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: Preparations containing lindane continue to be sold over the counter and may represent a hazard to poorly informed patients .
### Output Text:
Preparations containing <span class="chemical">lindane</span> continue to be sold over the counter and may represent a hazard to poorly informed patients . 