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: Yellow phosphorus , a component of certain pesticide pastes and fireworks , is well known to cause hepatotoxicity .
### Output Text:
Yellow <span class="chemical">phosphorus</span> , a component of certain pesticide pastes and fireworks , is well known to cause hepatotoxicity . 