(a) When wood burns, the carbon present in it is oxidised to carbon dioxide (C0₂), which is a non-poisonous gas. When the windows are closed, the air or oxygen cannot enter the room. In the incomplete supply of oxygen, carbon is oxidised to carbon monoxide (CO) which is a highly poisonous gas. It caused suffocation in a closed room. On opening the windows, the poisonous gas slowly diffused out the room and fresh air came inside.  
(b) Mohan is quite intelligent, quick in decisions and caring about his family.

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.

Carbon dioxide (CO₂) is the best fire extinguisher. Being heavier than oxygen, it covers the fire like a blanket. Since, the contact between the fuel and oxygen is cut-off, the fire is controlled. It also brings down the temperature of the fuel. That is why, it is an excellent fire extinguisher.

(a) Water is not used to control fires involving electrical equipments because water is a good conductor of electricity. So, it can easily conduct electric current and causes danger of electric shocks.
(b) LPG is a better domestic fuel than wood because LPG has more calorific value and produces no pollution. While wood produces smoke and causes more pollution.
(c) It is because the ignition temperature of paper is less, so it catches fire easily. It does not catch fire, when wrapped around aluminium pipe because aluminium absorbs the heat by conduction, so paper does not attain its ignition temperature

Necessary conditions for combustion are as follows:
(i) Presence of combustible substance.
(ii) Proper supply of air to provide oxygen.
(iii) Attainment of ignition temperature.

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.

LPG is a better fuel than wood. The colour of LPG flame during complete combustion is blue.

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

In jar B, the candle remains lighted for a longer time because oxygen is a supporter of combustion.

The substance which provides heat and light is called fuels

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.

It is because
(i) Charcoal burns easily as compared to wood.
(ii) Charcoal does not produce smoke on burning and so, it causes no air pollution.
(iii) The amount of heat produced by burning charcoal is much more than that produced by burning of wood.