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: Theophylline serum protein binding determined by equilibrium dialysis was lower in pregnant rats .
### Output Text:
<span class="chemical">Theophylline</span> serum protein binding determined by equilibrium dialysis was lower in pregnant rats . 