if a strong odour smell continuously for sometime, the sensation of smell of that substance weaken ... why?

Olfactory fatigue/adaptation-after being exposed to a particular smell for a prolonged time, the smell sense diminishes. This temporary and normal

Explanation:

This adaptation is important as it prevents our nervous system from information overload

Diffusion

Explanation:

The definition of diffusion is that: The movement of particles and ions from the area of higher concentrated region to the area of lower concentrated region due to the result of their random movement. When the strong odour particles are in one place it becomes concentrated in that area. Therefore, according to this, the odour diffuses from the higher concentrated area to the lower concentrated area, causing the particles to bounce off and spreading in all directions which results to the sensation of smell of the strong odour weakening.