Carbon and Fossil Fuels

Diatoms have several industrial applications due to their silica-based cell walls. They are used in insulation materials (diatomaceous earth for pipes and refrigerators), fire-resistant materials, and historically in dynamite preparation (to stabilize nitroglycerin). Diatoms are not used for food preparation - they are microscopic algae collected for industrial uses, not culinary purposes.

Light is not an essential condition for combustion to take place. 

Carbon dioxide is heavier than oxygen, so when sprayed on a fire, it settles down and forms a blanket over the burning material. This displaces the oxygen surrounding the fuel, cutting off the oxygen supply needed for combustion. It does not change the nature of oxygen or react with it - it simply physically separates fuel from oxygen.

Wax and kerosene oil burn with flames because they vaporize during combustion. When heated, these substances first melt (in case of wax) or form vapours (in case of kerosene), which then mix with oxygen and burn producing a flame. Flame is produced only when gaseous substances burn. Not all combustible substances produce flames - charcoal burns without a flame because it doesn't vaporize.

Water is not suitable for oil and petrol fires because water is heavier than oil and petrol. When water is poured on burning oil, it sinks below the oil surface due to higher density. The oil, being lighter, continues to float on top and keeps burning. Moreover, the water can boil and create steam explosions, spreading the burning oil further. This is why foam or CO2 extinguishers are used instead.

Charcoal is a solid fuel that undergoes combustion without vaporizing, so it only glows without producing a flame. Materials like candles, magnesium, and kerosene vaporize during burning, creating a visible flame when these vapors combust.

Cow dung cakes have the lowest calorific value (approximately 2000-2500 kCal/kg) among the given options, as they contain high moisture and ash content. In comparison, petrol has ~11000 kCal/kg, LPG ~12000 kCal/kg, and hydrogen has the highest at ~34000 kCal/kg.

Carbon monoxide is a colorless, odorless, and highly toxic gas produced during incomplete combustion of coal. It binds to hemoglobin 200+ times more strongly than oxygen, preventing oxygen transport in the body and causing asphyxiation, which can be fatal in closed spaces.

A candle flame has three zones: the dark zone (innermost, unburnt vapor), luminous zone (middle, partial combustion with glowing carbon particles), and non-luminous zone (outermost, complete combustion). 'Combustible zone' is not a standard term used to describe flame zones.

Charcoal is composed primarily of carbon and does not vaporize at combustion temperatures. Flame formation requires fuel vaporization; since charcoal only undergoes surface combustion without producing vapors, it glows without producing a flame.

Sulphur dioxide is produced when sulfur impurities present in coal and diesel burn during combustion. It is a suffocating gas that causes respiratory distress and is also corrosive as it dissolves in moisture to form sulphurous/sulphuric acid, irritating and damaging respiratory tissues and eyes.

We should keep in mind that the fuel selected must possess the following characteristic properties:

1. It should possess high calorific value.
2. It should have proper ignition temperature.
3. It should not produce poisonous products during combustion.
4. It should not burn quickly in air.
5. Combustion should be easily controllable.
6. It should not leave behind much ash on combustion.
7. It should be easily available in plenty.
8. It should have low moisture content.
9. It should be cheap and easy to handle and transport. 

The blue color of an LPG stove flame indicates complete combustion of the fuel, where LPG burns fully to produce carbon dioxide and water vapor. Blue flames are produced when combustion is efficient with minimal soot or unburnt carbon particles, while yellow flames indicate incomplete combustion.

In the luminous zone (middle zone) of a candle flame, partial combustion takes place. The hydrocarbons from the wax break down but do not fully oxidize, leaving tiny carbon particles that heat up and glow, producing the yellow luminous zone before reaching complete combustion in the outer zone.

Peak oil is the point in time when the maximum rate of petroleum extraction is reached, after which the rate of production is expected to enter terminal decline.