The Freezing Point

ByMs. Patricia Lombard

Objectives: • Investigate the effects of

temperature change on phase changes.

• Investigate the effects of changes in freezing point.

• Utilize the law of conservation of energy.

• Apply these concepts to make ice cream!

Thermal Energy Example:

Take a small piece of ice out of your fridge and hold it in your hand. The thermal energy content of your hand is higher than the thermal energy content of the ice cube. The atoms that comprise your hand are moving more rapidly than the atoms that make up the ice cube. Therefore, there will be a transfer of thermal energy from your hand to the ice cube.

This will cause the atoms in the ice cube to speed up while the atoms in your hand slow down. While this thermal energy is in transfer, it is called heat. This will cause the atoms in the ice cube to speed up while the atoms in your hand slow down. The increase in speed of the ice cube atoms changes the state of water from solid to liquid.

This transfer of thermal energy will continue until an equilibrium is reached between your hand, the ice (now water), and the air in the room.

By itself, melting ice has a temperature of 0° C (32° F).When heat flows into ice at that temperature(and heat always flows from a warmer place to a colder place), the ice doesn’t get hotter, it just transforms itself into water at that same temperature.

Salt lowers the freezing point of water from 0°C to - 30°C (32°F - -22°F). That’s why we can use it to melt ice on sidewalks and roads in winter. By putting salt on the ice, you encourage the melting process so much that the ice cream begins to use its own internal thermal energy to transform into water.

The temperature of the ice dropped well below its usual 0°C and yet it kept melting. Eventually, the drop in temperature stops– but by then the mixture is about -10°C (14°F) or so.

• To melt more ice, more heat must flow into the mixture. When you put your bag of ice cream mix nearby, heat begins to flow out of the bag and ice and salt water. More ice melts and the ice cream mix gets even cooler. Eventually, the water and ice cream mix start to freeze.

Then you get.......ice cream!

Procedures for Making Ice Cream

Combine in small bag: 1) ¼ cup milk 2) 2-3 teaspoons of sugar 3) ½ teaspoon vanilla Seal the bag tightly.

Place the small bag of ice cream mix in the large bag of ice and salt. Seal the large bag well.

Shake, shake, shake the bag for at least 5 minutes. It gets REALLY cold!

When the ice cream is frozen, remove it from the outside bag and enjoy!

Ice Cream and Thermal Energy Lab Sheet

Name__________________________________

Lab Questions:

1. Freezing is the physical change of a liquid to a _____________.

2. Thermal energy always flows from a ______________ temperature to a __________ temperature.

3. In order to make the milk into ice cream, what had to be removed?

4. What happened to the heat energy that left the milk?

5. Why was salt added to the ice?

6. Describe the transfer of energy that occurred in this lab:

From the milk mixture, to the _________________

From the ice/water, to your _________________

From the ice cream to your_____________(when you ate it)

I scream, You scream,We all scream for ice cream!