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: Previous reports have suggested that pain associated with the injection of lidocaine is related to the acidic pH of the solution .
### Output Text:
Previous reports have suggested that pain associated with the injection of <span class="chemical">lidocaine</span> is related to the acidic pH of the solution . 