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Biology Stpm Practical

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Biology Stpm Practical

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  • August 14, 2013
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Basic chemistry of a cell Properties of water – – – Life on Earth began in water and evolved there for 3 billion years before spreading onto land. Modern life, even terrestrial life, remains tied to water. All living organisms require water more than any other substance. Human beings for example, can survive for quite for a few weeks without food, but only a week or so without water.

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Water is deceptively simple. It is shaped something like a wide V, with its two hydrogen atoms joined to the oxygen atom by single covalent bond. Because oxygen is more electronegative than hydrogen, the electrons of the covalent bonds spend more time closer to oxygen than to hydrogen; in other words, they are polar covalent bonds. The uneven distribution of electrons of water molecules makes it an universal solvent.

In overall, water has four emergent properties: Cohesion 1. Water molecules stay close each other as a result of hydrogen bonding. 2. Although the arrangement of molecules in a sample of liquid water is constantly changing, at any given moment many of the molecules are linked by multiple hydrogen bonds. 3. These linkages make water more structured than most other liquid. 4. Collectively, the hydrogen bonds hold the substance together, a phenomenon called cohesion. 5. Cohesion means the hydrogen bonding between the water molecules that can form a continuous water column against the gravity without breaking. 6. Besides cohesion, water molecules can also perform adhesion between water molecules and cell walls by hydrogen bonds. 7. Both cohesion and adhesion are important in the transport of water in plants.

8. Related to cohesion is surface tension, a measure of how difficult it is to stretch or break the surface of a liquid. 9. At interface of water and air is an ordered arrangement of water molecules, hydrogen-bonded to one another and to the water below. 10. This makes the water behave as though coated with an invisible film. 11. You can observe the...