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: CaCl2 and 4 - aminopyridine .
### Output Text:
<span class="chemical">CaCl2</span> and <span class="chemical">4 - aminopyridine</span> . 