Combustion and Flame Question Answers: NCERT Class 8 Science

A chemical process in which a substance reacts with oxygen to give a product in the form of heat is called combustion. There are some conditions under which combustion takes place, which are as follows:

1. Air
2. Ignition temperature
3. Combustible substances must be present.

(a) Pollution

Burning of wood and coal causes pollution because when they burnt. Carbon dioxide is produced which is a polluted gas.

(b) LPG

We generally used LPG at home. LPG is a clean fuel. When LPG is burnt, no ash is produced behind.

(c) Ignition temperature

Ignition temperature is the minimum temperature at which any substance catches fire easily.

(d) Water  

Fire cannot be controlled by water which is due to oil because water has high density than the oil. So, when water thrown on it, water sinks below the oil.

The use of CNG in automobiles has reduced pollution in our cities because CNG is a clean fuel. It does not produce any harmful gas on burning. So, we should prefer the use of CNG.

The differences between LPG and wood are as follows:

1. Water is not used to control fires involving electrical equipments because water is the conductor of electricity. It produces ions in the presence of electricity. So, water is not used to control the fire involving electrical equipments.
2. LPG is a better domestic fuel than wood because LPG is a clean fuel. It does not produce any harmful gas behind burning. It is easy to burn whereas wood produces ash and it does not burns easily.
3. Paper by itself catches fire easily whereas a piece of paper wrapped around an aluminum pipe does not after the wrapping. Around an aluminum pipe its ignition temperature does not reach as the heat is transferred to the aluminum pipe; and the temperature of the paper lowers down.

A candle have three zones:

Outer zone, middle zone and innermost zone.

• Outer zone is of complete combustion which is blue in colour.
• Middle zone is of the partial combustion which is of yellow colour.
• Innermost zone is of the unburnt wax.

The calorific value of the fuel is expressed in the kilojoules per kilogram (kj/kg).

Calorific vale shows the fuel efficiency.

Carbon dioxide gas is able to control fire because it is heavier than the oxygen. It covers the fire like an blanket. As the contact between the fuel and oxygen is cut off, the fire can be controlled easily. Carbon dioxide gas does not harm the electrical appliances.

It is difficult to burn a heap of green leaves but dry leaves catches fire easily because green leaves  contains a lot of water. That water cools the leaves and the temperature becomes below its ignition temperature. This prevents the burning of green leaves. But in dry leaves, there is no water present. So, as the temperature raised above the ignition temperature, the leaves catches fire easily.

A goldsmith used the outermost zone of a flame for melting gold and silver with a metallic blow pipe.

The outermost zone of the flame has the highest temperature i.e. sufficient to melt the gold and the silver.

Mass of fuel = 4.5 kg

Heat produced = 180000 kJ

Calorific value = Total heat produced                                                                                                                                                                                                                              Total mass burnt

So, calorific value = 180000                                                                                                                                                                                                                                                   4.5

= 180000 x 10                                                                                                                                                                                                                                           45

= 40000 kJ/kg

Yes, the process of rusting is called as combustion because, this process of rusting also takes place in the presence of air and humidity as like combustion. Rusting is called as the slow combustion process because it takes longer time.

As we know the outermost zone of a flame have the highest temperature or we can say it is the hottest part. Here, the water of Ramesh’s beaker will get heated quickly in comparison to Abida’s beaker because outermost zone of a flame is the hottest part in comparison to the middle part of the flame.