Forces Exhibited by Water

Cohesive and Adhesive Forces:

Cohesive forces are forces which act between molecules of the same substance. Cohesive forces between water molecules result in creating a tension of the surface film in water and also resist separation. So, rain water falls in the form of droplets rather than a mist due to these strong cohesive forces.

Adhesive forces are forces which act between molecules of different substances. Adhesive forces cause water to stick to surfaces which it comes in contact with (example- water molecules stick to car side mirrors). A strong adhesive force between any particular surface with water will cause the water to spread over the surface.

Capillary action: If a thin straw is put in a glass of water, the water is pulled up to a greater height in the straw than the level of water in the glass. This is called capillary action. Capillary action is a net result of the cohesive forces between water molecules and the adhesive forces between molecules of the straw and water. If the adhesive force of water to the walls is stronger than the cohesive forces between the water molecules, the capillary action takes place, and the water will rise in the straw against the natural force of gravity. Plants also draw up water along with the dissolved minerals from the roots to the tip via thin narrow xylem vessels using capillary action.

BROWNIAN MOTION: In 1827, botanist Robert Brown suspended pollen grains in water and observed them under the microscope. He observed that the pollen grains continuously moved in random directions on their own accord, but he could not explain why this was happening. In 1905, physicist Albert Einstein explained that the pollen grains were being pushed around by even smaller water molecules hence they were moving continuously in random directions.

This effect is also observed in particles of smoke suspended in air or any other gas. Brownian motion is defined as a continues random movement of particles suspended in liquids or gases resulting from the collisions of molecules of the surrounding medium. The discovery of Brownian motion confirmed the theory that atoms and molecules exist as the constituents of matter. In addition, this motion tends to disperse particles throughout the medium and is the major force in diffusion.

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