During soccer practice one of your teammates sprains her ankle. You take an “instant cold pack” from the first aid box to use on her ankle. It is not at all cold when you take it out. The instructions on the cold pack tell you to punch the pack so that you break open a sack of chemicals inside it.

1. When you do that you notice that the cold pack rapidly becomes quite cold. What happened?

1. By punching the pack you gave the chemicals inside the energy they needed to permit the cooling down.
2. A chemical reaction took place absorbing thermal energy from the pack.
3. The cold pack forcefully expelled heat into its surroundings, which lowered its temperature.
4. Thermal energy was used up in forming chemical bonds, which made the cold pack colder.
5. None of the above answers are correct.

2. After a long time passes, the cold pack returns to room temperature. Why?

1. The chemical reaction that made it colder runs in the reverse direction, making it warmer.
2. Heat is transferred from your teammate’s ankle to the cold pack, making her ankle colder and the cold pack warmer.
3. Cold is transferred from the cold pack to your teammate’s ankle, making her ankle colder and the cold pack warmer.
4. Chemical bonds are formed, releasing energy and heating up the cold pack.
5. None of the above answers are correct.

3. After the cold pack returns to room temperature, what can you say about the total energy (chemical, thermal, and anything else) stored in the cold pack?

1. It is greater than when you first took the cold pack out of the first-aid kit.
2. It is less than when you first took the cold pack out of the first-aid kit.
3. It is equal to when you first took the cold pack out of the first-aid kit.
4. There is not enough information to tell

4. A long time after you punch the cold pack, which of the following is true:

     A.  The cold pack and the surrounding room do not exchange energy

     B.  The most likely energy of each degree of freedom in the cold pack is 1/2 k_BT

     C.  The average energy of each degree of freedom in the cold pack is 1/2 k_BT

5. Which of the following could NOT microscopically why a cold pack could get colder when punched:

     A. The chemicals in the cold pack mix, increasing the degrees of freedom

     B. The chemicals in the cold pack react, leading to a more bound state of the atoms in the system. 

     C. The chemicals in the cold pack react, increasing the degrees of freedom