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: We aimed to test whether HCFCs 123 and 124 can result in serious liver disease .
### Output Text:
We aimed to test whether <span class="chemical">HCFCs 123 and 124</span> can result in serious liver disease . 