For instance, I make myself some hot coffee and I want to turn it to iced coffee. Stirring it seems to cool it down much faster than letting the ice just sit in the coffee, however, from what I have read, stirring the ice melts the ice faster and more even though it cools it down faster. Why? Is this even correct?
Also, would the ice just sitting in the hot coffee melt roughly the same amount as the stirred once the coffees reaches the desired cold temperatures? Idk we can make one up, coffee from 160°f to 40°f, I guess. I also generally fill my coffee cup completely with ice as I’m sure the amount of ice changes this.
I hope this makes sense and I tagged the question correctly. Thank you,
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If you don’t stir, the water around the ice will stay cold, slowing the heat transport from the higher temperature to the lower. Stirring mix and accelerates the transfer of heat.
When objects that are colder or warmer than their environment sit in one place, they change the temperature of the environment immediately around them. In the case of an ice cube sitting in a glass, the ice cube takes in some heat and the water around it becomes colder.
Because the water around it is colder, it does not warm up as fast. The heat from outside that cold area has to work its way through the cold water to reach the ice.
If you stir, you make the water move. When it moves, the cold water around the ice is replaced with warmer water. The ice is now touching warmer water and warms up faster.
The same thing is what causes “wind chill”. A human standing in still air will warm the air around their body, creating a pocket of warm air around them that keeps away the colder air. If there is a wind, that warmer air is blown away and replaced by colder air, causing you to cool off faster.
There are two related areas of science at play here. Thermodynamics and Heat Transfer.
There are two states to the system here: “before ice is added” and “after the ice melts and the beverage reaches equilibrium temperature”. Thermodynamics can be used to determine the equilibrium temperature. Heat transfer can be used to describe the temperature transit. Simply, thermodynamics defines the starting point and the destination, and heat transfer defines the speed of the journey.
Thermodynamics defines the energy needed to get your car from A (hot coffee) to B (iced coffee), the “gas” for the journey, and heat transfer is the “gas pedal”. When you stir, you hit the accelerator, you get to B faster, and you run out of gas when you get there. If you don’t stir, you are simply letting the car idle all the way to B and you still run out of gas when you get there.
The rate of heat transfer is faster the larger the difference (delta) between the temperature of two objects of different temperatures. Ie. Hot coffee at 100°C and ice at 0°C will transfer heat faster than between coffee at 90° and water at 50°
If ice just sits there and melts, the water around the ice is close to the temperature of ice. If you wait for those molecules to diffuse through the rest of the coffee it will take a long time as it happens through what is called Brownian motion.
If you STIR the coffee, then you’re moving the ice, and the cold water from just melted ice, through the hot coffee, meaning the ice gets into contact with more of the hot 100°C liquid.
Think of the exchange of energy taking place as of a queue of toddlers. Which would be faster: tell each kid to take a piece of candy and go play in the sandbox, or ask them to take one and then to keep passing on the candy to the kid behind them until the candy bowl is empty?
Energy transfer happens faster the more of a difference there is between the sides taking place in the exchange. The higher the temperature, pressure, voltage, force, elevation, etc. difference the faster the process will go.
Hot water closest to the ice will cool down fast, but the rate at which it keeps cooling down further will also drop substantially. This will form a sort of an insulating layer. By stirring you are constantly introducing hot[ter] water and keeping the rate of the exchange up.
Heat flows from hotter to colder, with the rate of transfer proportional to the difference in temperature. A larger difference in temperature, a faster heat transfer, in this case more cooling of the coffee. Now if the coffee is just standing still, the coffee just adjacent to the ice cube will be colder than the rest of the coffee, so the heat transfer will be slower. There will be less cooling. If you are constantly stirring the coffee, then hot coffee will always be in contact with the ice cube and the heat transfer will be faster, the coffee will cool down faster. It is exactly the same reason why in hot weather you cool down faster in front of a fan than in still air.
A thermal gradient is a location where the temperatures change. In a glass of water with ice, the gradient changes suddenly between the ice and the water around it. The gradient likes to be smoother, so the water around the ice becomes colder and the ice next to the water becomes warmer. By stirring, the water is mixed to a more consistent temperature, causing a steeper thermal gradient around the ice.
For one thing, the ice melting is the main reason the coffee is cooling off to begin with. Ice melting is called an “endothermic reaction” which basically just means that when ice melts, it absorbs heat from its surroundings, cooling them down.
Consider two scenarios, one where you just leave ice sitting in a cup of hot coffee, and another where we are stirring our ice into the hot coffee. In the first situation, the ice starts to melt and cool off the coffee *immediately surrounding* it. Once that coffee has cooled off though, it has less heat for the ice to absorb for melting. That small amount of coffee in direct contact with the ice will slowly take heat from the rest of the drink in the mug and deliver it to the ice, but it will take a bit. However, if we stir our coffee, we’re constantly averaging out the temperature across our entire drink, keeping hotter coffee in contact with the ice constantly. This means the ice can melt faster and therefore the drink overall can cool at a faster temperature.
The coffee around the ice will cool as heat is absorbed by the ice. The rate of transfer depends on the temperature difference, so as it cools, the coffee melts the ice more slowly. Stirring the coffee brings the average temperature near the ice up, melting it faster. Ideally you would pour hot water over the ice, so that the water that is cooled by the ice flows away and you maintain as large a temperature difference as possible over the surface of the ice.
Heat transfer via a moving fluid is known as convection, as opposed to conduction, which is heat transfer through a material, like within the ice itself.
Melting the ice takes energy, which cools the coffee. Stirring simply causes the ice to melt faster, and therefore cool the coffee down. So when the ice is melted, the temperature will be the same, ignoring the ambient air. However, typically you consume iced drinks with the ice in them, and don’t wait till it melts. So stirring leads to a colder drink.
Ice in cold water melts more slowly than ice in hot water. When you leave ice in hot coffee the outside melts and creates a cushion of ice cold water around the ice cubes protecting it from melting. Stirring pushes the ice outside of the cushion into the hot coffee so it keeps melting and more ice cold water is mixing with the hot coffee.
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