature above the Ignition temperature.
Measures to control fire:
1. Fire Brigade Stations:
In case of fire, fire brigades will extinguish the fire by sprinkling the water on the affected areas. The water will bring down the temperature below its ignition temperature. As a result, fire will stop spreading. Water vapours also surround the combustible material, helping in cutting off the supply of air. So, the fire is extinguished.
2. Fire Extinguisher:
Water is the most common fire extinguisher. But, it works only on things like wood, paper, etc. However, in case fire is caught on electrical things then, water being good conductor of electricity will destroy those equipment. Even water is not good in case of fires due to oil, petrol, etc.
For such cases, Carbon dioxide (CO2) is best extinguisher. This extinguisher cut off the air supply and thus brings down the temperature below the ignition temperature as a result fire gets extinguished. Moreover, it usually does not damage electrical equipment.
3. Use of Blankets:
If a person catches the fire, then blankets can be used to extinguish the fire.
4. Forest Fires:
In summer season, when temperature rises too high then the regions having dry grasses will catch the fire. This fire spreads rapidly from grasses to trees and eventually entire forest is on fire. And it is difficult to manage such fires.
Different Types of Combustion:
1. Rapid Combustion
In this type of combustion, the substances burns rapidly and yield light and heat.
Example:Bring a burning matchstick or a gas lighter near a gas stove in the kitchen. Turn on the knob of the gas stove. We find that the gas burns rapidly and produces heat and light.
2. Spontaneous Combustion:
In this type of combustion, substances burst out into flames suddenly without any known reason.
Examples: Many disastrous fires in coal mines result due to this kind of combustion. The heat rays coming from the sun or a lightning strike might be responsible for this kind of combustion.
3. Explosion:
In this type of combustion, all of a sudden reaction results into heat, light and sound. Moreover, large quantity of gas also gets released.
Example: When a fire cracker is ignited, a sudden reaction takes place with the evolution of heat, light and sound with the large amount of gas.
Flame:
When something is burnt, a hot luminous gas emerges out of the substance. This gas is called as flame.Flames are result of the substances which vaporizes on burning. Example includes kerosene oil, wax, etc. which form flames on burning.
Flame structure:
When flames are observed carefully, one can notice different layers of flame as shown in figure below:
Outermost zone: It is blue in color and is hottest amongst all the zones. In this portion, complete combustion takes place.
Middle zone: It is yellow in color and is somewhat hot. In this portion, partial combustion takes place.
Innermost zone: It is black in color and is coolest amongst all the zones.
Explanation:
CHAPTER. 6
COMBUSTION AND FLAME
Exercise Questions
1.List conditions under which combustion can take place.
Answer:
Combustion can take place in the presence of:
(a) a combustible substance.
(b) oxygen, that is, the supporter of combustion.
(c) attainment of ignition temperature of the substance.
2.Fill in the blanks.
(a) Burning of wood and coal causes _____ of air.
(b) A liquid fuel, used in homes is ______
(c) Fuel must be heated to its ______ before it starts burning.
(d) Fire produced by oil cannot be controlled by ______
Answer:
(a) pollution
(b) LPG
(c) ignition temperature
(d) water
3.Explain how the use of CNG in automobiles has reduced pollution in our cities.
Answer:
The use of CNG in automobiles has reduced pollution in our cities as it is a quality fuel and has some benefits:
(a) It gives out less carbon dioxide gas, carbon monoxide gas, sulphur dioxide and nitrogen dioxide, which is beneficial as they play crucial role in global warming and acid rain.
(b) It leaves behind no residue after its combustion.
4.Compare LPG and wood as fuels.
Answer:
LPG Wood
(i) It does not cause pollution on combustion. (i) It pollutes air on its combustion.
(ii) No smoke is produced. (ii) It produces smoke.
(iii) It is a liquid fuel. (iii) It is a solid fuel.
(iv) It has more calorific value (55000 kJ/kg). (iv) It has less calorific value (17000 kJ/kg).
(v) It can be easily transported, as it is stored in cylinders. (v) It can’t be transported easily like LPG fuels.
5.Give reasons.
a.Water is not used to control fires involving electrical equipment.
Since water is a good conductor of electricity, it may result in electric shocks to the person trying to extinguish fire.
b.LPG is a better domestic fuel than wood.
LPG is better domestic fuel than wood because it does not produce gases, nor does it leave any residue behind. Moreover, it has more calorific value than wood.
c.Paper by itself catches fire easily whereas a piece of paper wrapped around an aluminium pipe does not.
As its ignition temperature is low, the paper by itself catches fire easily. But a piece of paper wrapped around an aluminium pipe does not catch fire easily, as the heat being given gets absorbed by the aluminium pipe and the piece of paper does not get its ignition temperature.
6. Make a labelled diagram of a candle flame.
7.Name the unit in which the calorific value of a fuel is expressed.
Answer:
The unit in which the calorific value of a fuel is expressed is kilojoules per kilogram (kJ/kg).
8.Explain how CO2 is able to control fires.
Answer:
As CO2 is heavier than oxygen, it forms a blanket around fire, because of which the supply of air is stopped. Men over, it brings down the temperature of the burning substance. In these ways, it plays a significant role in controlling fire.
9.It is difficult to burn a heap of green leaves but dry leaves catch fire easily. Explain.
Answer:
The green leaves hold some amount of water, so its ignition temperature gets increased and it does not burn easily. On the other hand, dry leaves are waterless, so they catch fire easily (having low ignition temperature).
10.Which zone of a flame does a goldsmith use for melting gold and silver and why?
Answer:
A goldsmith uses the outermost zone of a flame, which is non-luminous, to melt gold and silver as it is the hottest zone of the flame, having more temperature.
11.Can the process of rusting be called combustion? Discuss.
Answer:
The process of rusting emits heat during the formation of its oxide. So we can call the process of rusting as slow combustion.
12.Abida and Ramesh were doing an experiment in which water was to be heated in a beaker. Abida kept the beaker near the wick in the yellow part of the candle flame. Ramesh kept the beaker in the outermost part of the flame. Whose water will get heated in a shorter time?
Answer:
The water which was put by Ramesh will get heated in a shorter time; because he had put it nearer to the hottest zone of the flame.