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: In four patients hearing impairment on the side of the block was demonstrated after operation , in three measurements on the day of surgery and in one on the following day .
### Output Text:
In four patients hearing impairment on the side of the block was demonstrated after operation , in three measurements on the day of surgery and in one on the following day . 