Question

How do you know `HCl` is a strong acid?

Answer 1

Because it completely disassociates in solution another way of saying that is that it completely ionises in solution.

Explanation:

`HCl_(S)-> H^+ + Cl^-`

Have a look at this webpage

http://www.bbc.co.uk/schools/gcsebitesize/science/triple_ocr_gateway/how_much/strong_weak_acids/revision/1/

Hope it helps :)