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: After re - infarction , heart size increased in the placebo group and remained unchanged in the timolol group .
### Output Text:
After re - infarction , heart size increased in the placebo group and remained unchanged in the <span class="chemical">timolol</span> group . 