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: Median durations of response , time to next therapy and treatment - free interval were 8 , 11 . 2 , and 5 . 1 months , respectively .
### Output Text:
Median durations of response , time to next therapy and treatment - free interval were 8 , 11 . 2 , and 5 . 1 months , respectively . 