IV. Insert the missing prepositions into the blanks.

1. Hydrogen occurs …… nature as a diatomic molecule H_2.

2. Hydrogen constitutes over 10% of water …… mass and occurs in petroleum and all organic matter.

3. Oxygen …… air exists primarily as the diatomic molecule O₂.

4. Industrially, oxygen is produced …… the fractional distillation of liquid air.

5. Most of the oxygen in the atmosphere is the result …… . photosynthesis

6. Ozone absorbs the ultraviolet (uv) radiation …… sunlight.

V. Speaking

Give a short summary of the text.

VI. Reading

Text B

a. Guess if the following statements are true or false. If they are false correct them.

1. Liquid water has a number of unique properties. T/F

2. Water has high melting and boiling points. T/F

3. Covalent bonds give water a low heat capacity. T/F

4. The molecules are held together by tetrahedrally-directed hydrogen bonds.

5. Water is a polar molecule. T/F

Read and translate the text. The Composition and Structure of Water

Water was thought by the ancient to be an element. Henry Cavendish in 1781 showed that water is formed when hydrogen is burned in the air, and Lavoisier first recognized that water is a compound of the two elements – hydrogen and oxygen.

The formula of water is H₂O. The relative weights of hydrogen and oxygen in the substance have been carefully determined as 2.016:16.000. This determination has been made both by weighing the amounts of hydrogen and oxygen liberated from water by electrolysis and by determining the weights of hydrogen and oxygen that combine to form water.

Liquid water has a number of unique properties which indicate that the structure must be fundamentally different from that of most other liquids. Thus, water has high melting and boiling points, an unusually high heat capacity, and showed a characteristic decrease in molar volume on melting and subsequent contraction between 0 and 4⁰C. Quite apart from the behavior of aqueous solutions, any proposed structure for liquid water must be consistent with these observations.

From spectroscopic studies of isolated water molecules in the gas phase, it has been shown that the H-O-H bond angle is very nearly the tetrahedral angle of 105^o and the O-H internuclear distance is 0.97E; the observed dipole moment is 1.87x10^-18e.s.u. acting along the bisector of the H-O-H angle.

Bernal and Fowel in a classical work on the interpretation of these results showed that the net electronic density distribution was consistent with such a structure in which, in addition to the two protons carrying small positive charges, there are also two regions of negative charge. These four regions of charge, two positive and two negative, could be regarded as residing at the corners of a tetrahedron. If the oxygen is approximately sp³ hybridized then two of the orbitals on the oxygen atom are used for bonding the hydrogen atoms and the other two carry the lone pairs of electrons which can participate in hydrogen to two neighbouring water molecules. In terms of this picture the structure of ice, in which each molecule has four nearest neighbours can be represented in the following way. The molecules are held together by tetrahedrally-directed hydrogen bonds which are essentially electrostatic in character. The structure is an open one rather than a close-packed with a resulting increase in density. The characteristic increase in density with temperature continues until 4^oC when the expected decrease accompanying the increased thermal energy becomes apparent.