A substance with a pH of 2.5 is considered acidic. Among the options provided:
- Water has a neutral pH of around 7.
- Vinegar typically has a pH ranging from about 2 to 3, indicating that it is acidic.
- Ammonia has a basic pH, generally around 11 to 12.
- Baking soda (sodium bicarbonate) is also basic, with a pH around 8 to 9.
Given the pH of 2.5, the unknown substance is most likely vinegar.