Have you ever wondered what it is about throwing salt on ice that makes it melt? And just why does it melt? Where does the heat come from to melt the water? Why does it freeze again on the road at night?These questions and many more about freezing and thawing will be looked at in this lab.

To make ice cream, the ingredients—typically milk (or half and half), sugar and vanilla extract—need to be cooled down. One way to do this is by using salt. If you live in a cold climate, you may have seen trucks spreading salt and sand on the streets in the wintertime to prevent roads from getting slick after snow or ice. Why is this? The salt lowers the temperature at which water freezes, so with salt ice will melt even when the temperature is below the normal freezing point of water.

Technically, the temperature that the salt lowers is called the freezing point. When a freezing point is lowered, such as by adding salt to water, the process is called freezing-point depression. As we'll see in this activity, freezing-point depression is not unique to solutions made of water and salt; it also happens with other solutions. (A solution is made when a substance, such as salt, is dissolved and becomes a solute. The medium into which it is dissolved is a solvent—typically a liquid, like water.)

When you studied phase changes back in the day you learned that melting is an endothermic reaction whereas freezing is exothermic. These types of reactions discuss where the thermal energy is traveling, either into or out of the substance. In our experiment today, ice absorbs energy in order to melt. Heat is transferred from the cream solution to the ice, making the ice melt and the cream colder. Eventually as the ice absorbs more and more energy from the cream mixture it will begin to freeze.

We also learned in this unit that heat enjoys being in thermal equilibrium. This automatic transfer of energy and material will always work to balance the amount of energy and material. This is a natural law. From this we can deduce that heat energy will always flow from areas of higher temperature to areas of lower temperature. Knowing this simple rule, let’s look at a very tasty application of it.

Today we are going to make the ice cream, and you will record observations on your website.

1. The instructor will pre-mix the materials

   • 1 cup milk

   • 3 tbsp sugar

   • 3 tsp vanilla

   • ice

   • salt

2. Place the ice and salt in large gallon sized ziploc bag. Seal and shake for 30-45 seconds.

3. Obtain a second ziploc bag, and have the instructor scoop the milk mixture into your bag

4. Vigorously 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.

5. Continue shaking until mixture begins to harden

6. Take picture of final product

7. Eat and enjoy!

Would making ice cream be a physical or chemical change? (you may use the internet to help you answer this!) What were the struggles of this experiment? If you were to do this experiment again, what would you do differently? How would you describe the ice cream?

1. Pictures/ Videos of Ice Cream Creation

2. Ice Cream Journal