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: Animals surviving for 20 min were immediately stressed by a swim test in 25 degrees C water , and death - producing tonic seizures were scored for 2 min .
### Output Text:
Animals surviving for 20 min were immediately stressed by a swim test in 25 degrees C water , and death - producing tonic seizures were scored for 2 min . 