Preparing common laboratory gases
Oxygen is prepared by decomposing hydrogen peroxide with manganese(IV) oxide as a catalyst. Hydrogen is prepared by reacting a reactive metal (like zinc) with a dilute acid. Carbon dioxide is prepared by reacting a carbonate (like marble chips) with dilute hydrochloric acid. Ammonia is prepared by gently heating an ammonium salt with a base like calcium hydroxide.
Example 1: Write the word equation for preparing carbon dioxide from marble chips and dilute hydrochloric acid.
Calcium carbonate + hydrochloric acid calcium chloride + water + carbon dioxide.
Testing for gases
Each common gas has a simple, reliable test: a glowing splint relights in oxygen; a lit splint produces a "pop" sound in hydrogen; limewater turns milky (cloudy) when carbon dioxide is bubbled through it; damp red litmus paper turns blue in ammonia (confirming an alkaline gas); and damp blue litmus paper turns red then bleaches white in chlorine.
Example 2: A gas turns limewater milky. Identify the gas and explain the chemistry behind the test.
The gas is carbon dioxide. It reacts with the calcium hydroxide dissolved in limewater to form insoluble calcium carbonate, which is the fine white solid that makes the limewater look milky/cloudy.
General trends among non-metals
Non-metals typically have low melting and boiling points (compared to metals), are poor conductors of heat and electricity (except graphite), and form acidic or neutral oxides when burned in oxygen -- in contrast to metals, whose oxides are typically basic.