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: INTRODUCTION : Confusion is an adverse drug reaction frequently observed with valproic acid .
### Output Text:
INTRODUCTION : Confusion is an adverse drug reaction frequently observed with <span class="chemical">valproic acid</span> . 