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: Acute changes of blood ammonia may predict short - term adverse effects of valproic acid .
### Output Text:
Acute changes of blood <span class="chemical">ammonia</span> may predict short - term adverse effects of <span class="chemical">valproic acid</span> . 