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: Tests used to evaluate the changes in sexual behavior showed a significant decrease in sexual desire , sexual excitement phase ( erection ) , and frequency of sexual relations in the study group .
### Output Text:
Tests used to evaluate the changes in sexual behavior showed a significant decrease in sexual desire , sexual excitement phase ( erection ) , and frequency of sexual relations in the study group . 