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: These seven cases demonstrate that procainamide can produce an acquired prolonged Q - T syndrome with polymorphous ventricular tachycardia .
### Output Text:
These seven cases demonstrate that <span class="chemical">procainamide</span> can produce an acquired prolonged Q - T syndrome with polymorphous ventricular tachycardia . 