I PRE-READING TASK (ANTICIPATION )
l. Look at the picture and try to explain the use of the steam bath.
2. How often do you use it in the laboratory?
For reactions which do not require heating above about 90°C, the steam bath is the heating source of choice as it is inexpensive to purchase and operate and very safe. A steam baths important limitation is that its maximum temperature is dictated by the boiling point of water.
A steam bath is much more useful than a free flame for heating low-boiling liquids. Any vapors which escape from the distilling apparatus simply mingle harmlessly with water vapor rather than igniting. Since the maximum temperature of steam at atmospheric pressure is lOO 0 C, boiling is often less vigorous than with a free flame. A real advantage to using a steam bath is that a reaction mixture which is somehow forgotten cannot heat above 100 o
C and usually does not char. line 10
A reflux apparatus on top of a steam bath is a convenient way to reflux low-boiling organic solvents. Boiling chips in the reaction mixture usually provide enough agitation to make additional stirring unnecessary. The distance between the bottom of the flask and the bottom of the steam bath makes magnetic stirring difficult. Overhead stirring is common in research laboratories but too expensive for most teaching laboratories.
The several concentric rings supplied with a steam bath facilitate proper placement of the flask. Remove as many rings as necessary for the reaction flask to rest on the ring which is only slightly smaller than the flask. Do not completely immerse the flask in the steam bath.
Adjust steam pressure so that the flask heats but very little steam escapes. Turning up the steam very high has little effect on the reaction vessel. line 20
Control the rate of reflux by varying steam pressure or by using a towel to keep in the heat.
Oil baths are particularly useful for heating reaction mixtures. The contact is intimate because hot oil completely surrounds the bottom and sides of a flask. This results in even heating and effective control. Oil baths are relatively inexpensive and are usually safe since:
1) lack of a flame makes ignition of solvent vapors unlikely; and
2) the temperature of a bath is proportional to the amount of heat put in and usually cannot rise above some maximum value. Charring is thus less likely with oil bath than with either an electric mantle or a bunsen burner. line 28
Problems associated with use of oil baths are:
1) they are often slow to heat;
2) when very hot, they fume and can catch fire; and
3) they cool slowly after use. In addition, the flask retains an oily residue, which not only is sloppy but can cause burns when hot. Despite these difficulties, oil baths are very useful for heating reaction mixtures.
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Adapted from " Experimental Organic Chemistry" by H.Dupont Durst & C. W'.Coekel
Figure 8 Oil bath http://www-users.york.ac.uk/~dks3/facilities_frame_middle_files/oilbath.JPG
(2012-09-19)
Figure 9 Steam bath http://www.chem.wisc.edu/areas/organic/orglab/images/tech/Steambath.jpg
(2012-09-19)
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II TEXT ORGANIZATION
Skim the text and find the main topic sentences and their supporting details.
Topic sentence
Supporting details
___________________________________________________________________________
III TEXT COMPREHENSION
Decide whether the following statements are true or false.
1.
A steam bath is less useful than a free flame for heating low-boiling liquids.
2. The maximum temperature of steam at atmospheric pressure is 90° C .
3. Overhead stirring is usual in research laboratories.
4. Oil baths are useless for heating reaction mixtures.
5. Oil baths heat and cool quickly.
6. Charring is very likely to happen with oil baths.
IV FUNCTION WORDS
Scan the text to find the function words which show the relationship of:
CONTRAST
ADDITION
CA
USE
-
EFFECT
_________
_______
_________
T
F
T F
T
F
T
F
T F
T
F
V VOCABULARY
Match the words in the left-hand column with their explanations the right-hand column.
Immerse
Purchase
Ignition
- to heat so that the vapours condense and return
- the act of buying something
- to put into liquid so that it is covered
Vigorous
Agitate
Reflux
- to stir or shake strongly
- energetic
- the process by which an engine is started
VI WORD FORMATION
Common prefixes and suffixes that give negative meaning to adjectives are:
-un, -less, -in, e.g. undesired, helpless, ineffective. Scan the text to find the opposites for these adjectives :
Expensive __________________________
Harmful
Useful
___________________________
___________________________
Necessary ___________________________
Likely ___________________________
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VII WORD FAMILIES
Complete the following table.
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