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: CONCLUSIONS : Cocaine and ethanol in combination were more toxic than either substance alone .
### Output Text:
CONCLUSIONS : <span class="chemical">Cocaine</span> and <span class="chemical">ethanol</span> in combination were more toxic than either substance alone . 