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: Twenty - nine burn patients , each with two similar burns which could be separately treated , were given pairs of treatments at successive 12 - h intervals over a 3 - day period .
### Output Text:
Twenty - nine burn patients , each with two similar burns which could be separately treated , were given pairs of treatments at successive 12 - h intervals over a 3 - day period . 