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: The frequencies at which the impairment occurred varied between patients ; in one only low frequencies ( 0 . 25 - 0 . 5 kHz ) were involved .
### Output Text:
The frequencies at which the impairment occurred varied between patients ; in one only low frequencies ( 0 . 25 - 0 . 5 kHz ) were involved . 