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: AIM : This study was carried out to determine the effect of injection duration on bruising and pain following the administration of the subcutaneous injection of heparin .
### Output Text:
AIM : This study was carried out to determine the effect of injection duration on bruising and pain following the administration of the subcutaneous injection of <span class="chemical">heparin</span> . 