Question

Some substances catch fire very easily like alcohol, petrol, LPG, etc. These are called inflammable substances but some do not catch fire easily like water. What could be the reason for it?
A.Boiling point of inflammable substances is higher
B.Latent heat of vaporization is lower for inflammable substance
C.Ignition temperature of inflammable substance is lower
D.Both (B) and (C)

Answer 1

Hint:We have to know that certain substances such as petrol, LPG, alcohol catch fire easily and are known as inflammable substances and water and few others do not catch fire easily. This is due to the differences in temperature. Different substances catch fire in different temperatures.

Complete answer:
We can say ignition temperature is the least temperature during which fire is caught by a substance. The substances that contain very low ignition temperature and could easily catch fire with a flame are known as inflammable substances. Few of the examples of inflammable substances are petrol, alcohol, Liquefied Petroleum Gas (LPG) etc.
But this is not the case: a paper cup containing water is heated. The heat given to the paper cup is moved to water by process of conduction. So, in the presence of water, the ignition temperature of paper is not attained. So, it does not burn.
The reason why certain substances such as alcohols, petrol, LPG catches fires and some substances such as water do not catch fire is because of the ignition temperature.
Substances that are inflammable generally have low ignition temperature.
Therefore, the option (C) is correct.

Note: We have to remember that combustible substances do not catch dire as long its temperature is less when compared to ignition temperature. For example: Ignition temperature of phosphorus is $\text{3}{\text{5}}^{\text{o}}\text{C}$. So, until phosphorus is heated to temperature of $\text{3}{\text{5}}^{\text{o}}\text{C}$, it would not catch fire. But when the temperature of air is higher than $\text{3}{\text{5}}^{\text{o}}\text{C}$ or exactly $\text{3}{\text{5}}^{\text{o}}\text{C}$ phosphorus would catch fire without heating.