A free high-school chemistry trainer where you collect chemical formulas, ionic formulas and reaction equations as cards. It keeps re-quizzing the formulas you almost-but-not-quite know, turning fuzzy memory into solid marks.
Questions are in Japanese (interface in English). Collected formulas build up in your lab collection — a full chemistry review that fits in your phone.
Answer the same question correctly twice and it graduates into a mastery card; the ranking counts how many cards you have collected. You can drill only the questions you got wrong, and every day you play adds a study stamp. No sign-up is required and your records stay on your own device. (The questions themselves are in Japanese.)