= Solution
\b[<Oxygen> can be obtained by decomposing an oxygen-rich <chemical compound>, or by separating it from <air>.] For the historical laboratory, heating <potassium chlorate>, heating certain <metal oxides>, and <electrolysis> of <water> are useful examples. The substance collected is <molecular oxygen>, $\mathrm O_2$.
A common preparation heats <potassium chlorate>, often with <manganese dioxide> as a <catalyst>:
$$
\boxed{2\mathrm{KClO}_3\xrightarrow{\Delta}2\mathrm{KCl}+3\mathrm O_2.}
$$
A heat-resistant vessel and delivery tube conduct the evolved <gas> to an inverted collecting vessel over <water>, after the initial displaced <air> has been excluded. <Oxygen> is only slightly soluble in <water>, making this collection principle useful. The <manganese dioxide> changes the reaction rate and required heating conditions; it is not the net source of the <oxygen> in this catalysed balance. Without the <catalyst>, <potassium perchlorate> can occur as an intermediate.
Two older preparations use heated <metal oxides>:
$$
\boxed{2\mathrm{HgO}\xrightarrow{\Delta}2\mathrm{Hg}+\mathrm O_2,}
$$
$$
\boxed{4\mathrm{MnO}_2\xrightarrow{\text{strong heating}}2\mathrm{Mn}_2\mathrm O_3+\mathrm O_2.}
$$
The first explains the historical decomposition of <mercury(II) oxide>, with metallic <Mercury> deposited on cooler parts of the apparatus. It is historical chemistry, not a preferred present-day teaching preparation. The second is one stage in the reduction of <manganese dioxide>; at still higher <temperature> further oxygen loss can give $\mathrm{Mn}_3\mathrm O_4$, so the oxide residue depends on the conditions. Strong heating of <potassium nitrate> provides another net balance:
$$
2\mathrm{KNO}_3\longrightarrow2\mathrm{KNO}_2+\mathrm O_2.
$$
In <electrolysis>, suitable separated <electrodes> collect <oxygen> at the <anode> and <hydrogen> at the <cathode>. With a suitable electrolyte and electrodes, the net <chemical equation> is
$$
\boxed{2\mathrm H_2\mathrm O\longrightarrow2\mathrm H_2+\mathrm O_2.}
$$
The gases must be kept in separate collecting spaces if the experiment is to demonstrate their separate identities and the approximately $2:1$ <volume> ratio. Choice of electrolyte matters: a chloride-containing electrolyte can give <chlorine> instead of clean <oxygen> evolution.
For a modern small-scale comparison, catalytic decomposition of <hydrogen peroxide> gives
$$
2\mathrm H_2\mathrm O_2\longrightarrow2\mathrm H_2\mathrm O+\mathrm O_2.
$$
This is a different feedstock from <potassium chlorate>. Large-scale separation of <oxygen> from <air> by cryogenic fractionation belongs to later industrial practice and should not be substituted for an 1866 laboratory answer. A collected sample can be tested by its vigorous support of <combustion>, conventionally the relighting of a glowing splint; this observation identifies a property, while the preparation and exclusion of interfering gases establish the sample's provenance.
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