A combustible substance, oxygen or air, and heating the substance to its ignition temperature are necessary.
Pollution; kerosene; ignition temperature; water.
CNG burns more completely and produces less smoke, carbon monoxide and other harmful gases than petrol or diesel, so vehicular pollution is reduced.
LPG is cleaner, has higher calorific value, lights easily and leaves no ash. Wood produces smoke, ash and more pollutants, has lower heat efficiency, and cutting wood contributes to deforestation.
Water conducts electricity and may cause electric shock, so it is not used on electrical fires. LPG is cleaner and more efficient than wood. Paper wrapped around aluminium does not reach ignition temperature easily because aluminium conducts heat away.
The candle flame has three zones: innermost dark zone of unburnt vapours, middle luminous yellow zone of partial combustion, and outer non-luminous blue zone of complete combustion, which is the hottest.
Kilojoule per kilogram (kJ/kg).
Carbon dioxide is heavier than oxygen and forms a blanket over the fire, cutting off air. It also cools the fuel below ignition temperature.
Green leaves contain moisture, which absorbs heat and keeps them below ignition temperature. Dry leaves have little moisture and reach ignition temperature easily.
The outermost non-luminous zone, because it is the hottest zone due to complete combustion.
Calorific value $=180000/4.5=40000$ kJ/kg.
$40000$ kJ/kg.
Rusting is a slow oxidation process but not combustion in the usual sense because it does not produce heat and light rapidly like burning.
Ramesh's water heats faster because the outermost zone is the hottest.