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: The severity of reactions ( minimal , mild , and moderate , respectively ) was as follows : group 1 - - 6 , 6 , and 2 ; group 2 - - 1 , 5 , and 0 ; group 3 - - 5 , 1 , and 0 .
### Output Text:
The severity of reactions ( minimal , mild , and moderate , respectively ) was as follows : group 1 - - 6 , 6 , and 2 ; group 2 - - 1 , 5 , and 0 ; group 3 - - 5 , 1 , and 0 . 