One is amylose, a linear fraction with glucose units joined by a 1-4 glucosidic bond. The other is amylopectin, a branched component where each branch is relatively short and contains about 20-30 glucose units. Starch granules swell when exposed to water, but without applied heat, the swelling may not be immediately noticeable. Though swollen, the granules will settle to the bottom if not agitated.
Heating breaks enough of the intermolecular hydrogen bonds to allow the granule to swell noticeably.
Generally, starches with small granules swell at higher temperatures than do those with larger granules. Continued heating results in further swelling and in the starch-water mixture becoming translucent. As the starch granules swell, they result in an increase in viscosity or thickness. The viscosity is due to the swollen starch molecules colliding. It is possible to tear swollen starch molecules in which case the viscosity would decrease.