Name______Date______Period______
Phase Changes and Energy with Ice Cream
In order to have a phase change in matter, heat must be gained or lost. Phase changes occur all around us in everyday life. For instance, ice melts when a drink is left in a room at normal temperature. Water freezes when placed in a really cold temperature, as in the freezer. In this experiment, we will see how fast heat is lost in order to change the milk from a liquid to a solid state. This is also an example of a physical change in matter.
Transitions between solid, liquid, and gaseous phases typically involve large amounts of energy compared to the specific heat. If heat were added at a constant rate to a mass of ice to take it through its phase changes to liquid water and then to steam, the energies required to accomplish the phase changes (called the latent heat of fusion and latent heat of vaporization ) would lead to plateaus in the temperature vs time graph. The graph below presumes that the pressure is one standard atmosphere.
Specific Heat: The specific heat is the amount of heat per unit mass required to raise the temperature by one degree Celsius.
Heat of Fusion: The energy required to change a gram of a substance from the solid to the liquid state without changing its temperature is commonly called it's "heat of fusion". This energy breaks down the solid bonds, but leaves a significant amount of energy associated with the intermolecular forces of the liquid state.
Heat of Vaporization: The energy required to change a gram of a liquid into the gaseous state at the boiling point is called the "heat of vaporization". This energy breaks down the intermolecular attractive forces, and also must provide the energy necessary to expand the gas
Materials:
½ Cup Milk 1/8 Cup Sugar 1/8 Cup Rock Salt
Ice Spoons 2 Dixie Cups 2 Ziplock bags (small/large)
1tsb flavoring
Procedure:
1. Place ice cubes in one Ziploc bag.
2. Place ½ cup milk in second Ziploc bag. Add flavoring, 1/8 cup sugar.
3. Add 1/8 cup of rock salt in bag with ice. Seal and shake gently for 30 to 45 seconds.
4. Empty most of air from bag with milk in it. Carefully seal this bag and place it inside the bag with
the ice mixture. Seal the outer bag tightly to prevent leaks.
5. Gently shake the sealed baggies back and forth in your hands to make sure that the ice mixture coats
the entire surface of the milk bag. Shake for 15 minutes.
6. Carefully remove the inner bag and place on paper towels. Wipe salty water from around opening.
Open baggie and squeeze solid product into two cups for a treat! Enjoy with a spoon!
Discussion Questions:
- What state of matter was the milk when you began? When you were done?
- In order to change the phase of the milk, what had to be removed?
- List the possible sources of the heat needed for this phase change in your baggie. Which source do you think is the best possibility and why?
- Why was the salt added to the ice? What is the only factor that could have caused the changes shown in question 1? What does this tell you about the freezing point temperature of salt water compared to fresh water?
- If you did not add sugar would the ice cream have frozen faster? Why or why not?
- Why did the outside of the bag get wet? (Assume that your bag did not spring a leak.)
- What happened shortly after you added the salt to the ice cubes? Do you think that the temperature was above or below the freezing temperature for water?
- Heat energy is needed to change phase from a solid to a liquid. List the possible sources of the heat
needed for this phase change in your baggie. Which source do you think is the best possibility and
why?
- In the radiator of your car you put a combination of antifreeze and water to keep your car engine
cool in the summer and prevent the radiator from freezing in the winter. Explain how you think this
works in terms of what you saw in the experiment you just did.