\documentclass{ppex}
\nonstopmode
\iffalse *** Next code line only needed with older version
of the chemical macros if they produce slanted characters ***
\makeatletter \let \@@chemiechemiestyle\mathrm \makeatother 
\fi 
\usepackage {m-chemic}
\pagestyle{empty}
\setlength\textwidth{207.0pt}
\begin{document}
\ResetPreambleCommands
\ReadyForTheFray
\usepackage {m-chemic}
\[ \chemical{2H_2}{}
   \chemical{PLUS}{}
   \chemical{O_2}{}
   \chemical{GIVES}{}
   \chemical{2H_2O}{} \]
\end{document}
