Final answer:
The system consisting of water and battery experiences a transfer of energy, where the chemical potential energy of the battery is converted into heat, which warms up the water.
Step-by-step explanation:
When a fully-charged battery is dropped into a bucket of cold salt water and it shorts out, there is a transfer of energy within the system consisting of the water and the battery. The chemical potential energy stored in the battery is converted into electrical energy as a result of an internal chemical reaction. Shorting the battery causes the electrical energy to rapidly convert into heat due to the resistance in the water, leading to an increase in the temperature of the water. Thus, the change that has occurred is the conversion of the battery's chemical potential energy into heat energy, which is absorbed by the water, warming it up.