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: Using functional imaging and a face - learning task , we investigated neural correlates of encoding and recalling face - name associations in 20 recreational drug users whose predominant drug use was ecstasy and 20 controls .
### Output Text:
Using functional imaging and a face - learning task , we investigated neural correlates of encoding and recalling face - name associations in 20 recreational drug users whose predominant drug use was <span class="chemical">ecstasy</span> and 20 controls . 