Notes on the important physical properties of water

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1. There exists a strong cohesive force between water molecules. Because of this force of attraction the like molecules cling to one an­other rather tenaciously a prop­erty called cohesion. Cohesion provides high degree of tensile strength to water and keeps the water molecules together in a wa­ter column.

2. The high surface tension (76 erg/cm2 at liquid air interface) is also brought about by cohesion of water molecules at the surface. This causes water to occupy the least possible surface area. To some extent it helps in the formation of vacuoles, food droplets, etc. in the cell and explains as to why so many small organisms settle on water surface.

3. Water molecules adhere to sub­stances containing oxygen and ni­trogen atoms such as glass, clay, soil, wood, cellulose, protein, etc., which is largely due to its ability to form hydrogen bonds. Adhesion keeps the water column in capillary

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In fact, strong cohesion and high surface tension together with adhesive forces account for the capillary action of water. Rise of water in blotting paper, in capillary pores between soil par­ticles, in cellulosic cell wall and the ascent of sap (transport of water) through lumen of xylem are all ex­amples of capillary action. The hydro­gen bonding of water molecules at the water-air interface hold the water sur­face, which in turn pulls up the water.

Capillary ascent however, ceases when the weight of elevated water inside the tube equals the surface tension force. These forces together with transpira­tion pull provide satisfactory explana­tion for ascent of sap, even in giant trees.

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