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: RATIONALE : Nicotine and caffeine are widely consumed licit psychoactive drugs worldwide .
### Output Text:
RATIONALE : <span class="chemical">Nicotine</span> and <span class="chemical">caffeine</span> are widely consumed licit psychoactive drugs worldwide . 