Final answer:
The compound which solubility decreases most rapidly when the temperature increases from 50⁰ Celsius to 70⁰ Celsius is KNO₃, potassium nitrate.
Step-by-step explanation:
The compound which solubility decreases most rapidly when the temperature increases from 50⁰ Celsius to 70⁰ Celsius is KNO₃, potassium nitrate. The solubility of KNO₃ increases dramatically with an increase in temperature. On the other hand, the solubility of NaCl remains fairly flat and is not significantly affected by temperature changes.