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: Anaesthesia was induced with thiopentone 5 mg kg - 1 .
### Output Text:
Anaesthesia was induced with <span class="chemical">thiopentone</span> 5 mg kg - 1 . 