Why does a traffic policeman standing on a crowded square put a mask on his face while controlling the traffic ?

Raghav was returning with his family from a marriage party. On the way, a traffic policeman stopped their car. He asked Raghav’s father who was driving the car to exhale his breath into an instrument to check whether he has drunk or not. After checking from the instrument, he allowed them to go. As a student of chemistry: Name the instrument used by the traffic policeman.

Raghav was returning with his family from a marriage party. On the way, a traffic policeman stopped their car. He asked Raghav’s father who was driving the car to exhale his breath into an instrument to check whether he has drunk or not. After checking from the instrument, he allowed them to go. As a student of chemistry: Can you explain the theory behind this test ?

While watering a distant plant,a gardener partially closes the exit of the pipe by putting his finger on it.Why this results in the water streams going to a longer distance?

Explain why Water on a clean glass surface tends to spread out while mercury on the same surface tends to form drops. (Put differently, water wets glass while mercury does not.)

Abdul while driving to school, computes the average speed for his trip to be 20 km h^(-1) . On this trip along the same route there os less traffic and average speed is 40 km h^-(-1) . What is the average speed for Abdul's trip ?