Life on Earth began in water and evolved there for 3 billion years before spreading onto land.
Studied on its own, the water molecule is deceptively
simple. It is shaped like a wide V, with its two hydrogen
atoms joined to the oxygen atom by single covalent bonds.
Oxygen is more electronegative than hydrogen, so the electrons
of the covalent bonds spend more time closer to oxygen
than to hydrogen; these are polar covalent bonds. This unequal sharing of electrons and water’s
V-like shape make it a polar molecule, meaning that its
overall charge is unevenly distributed. In water, the oxygen
region of the molecule has a partial negative charge (δ-),
and each hydrogen has a partial positive charge (δ+).
The properties of water arise from attractions between
oppositely charged atoms of different water molecules: The
slightly positive hydrogen of one molecule is attracted to the
slightly negative oxygen of a nearby molecule. The two molecules
are thus held together by a hydrogen bond.
Cohesion of Water Molecules
Water molecules stay close to each other as a result of hydrogen bonding. 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. These linkages make water more structured than most other liquids. Collectively, the hydrogen bonds hold the substance together, a phenomenon called cohesion.
Cohesion due to hydrogen bonding contributes to the transport of water and dissolved nutrients against gravity in plants. Water from the roots reaches the leaves through a network of water-conducting cells.
Water’s High Specific Heat
The ability of water to stabilize temperature stems from its relatively high specific heat. The specific heat of a substance is defined as the amount of heat that must be absorbed or lost for 1 g of that substance to change its temperature by 1°C. Because of the high specific heat of water relative to other materials, water will change its temperature less than other
liquids when it absorbs or loses a given amount of heat.
Heat of vaporization is the quantity of heat a liquid must absorb for 1 g of it to be converted from the liquid to the gaseous state. For the same reason that water has a high specific heat, it also has a high heat of vaporization relative to most other liquids. Water’s high heat of vaporization is another emergent property resulting from the strength of its hydrogen bonds, which must be broken before the molecules can exit from the liquid in the form of water vapor. As a liquid evaporates, the surface of the liquid that remains behind cools down (its temperature decreases).
Water is one of the few substances that are less dense as a solid than as a liquid. In other words, ice floats on liquid water. when a deep body of water cools, the floating ice insulates the liquid water below, preventing it from freezing and allowing life to exist under the frozen surface,
Water is a very versatile solvent, a quality we can trace to the polarity of the water molecule. Suppose, for example, that a spoonful of table salt, the ionic compound sodium chloride (NaCl), is placed in water.
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