The average depth of the oceans is about 4000 m (13000 feet). The Pacific Ocean is the deepest. Just 30 years ago, two men found the deepest point of all the oceans in the Marianas Trench in the South Pacific.Using a specially designed underwater vessel named the Trieste, they went down 10900m (35800 feet), or about 11 km (7 miles). If you placed the highest mountain on land, Mt. Everest (8847 m or 29028 feet high), in this deep abyss, it would disappear without a trace.

The prefixes 'aqua' and 'hydro' both mean water. Try matching the words below to their meanings.
1. Aquanaut
2. hydrophone
3. aqueduct
4. hydroponics
5. aqualung

a. man-made channel for transporting water
b. a portable diving apparatus strapped to a diver's back
c. an underwater explorer
d. an instrument for detecting sound by water
e. growing plants in water

Answer: 1 c 2 d 3 a 4 e 5 b

Advanced knowledge

Molecular Structure

A water molecule is formed of one H atom and two O atoms linked by covalent bonds. Water is an unusual liquid. It has a very high boiling point and a high heat of vaporization. The maximum density is at 4ºC, and the water expands upon freezing. It has a very high surface tension and is a very good solvent for salts and polar molecules. These properties are a consequence of the dipolar character of H₂O. The electron cloud around H₂O results from the hybridization of s and p electrons, yielding two bonding orbitals between the O and the two H atoms and two nonbonding sp³ orbitals on the O. The molecule has a high negative charge density near 0 and high positive charge density near H.

In the vapor phase the equivalent size of a water molecule is 3.3 Å, or 0.33 nm (1 Å= 10^-10 m). The molecules move at high speed, and their translation energy is so high that during collisions the van der Waals forces are inefficient for creating bonds. The vapor expands when the temperature increases.

In the liquid phase the molecules are close together and occupy a volume of 29.7 ų, indicating a porosity of 36.7%. Liquid water forms a heterogeneous fluid structure of water molecules, clusters of molecules, and H⁺ and OH^- molecules. The molecular structure influences the density and the viscosity. Liquid water consists of an ice-tridymite structure, a quartzlike structure, and a close-packed ammonia-like structure. In the ice structure each 0 atom is bonded to four others by hydrogen bonds in a tetrahedral configuration. The H atoms in the O-H ... O bond are no longer 0.96 Å from the O atom but may be either 0.99 Å or 1.77 Å away. The volume per H₂O molecule in ice is 32.3 ų. The volume per gram-molecule is 19.56 ml, and the resulting density is 0.92.

The number of broken H bonds increases with temperature up to 50% at 40ºC. Broken hydrogen bonds are responsible to a considerable extent for its high dielectric constant (D = 78.55 at 250ºC), contributing to tire fact that water is the best solvent for polar compounds

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