AQA A Level Chemistry

What does acidify mean ? For example that H2SO4 can't be used to acidify BaCl2?

To acidify something literally means add an acid to it.

Sulphuric acid is problematic when acidifying BaCl2, as the sulphate ions from the sulphuric acid would react with the barium ions in the BaCl2 to form an insoluble precipitate of BaSO4. That is why HCl or HNO3 is typically used.