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: It has been proposed that modest changes in plasma potassium can alter the tendency towards cardiac arrhythmias .
### Output Text:
It has been proposed that modest changes in plasma <span class="chemical">potassium</span> can alter the tendency towards cardiac arrhythmias . 