Question

which proprty of gas help us to dectect LPG gas leakagte

Answer 1

The Ethyl Mercaptan gas is added to LPG ( as it don't have its own smell ), to identify if gas is leaked.
Diffusion is the property which let us identify gas leak

Answer 2

LPG is a colourless and odourless gas. So when there is any leakage, we will not be able to find out and this may cause destruction. So only Ethyl Mercaptan is added to the gas to find out if there is any leakage. As it is added, diffusion takes place and then if at all any leakage, with the smell of the LPG gas we can find it out! Hope this helps!