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: Pain recall after one week was similarly precise ( magnitude : p < 0 . 01 , duration : p < 0 . 05 ) .
### Output Text:
Pain recall after one week was similarly precise ( magnitude : p < 0 . 01 , duration : p < 0 . 05 ) . 