CHEM 343 QUIZ 2
-- ✅✅ -★ As vapor pressure of the unknown liquid increases, the vapors will force
AIR out the open end of capillary tube in the form of BUBBLES ★
-- ✅✅-★ As you heat your sample, its vapor pressure will begin to INCREASE ★
- - ✅✅-★ At room temperature, the unknown liquid should have a vapor pressure
of LESS THAN 1 ATMOSPHERE (ATM) ★
-- ✅✅ -★ At some point, all AIR within the capillary will have been displaced by
force of the increasing VAPORS from the unknown liquid... after this point, the
bubbles you see are no longer AIR BUBBLES—they are now VAPOR BUBBLES,
which move FASTER ★
-- ✅✅-★ At some temperature, the vapor pressure inside the capillary will EQUAL
ATMOSPHERIC pressure (1 ATM)... this temperature is known as the BOILING
POINT of the unknown liquid ★
-- ✅✅ -★ Once you remove your apparatus from the hot plate, the stream of
bubbles will SLOW as the hot bath cools, and at some temperature, the vapor
pressure in the capillary tube will DROP BELOW 1 ATM... at the instant this
happens, a VACUUM is created in the capillary tube... as a result, some of the
unknown liquid will BE SUCKED UP into the capillary ★
-- ✅✅-✰ BOILING POINT ✰
• the temperature at which the VAPOR PRESSURE of the liquid equals the
ATMOSPHERIC PRESSURE
-- ✅✅-✰ MICROSCALE METHOD (SIWOLOBOFF'S METHOD) ✰
1) Take a small test tube, strap it to a thermometer with a rubber band, and put ≈4-5
drops of your unknown into the test tube
2) Insert a melting-point capillary tube into your test tube OPEN END DOWN into
your unknown
3) Clamp the thermometer so that the thermometer bulb, the test tube, and the
open-end-down melting-point capillary in it are ALL in a WATER bath on top of a
HOT PLATE
4) As you heat your unknown, you will notice the AIR in the CAPILLARY EXPAND,
and a very SLOW stream of BUBBLES will start to escape the CAPILLARY as the air
leaves it
5) At some point, the BUBBLES will stream FASTER as the LIQUID UNKNOWN
begins to BOIL
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