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: Post - hoc t - tests revealed significant differences between placebo and ketamine for the amounts of words generated during lexical and semantic verbal fluency , while the phonological domain remained unaffected .
### Output Text:
Post - hoc t - tests revealed significant differences between placebo and <span class="chemical">ketamine</span> for the amounts of words generated during lexical and semantic verbal fluency , while the phonological domain remained unaffected . 