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: CONCLUSION : Milk - alkali syndrome may be a common cause of unexplained hypercalcemia and can be precipitated by small amounts of orally ingested calcium carbonate in susceptible persons .
### Output Text:
CONCLUSION : Milk - alkali syndrome may be a common cause of unexplained hypercalcemia and can be precipitated by small amounts of orally ingested <span class="chemical">calcium carbonate</span> in susceptible persons . 