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: However , the fractional excretion of uric acid was unchanged between the groups over the experimental period .
### Output Text:
However , the fractional excretion of <span class="chemical">uric acid</span> was unchanged between the groups over the experimental period . 