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: No significant differences were observed between quetiapine and placebo on SAS and BARS scores .
### Output Text:
No significant differences were observed between <span class="chemical">quetiapine</span> and placebo on SAS and BARS scores . 