Combustion and Flame

When magnesium burns in the presence of oxygen it produces magnesium oxide, light and heat.

Carbon dioxide is the best extinguishing agent for electrical fires and flammable liquid fires like petrol because: (1) It is non-conductive, so it won't cause electric shocks, (2) It is heavier than air and forms a blanket over the fire, cutting off oxygen supply, and (3) It doesn't leave residue or damage electronic equipment. Oxygen would fuel the fire, nitrogen is less effective, and hydrogen is highly flammable.

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

A matchstick does not catch fire until its temperature is raised to its ignition temperature and the ignition temperature of this type of substance is higher than room temperature.

 Oxygen is an essential gas for combustion.

Spontaneous combustion occurs when a material suddenly bursts into flames without an external ignition source, typically due to internal chemical reactions or heat buildup (like coal or oily rags). Rapid combustion is when a substance burns rapidly with a flame after being ignited by an external source. Option C correctly captures that spontaneous combustion is sudden (without external trigger) while rapid combustion is fast burning (with external trigger).

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.

The outermost zone is the hottest zone of the flame as oxygen is available in sufficient supply and complete combustion takes place here.

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 middle zone of a candle flame is called the luminous zone and appears yellow due to incomplete combustion of hydrocarbons. The incomplete combustion produces tiny carbon particles that heat up and glow, giving this zone its characteristic yellow color.

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.