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: In a randomized double - blind experiment performed in 2 occasions 7 days apart , 480 mg caffeine and a caffeine - free ( placebo ) solution were administered in a coffee form and anxiety scales were applied before and after each test .
### Output Text:
In a randomized double - blind experiment performed in 2 occasions 7 days apart , 480 mg <span class="chemical">caffeine</span> and a <span class="chemical">caffeine</span> - free ( placebo ) solution were administered in a coffee form and anxiety scales were applied before and after each test . 