Chemistry Unit 2

English for Chemistry - Unit 2

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UNIT 2

(A) DESCRIPTIONS

GENERAL AND SPECIFIC DESCRIPTIONS

As mentioned in Unit 1, a definition states what an object is and what it is used for. A description goes one step further. A description includes a definition plus it tells the reader, in detail, what the object looks like.

An accurate Technical Definition includes a physical description. Physical Descriptions tell the reader, in detail, what the object looks like. In order to make physical descriptions, you need to handle shapes, measures, and dimensions accurately.

There are two types of descriptions: General Descriptions describe a category of objects, such as cars. Specific Descriptions describe one specific item, such as a particular model of car.

General Descriptions There are four steps involved in writing a general description:

1. Write a clear definition. A general description should always start with a good definition. There is no point in telling the reader what something looks like if he does not know what it is.

2. State the shape, if it was not included in the definition. Descriptions of objects sometimes state the shape as part of the definition, as in, ''A can is a cylindrical container that is used ...''. If the shape has not been included in the definition, it should be stated next.

3. State what material(s) it can be made of, if this was not included in the definition. If the definition does not state what the object is made of, this is the time to do so. If the item being described can be made of different materials, list the most common ones, as in ''cans are usually made of steel or aluminium''.

4. Give typical dimensions for the object. Here is a general description of a can. The different parts of the description are indicated in brackets.

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(Definition) Cans are (shape) cylindrical containers that can be used to store and preserve food.They are also used to hold various other substances such as paint or grease. (Materials) They are usually made of steel or aluminium. (Typical dimensions) Cans come in a variety of sizes. For example, a typical soup can has a liquid capacity of 285 mL and a standard paint can has a liquid capacity of 4.55 L. ------------------------------------------------------------------------------------------


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Specific Descriptions It is easier to write a specific description than a general description because you are dealing with exact details. The steps to be followed when writing specific descriptions differ somewhat from those for general descriptions.

1.- A specific description should include a narrower definition than a general description: - A general description is written for someone who probably does not know what the object is.

- A specific description is written for someone familiar with the class of objects, but not with the specific model you are describing. The two definitions that follow illustrate this difference.

e.g.: An automobile is a vehicle that is used to transport people along roads. (general)

e.g.:The Model X is a conventional vehicle with a front-mounted engine and rear wheel drive. (specific)

2.- In a specific description you state the shape of the specific object you are describing, which may or may not be typical. In a general description you state the typical shape of the object being described. For example, most pens are cylindrical, so a general description would say that pens are cylindrical. However, in a specific description, you might have to describe a pen that was four-sided or pyramid shaped.

3.- In a specific description you state what the specific model you are describing is made of. In a general description you state what material(s) the object is commonly made of. For example, a table may be made of wood, plastic, glass or metal. In a specific description, you must be as specific as possible, saying, for example, that a particular table is made of pine wood.

4.- In a specific description you may give exact, not just typical dimensions.

EXERCISE ON DESCRIPTIONS:

The sentences below give you a lot of information about septic tanks. Some of the sentences refer to septic tanks in general. Others refer to a specific model, the SR2.

a) Using the information contained in the sentences that follow, write a general description of septic tanks. They come in a variety of shapes and sizes. Therefore the only dimensions that your ddescription should give is the capacity of a typical tank.


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b) Using the information from the sentences write a specific description of the SR2 septic tank. Remember, since this is a specific description your definition will not say what a septic tank is. It will state what kind of septic tank the SR2 model is.

Septic Tanks:

1. The SR2 is a rectangular tank.

2. Tightly sealed covered manholes provide access to the inlet and outlet devices in a septic tank

3. The SR2 has a capacity of 3.400 litres.

4. A septic tank is a watertight receptacle.

5. Septic tanks come in a number of shapes. The most common are horizontal or vertical cylindrical tanks and horizontal rectangular tanks.

6. A septic tank for a four bedroom house should have a capacity of at least 4.500 litres.

7. Septic tanks commonly have one or two compartments. A tank with one compartment is called a single compartment tank. A tank with two compartments is called a two compartment tank.

8. The outlet device must retain scum in the tank.

9. The purpose of a septic tank is to separate solid waste from liquid sewage. The solids are stored in the tank until there is sufficiently broken-down to be discharged for final disposal.

10. Septic tanks are generally made of pre-cast concrete or welded sheet steel.

11. The inlet device must divert the incoming sewage downwards.

12. The SR2 is made entirely of pre-cast concrete.

13. The outlet device may consist of a vented tee or baffle and an outlet pipe.

14. The inlet device may consist of an inlet pipe and a vented tee or baffle.

15. A septic tank should not be closer than 1.5 m to the foundation of a building.

16. In a gravity-feed system, the outlet pipe must be several centimetres lower than the inlet pipe.

17. A septic tank must have an inlet device at one end and an outlet device at the other end.

18. The SR2 is a single compartment tank.


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Text: Concentration and Dehydration of Fruit Juices Concentrated fruit juices are produced in very large quantity. There are two main advantages to concentration of fruit juices by removing between 60 and 99.5 percent of the water present: (1) a great economy in transportation and storage costs resulting from simple reduction in the volume and weight of the juice and (2) juice stability: a concentrated juice is more resistant to degradation of various kinds during storage than fresh juice under similar conditions.

A concentrated juice is reconstituted by the addition of cold water in the proper amount. Since the aim of juice concentration is to provide a reconstituted product that tastes and appears as much as possible like the original fresh juice, a juice-concentration process should remove water selectively. Ideally, components other than water should not be lost from the concentrate during processing, and no component should undergo chemical or biochemical change. This is a difficult goal to meet, in view of the fact that fruit juices are complex mixtures containing many substances.

Apple juice, for example, contains about 14 weight percent dissolved substances in the fresh juice. The most prominent dissolved species are sugars; apple juice contains 4 to 8% levulose, 1 to 2% dextrose, and 2 to 4% sucrose. Also present in apple juice are malic acid and lesser amounts of other acids, along with tannins, pectins, enzymes, and other substances. The taste and aroma of a juice reflect the synergistic contributions of a vast number of volatile compounds present in the juice, which have been identified in the vapor given off by apple juice through flame-ionization gas chromatography, mass spectrometry, and other techniques.

Orange juice contains about 12 percent dissolved substances and about 0,5 percent suspended material; 5 to 10 percent sugars are present. Sucrose is the most prominent sugar, levulose and dextrose being present to lesser extents. The most prominent acid is citric acid (about 1 percent). Numerous other nonvolatile components are present (pectins, glycosides, pentosans, proteins, etc.), along with a large number of volatile compounds. d-Limonene is the one compound which has been most directly related to


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characteristic orange aroma, although other compounds have also been shown to be prominent and important. Several hundred compounds have been identified in all.

The most common process for fruit-juice concentration is evaporation. Since the sugars and other heavier dissolved solids are all much less volatile than water, evaporation was a logical choice. It is a well-known and well-developed process and simple to carry out. Steam costs have always been reduced in practice through the use of multieffect evaporation. Despite the fact that evaporation is far and away the most common process, it has several problems:

1. Fruit juices have substantial thermal sensitivity and develop off-flavor and/or off-color when held at too high temperature for too long a time. Most berry and fruit juices can be kept 2 or 3 h at 328 K without detectable flavor change. At higher temperatures the time is much less, typically under 1 min at 367 K and about 1 s at 389 K. Vitamin C in citrus juices is similarly heat-sensitive.

2. Again because of the thermal sensitivity of juices, there is a strong tendency toward fouling of heat-transfer surfaces (buildup of a semisolid layer next to the surface) in evaporators. This fouling reduces the heat-transfer coefficient across the evaporator surface and accentuates tendencies toward off-flavor because of the long residence time of the fouling layer.

3. The volatile flavor and aroma compounds escape readily from the juice during evaporation, causing a flat lifeless taste.

Approaches to dealing with these problems have followed two paths: improvement of evaporation processes and development of other kinds of separation processes.

Considering improvement of evaporation processes first, the most obvious approach toward overcoming the problem of too high a temperature for too long a time is vacuum evaporation. When the evaporation is carried out under reduced pressure, the boiling point of the juice occurs at a lower temperature and there is less thermal degradation. Another approach is to reduce the residence time of the juice in the evaporator as much


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as possible and to make the residence time of different elements of juice as uniform as possible. For this purpose a high heat-transfer surface-to-volume ratio is required, along with high heat-transfer coefficients and an avoidance of pockets or corners giving a long residence time for some of the juice. Turbulent flow in low-diameter tubes gives a relatively uniform velocity distribution and a high rate of heat transfer into the juice, keeping the residence time small. In such an evaporator, condensing steam outside the tubes supplies the heat for evaporation.

Another way to obtain rapid heating and minimum residence time is to preheat the juice by direct injection of steam. The steam for this purpose must be clean, however. Rapid heating in evaporators can give conditions approaching those which are needed in any event of pasteurization.

The fouling problem can be minimized by clever evaporator design. A number of different approaches have suggested radio-frequency heating as a means of avoiding heat-transfer surfaces altogether during the later phases of evaporation.

The third problem in evaporation, that of the loss of volatile flavor and aroma species, is the result of using a separation process that does not provide the desired division of the many components present in fruit juice into the two products. For good product quality the volatile flavor and aroma species should remain with the juice-concentrate product, but instead they leave with the water vapor. Several approaches have been used for coping with this problem:

1. Adding fresh juice (called cutback) to the concentrate. 2. Obtaining flavor material from peels, cores, etc., and adding it to the

concentrate.

3. Separating the volatile flavor and aroma compounds from the water vapor and returning them to the concentrate.

4. Accomplishing the juice concentration by some process other than evaporation.

[taken from: King, C. Separation Processes. Mc Graw-Hill Inc.US]


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Exercises: Concentration and Dehydratation of Fruit Juices

Comprehension Skills: Reading comprehension exercise: Answer the following questions from the text:

1) How can a concentrated juice be reconstituted?

2) a) What should never happen to the concentrate during processing?

b) Should components suffer any change?

3) What other substances apart from malic acid can be found in apple juice?

4) Is d-Limonene the only compound present in orange juice?

5) Why has evaporation been preferently chosen as the most common process of fruit juice concentration?

6) How have chemists dealt with problems related to the evaporation process? What ways have they followed?


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7) How can you explain the importance of turbulent flow in low-diameter tubes

Rephrasing Skills

Rephrase the following sentences from the text. Make any necessary changes without

losing the original meaning.

1) A wide variety of volatile compounds have been identified to blend and give its good smell and right flavour to apple juice.

2) Due to changes in temperature and time of residence, fruit juices can modify their aroma and taste.

3) A different way to take is that of decreasing to a minimum the period of time dedicated to the juice when it is in the evaporator.

4) If you heat in advance the juice by means of a straight jet of vapour, you will get faster heating and reduce completely the time stage.


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5) In order to obtain an excellent produce, the volatile taste and good smell varieties should rest with the fruit juice concentrate.

Vocabulary Skills: Explain the following terms in your own words.

Cutback:

Fouling:

Volatile:

Evaporation:

Residence Time:

Grammar Skills: Multiple choice: Choose the correct answer/answers from those given below:

Alternative Water-Removal Processes

More possibilities for alternative water-removal processes ___(1)___ by a form

of morphological analysis. ___(2)___ water is the major component in a fruit juice, it makes sense for a separation process ___(3)___ the water ___(3)___ the juice solutes. It is very ___(4)___ that the alternative approach of removing everything else from the

water could be sufficiently selective. If water is to be removed from the feed mixture

___(5)___ that the water product ___(6)___ another phase, immiscible with the feed

(equilibration processes) ___(6)___ that it ___(6)___ from the feed by a barrier (rate-

governed processes). ___(7)___, the chemical potential or activity of water in this

product ___(8)___ that in the feed juice for transport of water into the second phase or across the barrier to take place. [taken from: King, C.Separation Processes. Mc Graw-Hill Inc.U.S]


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1. a) may be produced 5. a) is subject to b) can be generated b) it is necessary

c) may be yield c) it is bound to

2. a) even although 6. a) be or be separated

b) although b) is or is divided

c) even though c) is or be split

3. a) to remove from 7. a) In either case

b) to eliminate from b) In both cases

c) to transfer since c) In each case

4. a) unprobably 8. a) must be lower than

b) unliable b) should be lower that

c) unlikely c) has to be lower than

Writing Skills: Rewrite paragraphs 1,2,3, giving a brief description of those problems dealing with evaporation.

(B) PHYSICAL DESCRIPTIONS: SHAPES. MEASURES. DIMENSIONS

SHAPES

An accurate Technical Definition includes a physical description. In order to make Physical Descriptions you need to handle shapes, measures, and dimensions accurately.

The following chart lists most of the shapes you will need for writing descriptions in both their noun and adjective form:


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When the object described has no recognized geometric shape but does resemble a well-known object or a letter of the alphabet, it may be described in one of the following ways, i.e., an H-shaped antenna, a saw-tooth wave, etc.

MEASURES

The unit, standard or system used in stating size, quantity, or degree is called measure. Many pieces of apparatus are used for measuring. The glassware used in laboratories will often have units of volume marked on it. These are the same units that are used to measure the volumes of everyday things like medicines, liquids, and soft drinks.The units usually used are:

the litre, which has the symbol l

the millilitre, which has the symbol ml


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Two other units that are often used in chemistry are:

the cubic decimetre, which has the symbol dm

the cubic centimetre, which has the symbol cm

Beakers and conical flasks may have either cubic centimetres or millilitres printed on them. The beakers in Figure 1 both hold the same volume.

Measuring volume You can measure the volume of a liquid using a measuring cylinder. This is a glass container marked or graduated in either cm or ml. But there is a problem. When a liquid is poured into a narrow tube, its surface is not flat, but curved. This curve is called meniscus.

When you read the volume of a liquid in a measuring cylinder you must follow three rules:

1 Put the measuring cylinder on a flat surface

2 Have your eyes at the same level as the surface of the liquid

3 Take the reading from the bottom part of the meniscus

The burette and the pipette You can measure out a liquid more exactly using a burette or a pipette. Figure 3A shows a burette. When the tap at the bottom is opened, the liquid will run out slowly. By noting the change in level of the liquid, you can tell how much has run out.


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Figure 3A Figure 3B

Figure 3B shows a pipette. Liquid is sucked into the pipette, using a safety filler until the bottom of the meniscus just touches the line. You then know the exact volume of the liquid. For example, the pipette in figure 3A holds exactly 25 cm when it is filled. The liquid is then run out into a beaker or flask.

The thermometer

The thermometer is an instrument that measures temperature. Scientific thermometers are marked or graduated using the Celsius scale. At normal atmospheric pressure, the freezing point of pure water is 0 degrees Celsius on this scale. Its boiling point is 100 degrees Celsius. These are the two fixed points of the scale and there are 100 degrees in between.

Whenever you use a thermometer, remember to follow these rules:

Insert the right verb for each rule

1. __________ the thermometer carefully. It can easily roll off the bench and break.

2. __________ the scale first and make sure that you can read and understand it.

3. __________ the bulb of the thermometer in the substance while you are reading the temperature.

4. The mercury level will fall on its own whenever the thermometer is removed from the substance. Do not __________ it under the tap.

Weighing

The amount of a substance is called its mass. Mass is measured in these units:

the kilogram, which has the symbol

the gram, which has the symbol g


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The mass of an object is usually measured on an instrument called a top-loading balance. Look at Figure 4. The number in the little window shows the mass of the beaker, to one decimal place. It is 48.5 g. Now look at the black bar beside the number. Its top edge meets the diagonal scale at 6, so 6 is the second place of decimals. The full reading is _______ g.

Top-loading balances are very sensitive and must be placed on a firm bench away from draughts.

Figure 4

Weighing Out Powder Substances:

This sounds easy, but make sure you do it properly. First, you must weigh the test tube or beaker that will hold the powder. Next, you must take the powder from the bottle carefully, without spilling any. Then, when the powder is in the test tube or beaker, you must weigh both together, and find the mass of the powder by substraction.

Set your results out neatly, as in the example below:

Example Mass of beaker = 48.56 g Mass of beaker + powder = 72.06 g Mass of powder = 72.06 g 48.56 g = 23.50 g

DIMENSIONS

Dimensions mean measurements of any sort, i.e., figures about height, length, breadth, thickness, depth, etc. In order to make good descriptions you need to handle dimensions, measures, and shapes accurately. Here you have some examples of how to state dimensions correctly:


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Length

The bar is three metres long. The bar is 3 m. long. The length of the bar is three metres.

Width

The board is twenty centimetres wide. The board is 20 cm. wide. The width of the board is 20 centimetres.

Height

The pole is 20 metres high. The height of the pole is 20 m.

Depth

The trough is 50 cm deep. The depth of the trough is 50 centimetres.

Thickness

The board is two centimetres thick. The thickness of the board is 2 cm.

Diameter

The tube is six centimetres in diameter. The diameter of the tube is 6 cm.

Area (length x width) The room is 56 m in area. The area of the room is fifty-six square metres. Volume (length x width x height)

The crate is thirty-six thousand cubic centimetres in volume. The volume of the crate is 36,000 cm.


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Capacity (length x width x height of interior)

The container has a capacity of 24,336 cm. The capacity of the container is twenty-four thousand, three hundred and thirty-six centimetres. The capacity of the container is 24,336 cm.

Weight

The box weighs five kilograms. The weight of the box is 5 kg.

Density

The density of the woodlog is 12 g/cm. The woodlog has a density of twelve grams per cubic centimetre.

Similarities:

EXERCISE ON DIMENSIONS Each of the following sentences is written incorrectly. Based on the previous information, find the errors and rewrite the sentences correctly:

1. The surface is 30 cm. long x 20 cm. wide. (e.g.: The area of the surface is 600 cm. squared)

2. The base is a capacity of 1800 cm.

SQUARE (metres)

CUBIC (metres)

IN (preposition)

LENGHT WIDTH HEIGHT DEPTH THICKNESS DIAMETER X AREA X X VOLUME X X CAPACITY X WEIGHT DENSITY X


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3. The sheet of steel must be at least 0.02 mm. thicker.

4. The width of the beam is 11 cm. wide.

5. The deep of the building is 50 m.

6. The volume of the cage is 60 cm. x 20 cm.

7. The high of the bridge is two metres.

8. The steel rod is 2.5 cm. diameter.

9. The stone is 5 kg. weight.

10. The track is 7.8 m. in long.

Appendix: (Grammar in use) CONDITIONAL SENTENCES:

Possibility / Probability (Type 1)

The use of The Condition is the commonest way of showing that one event is dependent in some way on another event taking place.

Here, we shall deal only with conditional sentences Type 1 (Possibility / Probability), i.e.:

IF + PRESENT, + FUTURE

IF + PRESENT, + PRESENT (open condition)

IF + PRESENT, + MAY & INFINITIVE

e.g.: If it rains, the streets will get wet.

e.g.: The fission fragments are highly radioactive, if they are not remove periodically.

Note that both the -if- clause and the main clause can invert position.


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There are several other forms of dealing with The Condition, as for example, by starting the - If - clause using a different word such as: Unless (if not); Should; In the event of // In case( of); Provided that; The+comparative structure.

1. UNLESS + SUBJECT + PRESENT + MAIN CLAUSE (with affirmative verb)

e.g.: If the water is pure, it will not need further treatment.

Unless the water is pure, it will need further treatment.

2. SHOULD + SUBJECT + INFINITIVE + MAIN CLAUSE

e.g.: If the temperature drops, we will feel much colder

Should the temperature drop, we will feel much colder

If the temperature falls, condensation of the steam will result.

Should the temperature fall, condensation of the steam will result.

3. IN THE EVENT OF // IN CASE (OF) + SUBJECT + MAIN CLAUSE

e.g.: In the event of fire, all workers will leave the building.

In case the fire spreads to the chemicals, all workers will leave the building.

4. PROVIDED THAT + SUBJECT + PRESENT + MAIN CLAUSE

e.g.: Provided that the cost is reasonable, the design will be accepted.

5. THE + COMPARATIVE .. THE + COMPARATIVE

e.g.: The colder the water flowing into the pump, the longer the pump will take to be heated.

If / When / Once

-If- can be substituted by When or Once without any change, neither structurally nor grammatically.


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The Use of whether..

Whether means If but its use is restricted to a) indirect questions and b) correlations.

-Whether- is widely used in academic and technical writing, while -If- is preferred when speaking.

a) indirect questions:

e.g.: We wonder whether the new power plant is large enough.

e.g.: I am in doubt whether to give this plan my approval.

b) correlations:

whether or

e.g.: Canals, whether lined or unlined, are often useful and necessary.

whether or not

e.g.: She has not decided whether to apply for a new job or not.

e.g.: The importance of water will depend on local conditions, such as the

existence of large industries, and whether or not these industries use

public waterworks.

Complete these statements with the correct form of the verb:

1. If the turbine speed (increase) ,the governor automatically (come) into operation. 2. If the supply of coolant (fail) , emergency controls (operate) immediately.

3. If the nucleus (contain) an excess of neutrons, one or more of them (be converted) into protons.

4. Neutrons (be admitted) , if the uranium (be fissioned) . 5. The cylinder temperature (rise) , if the quantity of steam flowing through the

cylinders (be increased). 6. Unless the steam (be superheated) , higher pressures (be) necessary. 7. If an indicator (be fitted) , the pressure at any part of the stroke (be measured

/may). 8. If current (be passed) through a solenoid, a magnetic field (be set up).


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9. If no external forces (act) on a system, the momentum of the system (remain) constant.

10. Harmful radiations (result) unless the isotopes (be shielded) properly. 11. A sudden loss of lift (be experienced) , if the aircraft speed (fall) below a certain

level. 12. The conveyor belt (be) liable to slip off the drive, if it (strech). [taken from: Herbert, A., The Structure of Technical English, England, Longman]

INSTRUCTIONS:

Whenever you give an order or an instruction, the verb used is the imperative mood, that is, the infinitive without the preposition to. The imperative, basically, expresses a command or a request. The imperative has only two persons: the second person which covers both the singular and the plural, and the first person plural. The second person affirmative is formed using the infinitive without to and without a subject:

eg: Handle the thermometer very carefully

The negative is formed with the auxiliary do :

eg. Don't cool the thermometer under water

The first person plural is formed with the verb let followed by the personal pronoun us and the main verb:

eg. Let's use a solvent (let us)

There are also other ways to give instructions: with modal verbs should and must either using a Personal Construction (You) , or an Impersonal one with Passive Infinitive:

eg: You should keep the valve opened The valve should be kept opened

e.g.: Industries should not threaten to pollute air, water and soil Air, water and soil should not be threaten by industries

Exercise: The following instructions can be transcribed using the passive infinitive with 'should':

1. During reactor loading, add solids before liquids.

2. Use a solvent with a low vapour pressure.


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3. Employ a hopper specific to your solids.

4. During reactor operations, apply a vapour recycling system, if necessary.

5. Install gaskets on all vessel openings.

6. Use statistical process control (SPC) to regulate reactions.

7. Try to allow the reactor to cool as much as possible.

Instructions + The Condition

Whenever Instructions are given, there are things you must do, and things you must not do. We will refer to them as Conditional Instructions. The pattern to follow consists of two clauses (sentences): The conditional sentence comes first, this followed by the order, command, or request sentence. The following examples give us a precise idea about what has just been said:

e.g.: If the cylinder head is hot, wait until it cools.

e.g.: If you are checking your car batteries, dont smoke.

Now fill in the blanks with the most apropriate verb from those shown in the list below:

(dont use / tap / refrigerate / handle / try / provide)

1.- If the cost of laboratory equipment is high, please ------------ it very carefully.

2.- If the cylinder head is very loose, ------------ it with a hammer.

3.- If you need to keep food fresh for a long time, ---------------- it, so that it remains fit to eat.

4.- If the out-door temperature rises over 40 C, ---------------- to maintain a comfortable temperature in-door by regulating the air-conditioning adequately.

5.- On the other hand, if out-door weather is too cold, --------------- a warm environment in-door by installing a central heating system.

6.- If the engine is hot, --------------- cold water to flush the system.


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Different Uses of SHOULD: This auxiliary verb form is used very often in technical writing, with several slightly different meanings:

1. Instructions to operators, employees, etc.

These machines should be handled with great care.

Safety precautions should be observed at all times.

The results of the experiment should be plotted on a graph.

( N. B. This is sometimes used for politeness when must be is really meant )

2. Specifications (what is required of something)

The steel should not contain more than 0.5% of carbon.

The maximum internal diameter should be 40 thousandths of an inch.

3. Expectations (what is expected to happen)

The process of cooling should continue for several hours.

This building should be completed by the end of next year.

EXERCISE:

1.This experiment (..give..) us the answer to the problem.

2. Smoking (..permit..) within 50 yards of the store.

3. High tensile steels (..temper..) up to 600 C.

4. The new reactor (..be..) in operation by 2012.

5. A flux (..apply..) to the heated metal to prevent oxidation.


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6. The results of the experiment (..write..) up carefully.

7. The heated metal (..allow..) to cool slowly over a long period.

8. Construction workers (..wear..) safety helmets at all times.

REWRITE this passage, using should with passive forms instead of the imperative form.

Fill a test-tube half full of water and heat it nearly to boiling point. Support the tube on a stand and allow it to cool. Take the temperature every minute. Stir carefully with a glass rod. Record the readings you obtain, and plot them on a graph of temperature against time. Repeat this with a tube half-full of crystals. Allow the solid to melt. Heat the liquid to 100 C, fix the tube on the stand and allow it to cool. Record the results as before and plot them.

Chemistry Unit 2

Documents

descriptions general

general descriptions

specific descriptions

descriptions of objects

physical descriptions

definition cans

types of descriptions

narrower definition