When you shake a gallon container of milk, the milk inside gets colder due to a phenomenon known as evaporative cooling. Evaporative cooling occurs when a liquid evaporates, and the process of evaporation requires heat energy.
When you shake the container, you introduce air and increase the surface area of the milk. As a result, more milk is exposed to the air, and tiny droplets of milk are dispersed throughout the container. These milk droplets have a larger surface area than a single layer of liquid milk, which promotes faster evaporation.
As the milk droplets evaporate, they absorb heat energy from the surrounding milk to facilitate the phase change from liquid to vapor. This absorption of heat energy leads to a decrease in temperature within the milk, causing it to get colder.
Furthermore, the agitation caused by shaking the container also increases the rate of heat transfer between the milk and the container. The temperature of the container is usually lower than the temperature of the milk, so increased contact between the milk and the cold container results in further cooling of the milk.
Overall, shaking a gallon container of milk causes increased evaporation and enhanced heat transfer, leading to a drop in temperature and making the milk colder.