Main Idea

To prepare some of our favorite foods, such as ice cream, birthday cakes, and even barbecued steaks, we rely on chemical reactions. Some of these reactions occur when food ingredients combine, while others result from cooking or cooling processes. When we understand how science affects foods, we can create tasty new recipes.

Background

Certain substances affect the state of ice. When salt is sprinkled on ice, for example, it causes the ice to melt. This chemical reaction is actually a heat energy transfer.

Water freezes at 0 degrees C. A saturated saltwater solution freezes at -32 degrees C. If you sprinkle salt on ice, the ice will melt because the salt lowers the freezing point of water. When the salt makes contact with the ice, it starts a chemical reaction that transfers heat energy from the surrounding environment. Therefore, if a road is icy in the winter and we sprinkle salt on top of it, the salt-ice mixture will draw heat energy from the road, the air, and the friction from car tires and transfer it to the ice, which then melts.

Learning Objectives

• Develop observation skills.
• Understand the scientific process through active experimentation and comparison.
• Understand that chemicals can change the properties of water.

Time

1 hour

Introducing the Concept

Have the students sprinkle some coarse salt on a piece of ice. As the ice begins to melt, discuss what is happening. Ask them if they have ever seen this chemical reaction before. Perhaps some of the students have seen salt on the streets in winter. Introduce the idea of using the chemical reactions between salt and ice to super-cool the ice cream mixture when making home-made ice cream.

Materials

• 4 thin metal mixing bowls, two small and two large for each group of students
• coarse salt
• crushed ice
• ice cream mixture (below) for each group of students
• cooking elements

Vanilla Ice Cream Recipe

• 125 ml sugar
• 1.25 ml salt
• 250 ml milk
• 15 ml vanilla flavoring
• 500 ml chilled whipping cream (this mixture makes enough ice-cream for 10 students)

Note: raw eggs or egg yolks used to be common ingredients in vanilla ice cream recipes, but today this practice is not recommended, due to the possibility of salmonellosis. Two or three egg-equivalents of a pasteurized product such as Egg Beaters may be added instead.

Recipe Variations

• Chocolate Ice Cream: Reduce vanilla to 5 ml and add two squares of melted, unsweetened baking chocolate to the mixture.
• Strawberry Ice Cream: Reduce vanilla to 5 ml and add 250 ml of thawed frozen strawberries after mixing the ice cream.

Hands-On

1. Mix the first three Vanilla Ice Cream Recipe ingredients in a saucepan and stir constantly over a medium heat. Let the mixture cool to room temperature. Add the vanilla and the cream. Now the concoction is ready to be made into ice cream.
2. Place half of the ice cream mixture in one small bowl and the other half in the other small bowl.
3. Half-fill both large metal bowls with crushed ice.
4. Sprinkle coarse salt on the ice in only one of the two large bowls.
5. Place the two small bowls with the ice cream mixture into the large bowls, on top of the ice.
6. Stir the mixture in the bowls at the same time.
7. Continue to stir the mixture until it thickens to form ice cream.

Minds-On

• What differences did you notice as you made the two ice creams? What may have caused these differences?
• Discuss what happened to your classmates’ mixtures when they made different flavors. What were the differences? What might have caused the changes?
• What are some modern methods we use to prepare ice cream?

Teaching Tips

• When creating the ice cream, be sure that the students keep the ice mixture moving as well as the cream.
• Try to use very small ice cream bowls with small portions of the ice cream mixture to ensure freezing.
• For very young students you may want to prepare the ice cream mixture in advance.

Science All Around Us

Relate this activity to your community by organizing a tour of a local ice cream factory. If there are none in your neighburhood then you might look for another type of food manufacturer or even invite a food chemistry specialist into your classroom.

Interconnections

Experiment with baking powder and show how chemicals can be used not only in the preparation of food but also as key ingredients in recipes. Corn bread is a simple food, easily prepared by students, that illustrates basic chemistry. It includes active chemicals such as baking powder (a combination of corn starch, monocalcium phosphate and sodium bicarbonate) and buttermilk (acetic acid). Students can watch the reaction between these ingredients take place. You can talk about how the chemical reaction creates tiny air bubbles in most cakes.

Did you know that the process of freezing eliminates contaminants from water? Inuit people have tapped this phenomenon for centuries, making safe, clear drinking water from frozen salt water. Recently, Canadian engineers have created a sewage treatment system that relies exclusively on freezing, generating what they call “snowage”!

Engineering Toothpaste

Positively Paper

This activity copied from APASE of Vancouver, Canada, which has regrettably disappeared from the Web.

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