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 operating conditions regarding bleeding were estimated in a double - blind manner , and did not differ significantly between the groups .
### Output Text:
The operating conditions regarding bleeding were estimated in a double - blind manner , and did not differ significantly between the groups . 