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: The intragastric pressure during muscle fasciculations was significantly higher in the control group ( 16 + / - 0 . 7 ( SEM ) cm H2O ) than in the alfentanil group ( 7 . 7 + / - 1 . 5 ( SEM ) cm H2O ) .
### Output Text:
The intragastric pressure during muscle fasciculations was significantly higher in the control group ( 16 + / - 0 . 7 ( SEM ) cm <span class="chemical">H2O</span> ) than in the <span class="chemical">alfentanil</span> group ( 7 . 7 + / - 1 . 5 ( SEM ) cm <span class="chemical">H2O</span> ) . 