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: The average temporal delay between papaverine and the onset of an adverse BAEP change was 5 min .
### Output Text:
The average temporal delay between <span class="chemical">papaverine</span> and the onset of an adverse BAEP change was 5 min . 