If fire is due to oil, then water is not suitable fire extinguisher because it is heavier than oil and settles down below the oil. In such cases, sand or soil should be used.
They cut off the supply of air to the fire. Thus, fire can be controlled.

When a cracker is ignited, a sudden reaction takes place with the evolution of heat, light and sound and a large amount of gas is liberated. Such a reaction is called explosion. So, cracker on ignition produces sound because of sudden formation of large amount of gas due to chemical reaction.

The amount of heat energy produced on complete combustion of 1 kg of a fuel is called its calorific value.
Given, total mass of fuel = 4.5 kg
Total heat produced = 180000 kJ
Now, heat produced in burning 1 kg of fuel, i.e. calorific value
\(\frac{Total \ heat \ produced}{Total \ mass \ of \ fuel} =\frac{180000}{5}\) 
\(\frac{180000 \times 10}{45}=\frac{180000}{45}\) 
\(\frac{360000}{9}=40000KJ/Kg\) 
Hence, calorific value of fuel = 40000 kJ/kg.

Calorific value of a fuel is expressed in kilojoule per kilogram (kJ/kg).

No, all the substances do not catch fire at the same temperature. Different substances need different temperature to catch fire.

The substances which vaporise during burning produce flame. No, charcoal does not burn with flame, it glows only because it does not vaporise on burning.

The three examples of inflammable substances are petrol, alcohol, LPG.

In our body, food is broken down by reaction with oxygen. During such reactions, heat is produced. This heat keeps our body warm.

Three essential requirements to produce fire are as follows:
(i) Fuel
(ii) Air and
(iii) Heat to acquire the ignition temperature. The job of fire extinguisher is to cut off the supply of air or to bring down the temperature of fuel or both. The most common fire extinguisher is water. But water works only when things like wood and paper are on fire. For fires involving electrical equipment and inflammable materials like petrol, carbon dioxide (C0₂) 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, so the fire can be controlled. The added advantage of CO₂ is that in most cases, it does not harm the electrical equipment.

Reasons of forest fires are
(i) At high temperature, sometimes dry grass catches fire which spreads throughout the forest.
(ii) Campfire may also be a reason.
(iii) Lightning.
(iv) The use of fires by villagers to ward off wild animals.
(v) Fire lit intentionally by people living around forests for recreation.
(vi) Fires started accidentally by careless visitors to forests how discard cigarette butts.
(vii) The friction of bamboos due to high wind velocity and rolling stones.