Question

When does raisins swell ? When we put the raisins in tap water or rain water?

Answer 1

Raisins contain hypertonic solution which gains water by endosmosis across the semi-permeable cell membrane of raisins. So,they swell up in tap water. When they are put in rainwater,they lose water by exosmosis and as a result they shrink.

Answer 2

I think in both cases, the raisins will swell.

When you put raisins in water for couple of hours, the water, molecules gets inside the raisins due to the process of osmosis.  

The water molecules move inside from the walls of raisins and the raisins swells. Due to osmosis, the sugar content present inside the raisin will also flows out.