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: The frequency of visual loss decreased after the concentration of the ethanol diluent was lowered .
### Output Text:
The frequency of visual loss decreased after the concentration of the <span class="chemical">ethanol</span> diluent was lowered . 