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: For comparison , tests were conducted in two acute pain models , the hotplate and warm water tail - withdrawal procedures .
### Output Text:
For comparison , tests were conducted in two acute pain models , the hotplate and warm water tail - withdrawal procedures . 