Consult the TEXTS FOR SUPPLEMENTARY READING and learn about early attempts to find or get clean water (Text 47). Be ready to discuss the information you have read.

30. Read the text and give its brief summary:

Text 12 C

Water Purification

Fresh water is an essential ingredient of modern life. Though it’s often available as the result of natural processes, there are times when it must be extracted from impure water, typically salt water. In some countries where rain water is scarce, desalinated sea water is the main source of drinking water. Any extraction process that purifies water must separate water molecules from contaminating liquids, solids, or gases. You can purify water in your own kitchen. If you boil salt water in a pot, you’ll create steam and will see mist forming above the water as that steam condenses into water droplets in the air. If you place a cold surface in the steam, the steam will condense on that surface. The condensed steam is nearly pure water — distilled water. While the water in the pot contained salt, the distilled water does not. However, before you taste the distilled water, make sure that the water in the pot contained only non-toxic additives. Distillation isn’t perfect at removing contaminants and it can even concentrate them in some situations.

One way to purify water is by distillation. Distillation is a general technique for separating various chemicals from one another. The chemicals are heated to form a vapor and that vapor is condensed to form a new mixture of chemicals. Because the various chemicals have different tendencies to form vapors at a particular temperature, the newly formed mixture has a different balance of the chemicals from the original mixture. In some cases, the condensed liquid contains primarily a single chemical — all of the other chemicals are left behind in the original liquid. To understand how distillation can purify water, let’s remember the phases of water. At any temperature above absolute zero, there’s a possibility of finding gaseous water molecules above the surface of ice or water. These water molecules have acquired enough thermal energy to break free of the solid or liquid and become a gas.

If you place some water in an enclosed container, water molecules will evaporate until there are enough of them in the gas phase that they return to the liquid’s surface as often as they leave it. At that point the two phases, liquid and gas, are in phase equilibrium. Although molecules constantly shift back and forth between the two phases, neither phase grows at the expense of the other. Overall, there’s no net movement of molecules from one phase to the other.

At this phase equilibrium, the relative humidity is 100% — the water vapor has reached its saturated vapor pressure. But we have forgotten to pay attention to temperature. Since water’s vapor pressure depends on temperature, the present balance of gas and liquid in the container is ideal only at its current temperature. If you warm up the container, more water molecules will enter the gas phase and the amount of liquid water will decrease. If you cool down the container, more water molecules will enter the liquid phase and the amount of gaseous water will decrease. This connection between temperature and vapor pressure is the central principle behind distillation. What happens if there is air inside the container, along with the water and water vapor? Surprisingly, the air doesn’t matter. The density of water molecules in the gas phase is the same, whether the air is there or not. This interesting observation makes it possible to perform distillation with or without air around, although air’s presence affects the total pressure on the water and thus its boiling temperature.

TEXT AND VOCABULARY EXERCISES

31. Find in the text words or phrases which mean the same as:

32. Find the correct words in the text above to complete the sentences:

1. Fresh water is an ___ ingredient of modern life.

2. In some countries where rain water is ___, desalinated sea water is the main source of drinking water.

3. The condensed steam is nearly ___ water — distilled water.

4. ___ is a general technique for separating various chemicals from one another.

5. In some cases, the ___ liquid contains primarily a single chemical – all of the other chemicals are left behind in the original liquid.

6. Although molecules constantly shift ___ and ___ between the two phases, neither phase grows at the expense of the other.

7. Since water’s vapor pressure ___ on temperature, the present balance of gas and liquid in the container is ideal only at its current temperature.

8. The ___ between temperature and vapor pressure is the central principle behind distillation.

9. The density of water molecules in the ___ phase is the same, whether the air is there or not.

10. This interesting observation ___ it possible to perform distillation with or without air around.

33. Match each word in A with the Russian equivalent in B:

34. Render Text 12С using the expressions below:

§ The aim of this text is to …

§ It is based on …

§ This text is intended to …

§ This text looks at/describes …

§ According to …

§ In the words of …

§ In general / On the whole / In the main

§ Interestingly/Curiously/Strangely /Surprisingly/It is interesting that …

§ To sum up / To summarize /In short

35. Fill in the gaps with the words from the box:

The problems of the 1___ use of natural resources, energy 2___, and protection of the environment, prevention of technological 3___, and risk management all require the active participation of engineers, with their particular knowledge and 4___.

So the goal of engineering education must be to train specialists within a holistic life paradigm, so that they can be 5___ citizens. The special role of engineer should be mentioned, because environmentally 6___ technologies and green chemistry could provide real help in the transition to sustainability.

Check your answers on p. 280.

36. Work in pairs, make as many questions as possible to review the contents of the text below and ask each other:

History of Drinking Water Disinfection

The link between water quality and health has been known since the early ages. Clear water was considered clean water. Swamp areas were associated with fever.

Disinfection has been applied for centuries. Two basic rules dating back to 2000 B.C. state that water must be exposed to sunlight and filtered with charcoal and that impure water must be purified by boiling the water and than dipping a piece of copper in the water seven times, before filtering the water. Descriptions of ancient civilisations were found about boiling water and water storage in silver jugs. To realize water purification copper, silver and electrolysis were applied.

Disinfection has been applied for several decades. However, the mechanism has been known for only one hundred years.

In 1680 Anthony van Leeuwenhoek developed the microscope. His discovery of microorganisms was considered a curiosity. It took scientists another two hundred years before they started using the microscope to distinguish microorganisms and other pathogens.

The first multiple filter was developed in 1685 by the Italian physician Lu Antonio Porzo. The filter consisted of a settling unit and a sandfiltration unit. In 1746 the French scientist Joseph Amy received the first patent for a filter design, which was applied in households by 1750.

The filters consisted of wool, sponges and charcoal. For the past centuries humans have suffered from diseases such as cholera and the plague. The origin of these diseases was misinterpreted. It was said that the diseases were a devine punishment or were caused by impure air or the alignment of the planets.

In 1854 a cholera epidemic caused many deaths in the city of London. John Snow, an English doctor, discovered that the cholera epidemic was caused by a contaminated water pump. He prevented a spread of the epidemic by closing down the contaminated water pump. After that scientists have performed bacteriological studies to research the development, existence and identification of microorganisms and the removal of microorganisms from drinking water.

In the nineteenth century the effect of disinfectants, such as chlorine, was discovered. Since 1900 disinfectants are largely applied by drinking water companies to prevent the distribution of diseases and to improve water quality.