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: Specific gravity was greater in the 15 / HTN group ( 1 . 043 + / - 0 . 002 ) versus the 90 / HTN ( 1 . 036 + / - 0 . 003 ) and control ( 1 . 037 + / - 0 . 003 ) groups .
### Output Text:
Specific gravity was greater in the 15 / HTN group ( 1 . 043 + / - 0 . 002 ) versus the 90 / HTN ( 1 . 036 + / - 0 . 003 ) and control ( 1 . 037 + / - 0 . 003 ) groups . 