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 median ( IQR [ range ] ) time after surgery when the seizure occurred was 7 ( 6 - 12 [ 1 - 216 ] ) h and 8 ( 6 - 11 [ 4 - 18 ] ) h , respectively .
### Output Text:
The median ( IQR [ range ] ) time after surgery when the seizure occurred was 7 ( 6 - 12 [ 1 - 216 ] ) h and 8 ( 6 - 11 [ 4 - 18 ] ) h , respectively . 