Mixing Hot and Cold


Students came into my class to see the comic above projected on the screen. The comic reminds me of frustrations at my grandmother’s house trying to get the temperature just right. A slight adjustment would either freeze or scald me. My students looked with blank expressions. It seems as if none of them have this experience. Given the homes of many, I wonder if they simply program in the water temperature of choice. What are they missing from this daily experiment?

With the comic intro a flop, I asked each to individually answer the following:

  • Water at two temperatures are mixed together. The two temperatures are 50 C and 10 C. Please make a selection that best fits what you believe will happen to the temperature once they are mixed.
  1. The temperature of the “colder” water will be subtracted from the “warmer” water, resulting in a final water temperature of 40 C.
  2. The temperatures of the two waters will be averaged, resulting in a final temperature of 30 C.
  3. The temperatures of the two will be added, resulting in a final temperature of 60 C.

Source Article: Teaching about Heat and Temperature Using an Investigative Demonstration, Science Scope , December 2011

Students were also asked to explain why they chose the subtraction, average or addition conception. In my class of 20 students, an even selection was made with 10 students selecting subtraction and 10 selecting average. For me, the line was almost predictable. Before class, I put students in groups that I anticipated would have similar conceptions. That way students would share initial ideas as they then moved forward and planned an investigation. My predictions were almost spot on. 

For me, the interesting information was found in the explanations of reasoning. Support for the subtraction conception:

  • I believe (a) because when cold hits something hotter it mixes together.Then the cold changes to warm because it mixes with the hot.
  • a, because there are more heat molecules than cool molecules and they will end up subtracting each other.
  • I chose “a” because I had mixed hot and cold water before and the temperature dropped.
  • A. The hot water is too hot that it will break those coldness in cold water. That the water temperature would stay at a higher temperature.
  • I believe that it is “A” because I made it into real life. When the two waters are mixed, the water is hotter but there is cold water which ends up having a hot water, but it’s colder than the original one.
  • A, because when the cold water meets the hot water the cold water is not going to become hot water. It is going to mix, so the temperature should become 40C.
  • You cannot add both of them up because they are like positive and negative numbers. Add more hot water and the water will be warmer.

Support for the average conception:

  • Because the energy of the hotter water would mix with the colder water and they will get averaged.
  • The water will cool down until both temperatured water becomes the same temperature.
  • Because the hot water will heat the cold water up and the cold water will cool the hot water down until they have the same temperature.
  • There is more energy in the hot water than the cold water. When the hot & cold water mix, the energy evens out so the temperature will be averaged.
  • B, because I believe the two waters are the same volume (mass) and the temperatures would come together and be divided by 2.
  • The temperature of the two waters will be averaged if the amount of water on both sides are the same. Temperature is a unit to show an amount of energy. If two energies get mixed, the temperature we get will be their average.
  • I chose “b” because the two cups of water contains the same amount of water mixed together. After the two cups of water mix together the temperature would be mixed and averaged.
  • Assuming that both are the same amount, the water mixed would result in 30C. Heat transfers quickly, the cold water drains the hot water’s heat while the hot water has cold temperatures spread through it. Both balance out each other.
  • I think the best choice is b because neither the cold water or hot water should be subtracted or added into each other so the best choice is to average both.
  • If the amount of both water is the same, the water will average out. The water is not “negative” or “positive”. There is a colder/hotter temperature for both waters so therefore they will average.

From this point, I asked students to grab a whiteboard to sketch, write a procedure and make a data collection table. The stumbling block on procedures was when to take the final  “mixed” temperature. In the end, each of the five groups produced data that supported the average conception.

Where now? Students made a prediction, planned an investigation and collected data. Next class involves making a claim, using evidence to support the claim, and reasoning to tie together their ideas. I’m looking forward to reading the students’ reasoning. They were given background reading that I hope will support the experimental data. Will I be treated with more insight into their thoughts on this concept?


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