

Subject:
Mixing Hot and Cold Liquids
Category: Science > Physics Asked by: pdq2ga List Price: $10.00 
Posted:
08 Feb 2004 10:42 PST
Expires: 09 Mar 2004 10:42 PST Question ID: 304721 
A problem occured to me over my morning coffee. The waitress brought the hot coffee in a cup, and the cold cream in a small metal pitcher. After I added some cream to the coffee, the mixture was still too hot to drink for a few minutes. Would the waiting time for an acceptable temperature have been different had I not mixed the two liquids right away, and let them both settle separately from opposite directions toward the room temperature? Upon reflection, it seems there are actually two questions. The first is practical  there are two different types of liquids in two different types of containers (the coffee cup is tapered, so as the total amount of liquid in the cup increases, the surface area of the liquid grows larger, perhaps increasing the rate of evaporation). The second scenario is for two amounts of the same liquid, such as water, in two identical containers, at two different temperatures. Would mixing make a difference in this case? 

Subject:
Re: Mixing Hot and Cold Liquids
Answered By: supermacmanga on 08 Feb 2004 13:42 PST Rated: 
Hello pdq2ga: Question one  I will assume that:  the coffee is hot (say, 90degC)  the cream is cold (say, 5degC)  the final temperature shall be 60degC. Consider case 1, where you don't add the cream to the coffee. You wait, and let the coffee cool to 65degC, and then you add the cream. In the same time interval, the cream warms to 10degC. You then mix the two, and suppose the coffee + cream cools to 60degC. Consider case 2, where you add the cream to the coffee immediately. Then, you wait for the coffee + cream to cool to 60degC. In case 1, the cream absorbs LESS heat from the coffee than in case 2, because the cream has already warmed a bit. Thus, the effectiveness of the cream as a heatabsorber is reduced. In addition, in Case 2, the surface area of the coffee + cream is greater than the surface area of coffee + cream in Case 1  meaning Case 2 dissipates heat slightly faster than Case 1. Therefore, by this analysis, Case 2 would let the coffee cool faster. If we do not consider the warming of the cream, then Case 2 is still faster, because of slightly higher surface area and faster cooling. Here, I assume that the heat capacity (the ability to store energy by heat*) for coffee alone is the same as coffee + milk. This is a reasonable assumption, because both coffee and milk are mostly water, with solutes such as calcium or proteins or carbohydrates or fat making up a very small part. Question two  Suppose that: Case 1) You have two containers with 10 L of water at 90degC and you want it to cool. Case 2) You have one container with 20 L water at 90degC and you want it to cool. (i.e. "mixed") Under Case 2, you have a limited surface area, so the water takes longer to cool than Case 1, where there is a greater interface between the water (and its container) with the air  with more surface area in two containers, Case 1 cools faster. Consider a larger case, if the 20 L of water was divided into 20 containers. In such a situation, the surface area would be very high, resulting in even faster cooling. In the limiting case, if the 20 L of water was divided into individual molecules and spread over an infinite area, the cooling would take place extremely quickly. For a practical example, think of ice cubes. It would take longer to cool and freeze a glass of water placed outside in the snow than if the water was poured on the pavement and let to freeze. All of my solutions assume that the environment is large (i.e. the containers are separated and their individual cooling has no effect on the temperature of the ambient air) and has a reasonable temperature (i.e. 20degC). Let me know if you have questions about any part of my answer  please use the "Clarify Answer" function. Thanks!!  supermacmanga Fine print: * Heat capacity, strictly speaking, is actually the ability to store energy per degreegram. I'm using "heat" in a very liberal way here, by its popular definition.  
 
 
 

pdq2ga
rated this answer:
Excellent, clear answer! Now I can enjoy my coffee without worrying. 

Subject:
Re: Mixing Hot and Cold Liquids
From: markjga on 08 Feb 2004 10:49 PST 
Your interesting question reminds of a very old joke in which Mike says to Pat, "I just got one of those things  I think they call it a Thermos bottle  that keeps hot things hot and cold things cold." Says Pat, "But how do it know?" markjga 
Subject:
Re: Mixing Hot and Cold Liquids
From: supermacmanga on 08 Feb 2004 13:54 PST 
For mathematical and physics background to do with heat: http://www.taftan.com/thermodynamics/CP.HTM http://scienceworld.wolfram.com/physics/HeatCapacity.html http://en.wikipedia.org/wiki/Specific_heat_capacity 
Subject:
Re: Mixing Hot and Cold Liquids
From: racecarga on 09 Feb 2004 15:49 PST 
I think that the researcher's answer to this question is quite wrong. Under ordinary circumstances, drinkable temperature will be reached faster if the coffee and cream are not mixed immediately. There are 2 reasons for this. 1) The rate at which heat is transfered between a cooling (or warming) body and its environment is proportional to the temperature difference. If the constant of proportionality is the same for the coffee alone, the cream alone, and the coffeecream mixture (that is, if the temperature difference between each liquid and its environment drops to half its former value in, say, 10 minutes) then the length of time to reach a given temperature is exactly the same whether you mix immediately or after waiting. However, the constants of proportionality will not all be the same. It would take the coffeecream mixture longer to halve its temperature difference from the environment than it would take the coffee alone, because the mixture has more thermal inertia. It's obvious from experience that half an inch of hot coffee in the bottom of a cup cools much faster than a full cup. So since the rate of cooling of the mixture is slower than than that of the coffee alone, and since the cream begins much closer to room temperature than the coffee, it's better to wait than to mix immediately. As an example, let's say the coffee is 90C, the cream is 6C and room temperature is 20C. Then the coffee is 70 degrees warmer than equilibrium and the cream is 14 degrees cooler than equilibrium. That is, the temperature difference is 5 times greater for the coffee (that's why I chose the 'weird' temperature of 6C for the cream). Now let's say you mix equal amounts of coffee and cream (obviously, no one drinks this much cream in their coffee, but it makes the calculations simple, and it doesn't change the outcome). Assuming similar thermal properties of the containers, the temperature of the coffee will decrease 5 times as fast as the temperature of the cream will increase. If we wait for the coffee to cool by 10C (one seventh of its temperature difference from the environement) to 80C, the cream will have warmed to 8C (also changing its temperature difference by one seventh), and when we mix, we get 44C. Now if we mix immediately, we get 48C, which is 28C warmer than room temperature. To 'catch up' to the case where we waited to mix, the mixture has to cool by 4C, or one seventh of its temperature difference. Since the coffeecream mixture will have a slower cooling rate than either liquid alone, it will take it longer to achieve this oneseventh cooling factor than it took the two liquids separately. 2) More steam will rise from the hot coffee than from the cooler coffeecream mixture. Steam has a very large amount of latent heat, so it carries away lots of heat from the coffee. 
Subject:
Re: Mixing Hot and Cold Liquids
From: supermacmanga on 09 Feb 2004 16:30 PST 
Hello racecarga, I see where you're coming from, but: 1) You mentioned the following: > Since the coffeecream mixture will have a slower cooling rate than either liquid alone, it will take it longer to achieve this oneseventh cooling factor than it took the two liquids separately. a) Since the temperature difference is oneseventh, and since the coffee, cream after waiting also cooled oneseventh, and since the rate of cooling is proportional to the difference in temperature, by your argument, the time to cool is the same. Suppose it takes one time unit to cool or warm oneseventh of a liquid's temperature difference. Thus, it takes one time unit for the coffee to cool from 90degC to 80degC, and one time unit for the cream to warm from 6degC to 8degC. Mixing results in a 44degC solution. Then, consider mixing immediately. We get a 48degC solution. Then cooling oneseventh of its temperature difference, to 44degC, takes one time unit. The time is the same for both, by this argument. However: b) The coffeecream mixture has a higher surface area, higher interface between the environment, and faster cooling rate. Thus, mixing wins, not because of temperature differences but because of a surface area increase. 2) You are correct that steam from the coffee will cool down the solution, but consider this  We have a 90degC coffee that cools to 80degC by evaporation. It takes a unit time t to do this. We also have 90degC coffee that could cool immediately to 80degC by mixing. It takes zero time to do this. Given, after mixing, the mass of the coffee + cream would be greater than the mass of the coffee alone, but since cream is a small part of the overall coffee + cream, it can be disregarded. As well, the higher surface area may justify this simplification.  supermacmanga 
Subject:
Re: Mixing Hot and Cold Liquids
From: supermacmanga on 09 Feb 2004 16:57 PST 
Here's an answer giving a mathematical proof of my answer. http://faculty.salisbury.edu/~kmshannon/math202/coffee.htm They examine two scenarios, one where the cream is kept cold, the other where the cream is left out. In the former, the temperature is the same; in the latter, the temperature is colder without mixing. However, the former is applicable to this question only. Note that this solution does not consider an increase in surface area. If we do consider the increase surface area, then mixing immediately means the coffee will cool faster. This is basically a mathematical confirmation of my intuitive reasoning based on Newton's Law of Cooling, above.  supermacmanga 
Subject:
Re: Mixing Hot and Cold Liquids
From: racecarga on 11 Feb 2004 11:29 PST 
Hello. That's a cool link. But it says the reverse of the comment above. If the cream is left in the fridge, the coffee will be cooler if you mix later, whereas if the cream is left out, the temperatures will be the same. Just a typo. The case here is that the cream and coffee are both on the table (not in the fridge), so under the assumptions in the link, the temperature would be the same with or without mixing. My opinion that in reality waiting to mix will result in a lower temperature is based on the assumption that a larger volume of liquid will cool more slowly. Supermacman is right that if surface area increases by more than volume does when you mix, mixing immediately could result in a lower temperature. But if you think about it, it's hard to imagine a shape of cup that would allow this to happen. 
Subject:
Re: Mixing Hot and Cold Liquids
From: supermacmanga on 11 Feb 2004 15:26 PST 
I think the best way to answer this for certain is to do an experiment =) 
Subject:
Re: Mixing Hot and Cold Liquids
From: pinkfreudga on 11 Feb 2004 16:16 PST 
Somehow I am reminded of this old story... Three college physics students were given this problem: If you have a 3 pound roast at room temperature (70 degrees Fahrenheit), and you put it in a 350 degree oven, how long will it take for the temperature at the center of the roast to reach 160 degrees? The first student determined that a roast is mostly water, and used the formula for the transmission of heat through water to calculate an answer. The second student bought a 3 pound roast, left it out at room temperature for several hours, then placed it in a 350 degree oven with a meat thermometer in it. He recorded the amount of time it took for the thermometer to reach 160 degrees. The third student called his mother and asked, "Mom, how long do I cook a 3 pound roast?" 
Subject:
Re: Mixing Hot and Cold Liquids
From: newberyga on 07 Apr 2004 01:55 PDT 
Regarding the analysis at the link quoted by supermacman, http://faculty.salisbury.edu/~kmshannon/math202/coffee.htm This derivation is incorrect because among other things one of the critical assumptions is faulty. The author assumed that the proportionality constant would be the same for coffee with cream as without cream. This could be approximately true only if the mass of the cream was very much smaller than the mass of the coffee and the surface area available for heat transfer was relatively constant. What the author labels the proportionality constant, k, is actually equal to k = (K * A) / (M * C) K = the thermal conductivity (in this case, equivalent to the combined effect of conduction, convection, and radiation) A = the area available for heat transfer M = the mass of the liquid C = the heat capacity of the liquid According to the author, the formula to calculate the final temperature for the two cases would be, Case 1: Add room temperature cream early T(t) = C + [(aH + (1a)C)  C]e^(kt) = C + [aH  aC]e^(kt) Case 2: Add room temperature cream late T(t) = a[C + (H  C)e^(kt)] + (1a)C = C + [aH  aC]e^(kt) where, C = room and cream temperature H = starting black coffee temperature a = mass of coffee / mass of cream The author's equation for Case 1 is incorrect. If we assume no change in heat loss area (which may be true if container is a cylindrical styrofoam cup), then the Case 1 equation would be, T(t) = C + [aH  aC]e^[akt/(a+1)] where k is the proportionality constant for the original black coffee. Plug in the numbers and it should be clear that the total heat loss in time, t, for Case 1 is LESS than for Case 2. newberyga 
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