Question

why when we open a gas tap for a few second the sound of escapping gas is heard first but the smell of gas comes later

Answer 1

the answer-

because the speed of light is faster then the smell........

Answer 2

when we open the gas tap.. we hear the sound first because the particles takes much time to fill the spaces between the air particles...where as the sound propagate much faster due to its faster speed...that's why we hear the sound of gas leaking first.