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: Although intravenous caffeine is generally well tolerated , the clinician should be aware of the potential for unpredictable and serious ventricular arrhythmias .
### Output Text:
Although intravenous <span class="chemical">caffeine</span> is generally well tolerated , the clinician should be aware of the potential for unpredictable and serious ventricular arrhythmias . 