In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: In this study naloxone reversed respiratory paralysis induced by thiopental in rats .
### Output Text:
In this study <span class="chemical">naloxone</span> reversed respiratory paralysis induced by <span class="chemical">thiopental</span> in rats . 