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: In a randomized , double - blind , placebo - controlled , crossover study , we studied 12 volunteers in three experiments .
### Output Text:
In a randomized , double - blind , placebo - controlled , crossover study , we studied 12 volunteers in three experiments . 