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: During therapy the patients rated their pain on a visual analogue scale ( VAS ) and a verbal outcome scale .
### Output Text:
During therapy the patients rated their pain on a visual analogue scale ( VAS ) and a verbal outcome scale . 