# What is wrong with \ch{CaO$_{(s)} + H2O$_{(l)}->Ca$^{2+}_{(aq)} + 2 OH$^-_{(aq)}}

In the interest of providing a complete question, I have the following code:

\documentclass[10pt,a4paper]{article}
\usepackage[utf8]{inputenc}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{chemmacros}
\begin{document}
\ch{CaO$_{(s)} + H2O$_{(l)}->Ca$^{2+}_{(aq)} + 2 OH$^-_{(aq)}}
\end{document}

When compiled does it produces lots of error messages (it does still compile though) how can I fix this?

• I don't get an error with this code. But you seem to be using $to mean "the next symbol is mathy". It actually toggles between text-mode and math-mode (which is why H2O is italicized, but the rest isn't). What output are you trying to get? Generally, you want an entire math expression to be within one set of$. – Teepeemm Nov 12 '16 at 20:41

You're misunderstanding the purpose of $. In TeX,$ means "switch between math mode and text mode". So with your original expression, you have the text CaO, the math expression _{(s)} + H2O, the text expression _{(l)}->Ca, math ^{2+}_{(aq)} + 2 OH and text ^-_{(aq)}. (You can tell because the math characters are italicized.) This all gets confused because ^ and _ normally aren't allowed in text mode (which causes an error and forces you into math mode), but chemmacros changes that.

Because we're using chemmacros, we can use ^ and _ within \ch without bothering with \$, and everything works out fine (but we need spaces around the ->).

But chemmacros already knows about phases, and wants to help us out (see "The phases Module" in the documentation). We can simply have

\documentclass{article}
\usepackage{chemmacros}
\chemsetup[phases]{pos=sub}
\begin{document}
\ch{CaO\sld{} + H2O\lqd{} -> Ca^{2+}\aq{} + 2 OH^-\aq{}}
\end{document}

I suppose you want

\documentclass[10pt,a4paper]{article}
\usepackage[utf8]{inputenc}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{chemmacros}
\begin{document}

\ch{CaO_{(s)} + H2O_{(l)} -> Ca^{2+}_{(aq)} + 2 OH^-_{(aq)}}

\end{document}