Identify P and Q:

P: I am the most common fire extinguisher but not suitable for oil, petrol, and electrical fires. I am heavier than oil and also, conduct electricity.
Q: I am an excellent fire extinguisher and most suitable for oil, petrol, and electrical fires. I also, form a blanket around the fire and bring down the temperature of the fuel.

P is  water ( H₂O)  and Q is carbon dioxide (CO₂ )

Water is heavier than oil and petrol and sinks below them, and oil and petrol keep burning on top. Water is a good conductor of electricity. So P is water. For fires involving electrical equipment and inflammable materials like petrol, carbon dioxide (CO₂) is the best extinguisher. So Q is carbon dioxide.