Final answer:
Acidic substances have a high concentration of H+ ions, resulting in a pH lower than 7, which signifies their acidic nature.
Step-by-step explanation:
The concentration of H+ ions in acidic substances is a high concentration. In a neutral substance, like pure water, the concentration of H+ ions is 1 × 10-7 moles per liter, which corresponds to a neutral pH of 7.0. Acids increase the H+ concentration when one of their hydrogen atoms dissociates, leading to a pH less than 7. The lower the pH, the higher the concentration of hydrogen ions and the more acidic the solution is. For example, vinegar contains acetic acid and has a pH lower than 7, indicating a high concentration of hydrogen ions.