Why dry raisins placed in water swell up?

Raisins are dried grapes. When they are placed in water, the water from the surrounding diffuses into the raisins and so the raisins swell up. The phenomenon involved is called osmosis.

When raisins are kept in water for a few hours, the raisins will swell up or they will expand in size.

This happens due to osmosis, which is the movement of molecules from a region of its higher to its lower concentration through a semi or selectively permeable membrane.

Osmosis can be of two types, endosmosis (the entering of solvent from high to low concentration) and exosmosis (the removal of solvent from high to low concentration)

When raisins are kept in water, the raisins has less solvent concentration and the water has comparatively more solvent concentration, which means that it will act as a Hypotonic Solution.

The raisins will act as a semi permeable membrane and water will move from the outer surroundings into the raisins (endosmosis) due to which the raisin will increase in size or swell up.

This happens due to imbibitions of water and endosmosis.

Imbibitions are the adsorption of water or any other liquid by the solid particles of a substance causing it to increase in volume without forming a solution.

The solid particles which adsorb water or any other liquid are called imbibants. The liquid which is imbibed is called imbibate. The phenomenon of imbibitions occurs due to presence of hydrophilic colloids. Proteins, starch and cellulose are some examples of hydrophilic substances. Different substances have different imbibing capacity. Proteins have very high imbibing capacity, starch has less imbibing capacity and cellulose is the weakest imbiber. Because of difference in imbibing capacities, proteinaceus pea seeds swell more on imbibitions than starchy wheat seeds.