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: Cocaine causes memory and learning impairments in rats : involvement of nuclear factor kappa B and oxidative stress , and prevention by topiramate .
### Output Text:
<span class="chemical">Cocaine</span> causes memory and learning impairments in rats : involvement of nuclear factor kappa B and oxidative stress , and prevention by <span class="chemical">topiramate</span> . 