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: Infants were categorized into 1 of 2 groups : those exposed to cocaine and those not exposed to cocaine .
### Output Text:
Infants were categorized into 1 of 2 groups : those exposed to <span class="chemical">cocaine</span> and those not exposed to <span class="chemical">cocaine</span> . 