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: Urinary concentration defect appeared frequent but the extent of the impairment is difficult to assess because of the uncertainty about the norms applicable to this group of patients .
### Output Text:
Urinary concentration defect appeared frequent but the extent of the impairment is difficult to assess because of the uncertainty about the norms applicable to this group of patients . 