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: Ketamine reduced both the area of brush - evoked and punctate - evoked hyperalgesia significantly and it tended to reduce brush - evoked pain .
### Output Text:
<span class="chemical">Ketamine</span> reduced both the area of brush - evoked and punctate - evoked hyperalgesia significantly and it tended to reduce brush - evoked pain . 