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: Valproic acid I : time course of lipid peroxidation biomarkers , liver toxicity , and valproic acid metabolite levels in rats .
### Output Text:
<span class="chemical">Valproic acid</span> I : time course of lipid peroxidation biomarkers , liver toxicity , and <span class="chemical">valproic acid</span> metabolite levels in rats . 