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: There were no differences in demographic data , growth , and socio - economic status between the two groups .
### Output Text:
There were no differences in demographic data , growth , and socio - economic status between the two groups . 