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: When QTc interval prolongation is observed , early magnesium supplementation is advocated .
### Output Text:
When QTc interval prolongation is observed , early <span class="chemical">magnesium</span> supplementation is advocated . 