打化学式的艺术
$\xlongequal{\quad\triangle \quad}$
\xlongequal{\quad \triangle \quad}
$\xlongequal[ \triangle]{\quad MnO _2 \quad}$
\xlongequal[ \triangle]{\quad MnO_2 \quad}
$\ce{<=>}$
\ce{<=>}
$\ce{<=>>}$
\ce{<=>>}
$\downarrow$
\downarrow
$\uparrow$
\uparrow