Class 8 - Chemistry - Combustion and Flame

Question 1:

 List conditions under which combustion can take place.

Answer:

The conditions under which combustion takes place are as follows:-

  1. Presence of combustible substance.
  2. Presence of supporter of combustion i.e., oxygen.
  3. Attainment of ignition temperature. (The lowest temperature at which a substance catches fire is called its ignition temperature).

 

 

Question 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) 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.

 

 

 

 

Question 3:

Explain how the use of CNG in automobiles has reduced pollution in our cities.

Answer:

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.

 

 

Question 4:

Compare LPG and wood as fuels.

Answer:

             LPG

   Wood

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.

 Class_8_Combustion_&_Flame_LPG_Flame  Class_8_Combustion_&_Flame_Wood

Question 5:

Give reasons:

(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.

Answer:

  • Water is not used to control the fire involving electrical equipment because it is a good conductor of electricity.
  • The person using it can get a shock.
  • LPG is a good fuel as compared to wood as it readily available and it is cheap.
  • It burns easily in air at a moderate rate and it produces a large amount of heat. It does not leave behind any undesirable substances.
  • Paper by itself catches fire easily because it is a highly combustible substance.
  • But when it is wrapped on aluminium, it does not bum because aluminium is a good conductor of heat and it takes the heat from paper.

 

 

Question 6:

Make a labelled diagram of a candle flame.

Answer:

Different Zones of Candle Flame

 Class_8_Combustion_&_Flame_CandleFlame

 

 

Question 7:

Name the unit in which the calorific value of a fuel is expressed.

Answer:

The calorific value of a fuel is expressed in a unit called kilojoule per kg (kJ/kg).

Question 8:

Explain how CO2 is able to control fires.

Answer:

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.

                       

Fire Extinguisher

 

 Class_8_Combustion_&_Flame_FireExtinguisher     Class_8_Combustion_&_Flame_Person

 

 

 

Question 9:

It is difficult to burn a heap of green leaves but dry leaves catch fire easily. Explain.

Answer:

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.

 

 

 

Question 10:

Which zone of a flame does a goldsmith use for melting gold and silver and why?

Answer:

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

 Class_8_Combustion_&_Flame1

 

Question 11:

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.

Answer:

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)

=40000 (kJ/kg)

The calorific value of the fuel is 40000 (kJ/Kg)

 

 

Question 12:

Can the process of rusting be called combustion? Discuss.

Answer:

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.

 

 

Question 13:

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 outermost part of the flame is the hottest one, so Rakesh’s water will get heated in a shorter time.

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