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: PARTICIPANTS : Ten healthy elderly ( mean age 70 ) and 9 young ( mean age 33 ) volunteers .
### Output Text:
PARTICIPANTS : Ten healthy elderly ( mean age 70 ) and 9 young ( mean age 33 ) volunteers . 