Final answer:
A pure glass of ice water will be at 0°C due to the thermal equilibrium between the ice and the water. The ice absorbs heat during melting, a process that occurs at a constant temperature, thanks to the enthalpy of fusion, keeping the water at the freezing point until all ice has melted.
Step-by-step explanation:
A pure glass of ice water, consisting of ice cubes floating in water, would have a temperature right at 0°C. This is because the ice and the water are in thermal equilibrium. Ice has the unique property of staying at 0°C while it is melting. The ice absorbs heat from the surrounding water (or any warm environment) without increasing its temperature, a process requiring energy known as enthalpy of fusion. Only when all of the ice has melted does the temperature of the water begin to rise. This is why, even on a hot summer day, a glass of water with ice in it remains at the freezing temperature of 0°C as long as there is some ice left.
For example, when ice is added to a warmer drink, the drink cools down because the ice absorbs heat as it melts. This heat absorption, necessary to change the state of the ice from solid to liquid at a constant temperature, is responsible for the cooling effect. The rate of ice melting is limited by the rate of heat transfer, which prevents the ice from melting all at once and keeps the water around it very close to 0°C.