mhchem - bold part of equation typeset with \ce{}

An \textbf is a hard break inside a \ce. What goes inside \textbf's argument is not processed by the mhchem package.

This is the way to go.

\ce{NH3(g) + HCl(g) -> $\textbf{\ce{NH4Cl}}$(s)}

Use $ to indicate that you want to escape mhchem parsing (mhchem does the correct guessing that \textbf and the next {} belong together, but using $ is much clearer). Then use \textbf, then use \ce inside.

The $ part (or the \textbf for that matter) might interrupt the mhchem flow. The succeeding (s) works fine, here, but you might not be always so lucky (for instance, a $\textbf{4}$ would not be recognized as a number).

Try

• using \ce in math mode (this dose change the output when you use a text font that looks different from the math font, or in headings, as commented by @mhchem) and
• using \mathbf instead of \textbf

See this example:

\documentclass{article}
\usepackage[version=4]{mhchem}

\begin{document}
$\ce{NH3(g) + HCl(g) -> \mathbf{NH_4Cl}(s)}$
\ce{NH3(g) + HCl(g) -> NH_4Cl(s)}
\end{document}