Question

Why, water cannot be used to extinguish petrol fires?

Answer 1

Hint:A fire needs three elements to burn which are oxygen, heat and fuel. We can cut the supply of any of these three elements to extinguish the fire.


Complete step by step answer:
When we are applying water on a petrol fire, we cannot stop the supply of the basic three elements which are oxygen, fuel and heat. Since water is heavier than petrol, petrol will come above the water and continue the contact with the atmosphere (oxygen). Already the existing temperature of the atmosphere will be high due to the fire, most of the water poured will evaporate faster. So, we cannot stop the petrol fires using water.
Therefore, the volume occupied by 1 molecule of $H_{2}O$ is $2.99\times {10}^{23}$ ml.

Additional Information:
We all know that there are different classes of fires like class A, class B, class C, class D and class K. Class A fire is caused by the combustion of ordinary substances like wood, paper, trash. Class B fire is caused by the combustion of fuels like petrol, diesel and inflammable gases. Class C fire is caused by the electrical devices mainly due to short circuits. Class D fire is caused by the combustion of metals like sodium. Also, class K fire is caused by the combustion of cooking oils and fats or known as kitchen fires.


Note:
The petrol fires are also known as class B fires. Class B fires can be extinguished by foam, powder or carbon dioxide extinguishers.Fire extinguishers with a Class B rating are effective against flammable liquid fires. These can be fires where cooking liquids, oil, gasoline, kerosene, or paint have become ignited. Two commonly used chemicals are effective in fighting these types of fires.