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: Hexabrix and Vasurix polyvidone are considered the best contrast media for hysterosalpingography and perhaps because of its low toxicity Hexabrix should be preferred .
### Output Text:
<span class="chemical">Hexabrix</span> and <span class="chemical">Vasurix polyvidone</span> are considered the best <span class="chemical">contrast media</span> for hysterosalpingography and perhaps because of its low toxicity <span class="chemical">Hexabrix</span> should be preferred . 