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: Pethidine - associated seizure in a healthy adolescent receiving pethidine for postoperative pain control .
### Output Text:
<span class="chemical">Pethidine</span> - associated seizure in a healthy adolescent receiving <span class="chemical">pethidine</span> for postoperative pain control . 