Descriptive Chemistry of Some Common Gases

There are certain miscellaneous facts on the properties of some common gases that one needs to
be aware of for the SAT Subject Test: Chemistry. These properties are exploited in qualitative tests
designed to detect their presence:

CO 2    (g) +   H 2 O   (l)     H 2 CO 3    (aq)

When carbon dioxide is passed through limewater, Ca(OH) 2 , the solution turns cloudy from the
formation of insoluble calcium carbonate:

CO 2    +   Ca(OH) 2    →   CaCO 3  +   H 2 O

The precipitation of calcium carbonate, however, does not go on indefinitely. As just mentioned,
water containing CO 2 is slightly acidic, and this causes the calcium carbonate to dissolve:

CaCO 3  (s) +   H 2 O   (l) +   CO 2    (g) →   Ca2+    (aq)    +   2HCO 3 –    (aq)

THINGS TO REMEMBER

Oxygen: molecular   oxygen, O 2 ,   is  a   reactant    in  combustion  reactions.  If  a   glowing splint  is

lowered into    a   test    tube    containing  oxygen, it  will    reignite.

Hydrogen:   when    ignited in  air,    hydrogen,   H 2 ,   burns   with    a   blue    flame.

Nitrogen:   N 2 ,   the largest component   of  air (a  little  less    than    80% by  volume),    is  relatively  inert.

Carbon  dioxide:    CO 2    gives   a   moderately  acidic  solution    when    dissolved   in  water   because of  the

reaction:

Measurement of  Gas Pressures

Boyle’s Law