3. Render the following text.

After Biot's discovery of optical activity in 1815, little was done until Louis Pasteur entered the picture in 1849. Pasteur, who received his formal training in chemistry, had become interested in the subject of crystallog­raphy. He began work on crystalline salts of tartaric acid derived from wine and was repeating some measurements published a few years earlier when he made a surprising observation. When he recrystallized a concentrated solution of sodium ammonium tartrate below 28°C, two distinct kinds of crystals precipitated. Furthermore, the two kinds of crystals were nonsuperimposable mirror images of each other. That is, the crystals were not symmetrical, but were related to each other in exactly the same way that a right hand is related to a left hand.

Working carefully with a pair of tweezers, Pasteur was able to separate the crystals into two piles, one of "right-handed" crystals and one of "left-handed" crystals. Although a solution of the original salt (a 50:50 mixture of right and left) was optically inactive, solu­tions of the crystals in each of the individual piles were optically active, and their specific rotations were equal in amount but opposite in sign.Pasteur was far ahead of his time. Although the structural theory of Kekule had not yet been proposed, in explaining his results Pasteur spoke of the molecules themselves, saying: "It cannot be a subject of doubt that [in the dextrotartaric acid] there exists an asymmetric arrangement having a nonsuperimposable image. It is no less certain that the atoms of the levoacid possess precisely the inverse asymmetric arrangement." Pasteur's vision was extraordinary, for it was not until 25 years later that the theories of van't Hoff and Le Bel confirmed his ideas regarding the asymmetric carbon atom.

Today, we would describe Pasteur's work by saying that he had discov­ered the phenomenon of optical isomerism, or enantiomerism. Enantiomers (pronounced e-nan-tee-o-mers; from the Greek enantio, "opposite") are molecules that are mirror images of each other. The two "right-handed" and "left-handed" tartaric acid salts that Pasteur separated are identical in all respects except for their interaction with plane-polarized light. They have the same melting point, the same boiling point, the same solubilities, and the same spectroscopic properties. They are, however, related to each other as a right hand is to a left hand.

4. MODAL OPINION: express your opinion about the text given in the previous exercise using each of the following modal verbs:

• Should

• Shouldn’t

• Have to

• Don’t have to

• Could

• Couldn’t

• Must

• Mustn’t

5. ONE MONTH LATER: write down and speculate five things you think happened in the month following the Pasteur’s discovery.

Listening

1. Imagine that you are starting a presentation. What phrases might you use?

2.  Listen totwowaysofopeningpresentationsandseeifyoucanhearsomeofthephrasesabove.

3. Read some advices on delivering effective presentations in the Appendix 7 and write your own opening for the topic “Stereochemistry”.

Writing