Final answer:
A cup of water with ice remains at 0°C due to thermal equilibrium; heat is absorbed by the ice as it melts, maintaining the water's temperature. Ice is an efficient cooling agent as it absorbs energy during melting. Once all the ice is melted, the water temperature will increase.
Step-by-step explanation:
A cup of water or soda with ice cubes remains at 0°C even on a hot summer day due to the thermal equilibrium between the ice and the liquid water. While there is ice present in the liquid, the system stays at the freezing temperature. Heat from the warmer environment or liquid is absorbed by the ice as it melts, maintaining the overall temperature at 0°C. This process continues until all of the ice has melted, after which the temperature of the water will begin to rise.
The efficiency of ice chips or an ice bag in cooling is due to the energy required for the phase change from solid to liquid at 0°C. The specific heat capacity of ice is lower than that of the surrounding air in a freezer, meaning ice can absorb more energy from whatever it is cooling, such as an ice-cream mixture, thereby speeding up the cooling process.