Arsenic trioxide reacts with hydrogen sulfide to form arsenic trisulfide \( \text{As}_2\text{S}_3 \), which is a colloidal sol:
\[
\begin{align}
\text{As}_2\text{O}_3 + 3\text{H}_2\text{S} \rightarrow \text{As}_2\text{S}_3 + 3\text{H}_2\text{O}
\]
Arsenic trisulfide appears as a yellow precipitate and forms a sol in water.