Class 8 - Chemistry - Combustion and Flame
List conditions under which combustion can take place.
The conditions under which combustion takes place are as follows:-
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 _________ .
(a) Burning of wood and coal causes pollution of air.
(b) A liquid fuel, used in homes is Liquefied Petroleum Gas (LPG).
(c) Fuel must be heated to its Ignition temperature before it starts burning.
(d) Fire produced by oil cannot be controlled by water.
Explain how the use of CNG in automobiles has reduced pollution in our cities.
Combustion of fuels like petroleum causes formation of unburnt carbon particles along with CO gas.
These gases cause respiratory diseases. Therefore, the use of diesel and petrol as fuels in automobiles is
being replaced by CNG (Compressed Natural Gas), as CNG produces harmful products
such as: - sulphur dioxide, oxides of nitrogen, and oxides of sulphur etc.in a very small amounts. CNG is a cleaner fuel.
Compare LPG and wood as fuels.
1.It has more calorific value about 55000 kJ/Kg.
1. It has less calorific value about 17000 to 22000 kJ/Kg.
2. It causes less pollution.
2. It causes more pollution.
3. It is easy to store and easily transported in cylinders and pipelines.
3. It needs lot of space to store and also very difficult to transport.
4. Leaves no residue.
4. Leaves ash as residue.
(a) Water is not used to control fires involving electrical equipment.
(b) LPG is a better domestic fuel than wood.
(c) Paper by itself catches fire easily whereas a piece of paper wrapped around an aluminium pipe does not.
Make a labelled diagram of a candle flame.
Different Zones of Candle Flame
Name the unit in which the calorific value of a fuel is expressed.
The calorific value of a fuel is expressed in a unit called kilojoule per kg (kJ/kg).
Explain how CO2 is able to control fires.
For fires involving electrical equipment and inflammable materials like petrol, carbon dioxide (CO2) is the best extinguisher.
CO2, being heavier than oxygen, covers the fire like a blanket. Since the contact between the fuel and oxygen is cut off, the fire is controlled.
The added advantage of CO2 is that in most cases it does not harm the electrical equipment.
It is difficult to burn a heap of green leaves but dry leaves catch fire easily. Explain.
Green leaves contain moisture, so they do not catch fire. Dry leaves do not contain moisture, so they catch fire easily.
Moisture increases the ignition temperature of leaves.
Which zone of a flame does a goldsmith use for melting gold and silver and why?
Goldsmiths blow the outermost zone of a flame with a metallic blow-pipe for melting gold and silver. As it is the hottest zone (complete combustion) of the flame.
Goldsmith blowing through a metallic pipe
In an experiment 4.5 kg of a fuel was completely burnt. The heat produced was measured to be 180,000 kJ. Calculate the calorific value of the fuel.
Heat produced = 180,000kJ
Mass of fuel = 4.5 kg
Calorific value = (Heat produced) (kJ) / (Mass of Fuel) (in kg)
= (180,000kJ) / (4.5 kg)
= (1800000)/ (45)
The calorific value of the fuel is 40000 (kJ/Kg)
Can the process of rusting be called combustion? Discuss.
No, as rusting is a very slow process as compared to combustion and the heat evolve in combustion is much more than rusting.
Rusting can take place at room temperature but combustion need an ignition temperature.
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?
The outermost part of the flame is the hottest one, so Rakesh’s water will get heated in a shorter time.