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: CONCLUSION : This work shows that confusion with valproic acid is a serious , rather frequent but reversible adverse drug reaction .
### Output Text:
CONCLUSION : This work shows that confusion with <span class="chemical">valproic acid</span> is a serious , rather frequent but reversible adverse drug reaction . 