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: He bought 25 g of carbachol as pure substance in a pharmacy , and the father was administered 400 to 500 mg .
### Output Text:
He bought 25 g of <span class="chemical">carbachol</span> as pure substance in a pharmacy , and the father was administered 400 to 500 mg . 