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: Steroid structure and pharmacological properties determine the anti - amnesic effects of pregnenolone sulphate in the passive avoidance task in rats .
### Output Text:
<span class="chemical">Steroid</span> structure and pharmacological properties determine the anti - amnesic effects of <span class="chemical">pregnenolone sulphate</span> in the passive avoidance task in rats . 