To boil water, we place the kettle directly over the flame, not beside it. So, how should we cool an object with ice water? Many people, out of habit, place items on top of the ice. Doing so is futile…
When heating water, placing the kettle directly over the flame is correct because the air heated by the flame becomes lighter and rises around the kettle. This method allows us to utilize heat efficiently.
However, when it comes to cooling items with ice water, many still place objects on top, such as putting a milk carton on top of the ice. This is incorrect because the air above the ice, once cooled, will sink and be replaced by the warmer air surrounding it. From this, we can conclude: if you want to cool food or drinks, do not place them on top of the ice; instead, put them underneath the ice.
If you place a pot of water on top, only the lowest layer of water will cool down, while the upper parts remain surrounded by warmer air. Conversely, if you place ice cubes on top of the pot lid, the water inside the pot will cool down quickly because the cold layer on top will sink, causing the warmer water below to rise and replace it. This process continues until the entire pot of water is cold (note that pure water does not cool down to 0 degrees Celsius but only to 4 degrees Celsius, at which point water has its highest density). Additionally, the cold air surrounding the ice will also descend and envelop the pot of water.
