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