Name____________________________________
Hour_____
Directions: Answer the questions in complete sentences. You will need to use the articles to answer some of the
questions below. Questions 1-3 are time sensitive. You must take the temperature before and after, and make
observations before you eat your ice cream in class.
1. What is the temperature of your ice/salt before you shake the ice? What is the temperature of the ice/salt after
you have made your ice cream?
Before Temperature ______
After Temperature ______
2. Make qualitative observations of your ice cream (before you add toppings).
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3. How does your ice cream compare to ice cream you can buy at the store? (Include how the taste and texture
compare to other ice cream you’ve had.)
4. What is the purpose of using salt to make ice cream? Why do we use salt?
5. Describe the direction heat transfers between the bag of ice/salt and the bag of ice cream ingredients.
6. Is making ice cream an example of conduction, convection, or radiation? Explain how the heat is transferred. Be
specific.
7. Why is a towel included in the process of making ice cream? Explain the function it serves.
8. Describe how the molecules of the ice cream ingredients change as it goes from a liquid to a solid as it freezes.
9. According to the articles, how does salt make water colder?
10. According to the articles, what temperature can water reach when it contains salt? ___________
11. At what temperature does a 10-percent salt solution (10% salt) freeze?______________
12. At what temperature does a 20-percent salt solution (20% salt) freeze? ______________
13. Explain why we put salt on the roads to keep the roads safe in the winter.
14. At what temperature does the salt have no effect on ice? _________________
15. According to the article, what is more useful than salt in extremely frigid temperatures? Why would they use this
alternative?
Why do they use salt to melt ice on the road in the
winter?
If you live in a place that has lots of snow and ice in the winter, then you have probably seen the highway department
spreading salt on the road to melt the ice. You may have also used salt on ice when making home-made ice cream. Salt
lowers the freezing/melting point of water, so in both cases the idea is to take advantage of the lower melting point.
Ice forms when the temperature of water reaches 32 degrees Fahrenheit (0 degrees Celsius). When you add salt, that
temperature drops: A 10-percent salt solution freezes at 20 F (-6 C), and a 20-percent solution freezes at 2 F (-16 C). On a
roadway, this means that if you sprinkle salt on the ice, you can melt it. The salt dissolves into the liquid water in the ice
and lowers its freezing point.
If you ever watch salt melting ice, you can see the dissolving process happen -- the ice immediately around the grain of
salt melts, and the melting spreads out from that point. If the temperature of the roadway is lower than 15 F or so, then
the salt really won't have any effect -- the solid salt cannot get into the structure of the solid water to start the dissolving
process. In that case, spreading sand over the top of the ice to provide traction is a better option.
When you are making ice cream, the temperature around the ice cream mixture needs to be lower than 32 F if you want
the mixture to freeze in a timely manner. Salt mixed with ice creates a salt solution that has a temperature lower than
32 F. When you add salt to the ice water, you lower the melting temperature of the ice down. The solution is so cold
that it easily freezes the ice cream mixture.
Why Does Adding Salt to Water Make it Colder?
Freezing Point
So how does salt (sodium chloride) make water colder? In essence, it does not. Salt works to depress the freezing point
of water so the water can become colder than 32 degrees Fahrenheit (zero degrees Celsius) before it turns to ice. In fact,
water containing salt can reach temperatures of nearly minus 6 degrees F.
When ice cream is made, cream is placed into a canister and rotated within an ice bath. If no salt is added to the ice
bath, the lowest temperature it can reach is 32 degrees F. While the cream can freeze at this temperature, it can do so
more quickly at a lower temperature. When salt is added to the ice bath (usually rock salt in ice cream making), it comes
into contact with the thin layer of water on the surface of the melting ice. The salt dissolves and the water becomes
salty. This salt water has a lower freezing point, so the temperature of the ice bath can get even colder, thus freezing the
ice cream more quickly.
Uses of Salt to Melt Ice
The principle of salt lowering the freezing point of water is used frequently to keep roads safe in winter. During snow
and ice events, trucks spread a thin layer of salt on roadways. This causes snow and ice to melt on impact rather than
freeze and makes the roads wet rather than icy and dangerous. However, there is a limit to how cold water can become
before freezing; in extremely frigid temperatures, applying sand to the roads to increase friction is more useful than
applying salt. Types of salt other than sodium chloride can be used in colder temperatures. Calcium chloride and
magnesium chloride, for example, can melt ice at low temperatures. However, some of these compounds can be
detrimental to the environment and are used only occasionally.
Effects of Crushing Ice
Using crushed ice with salt will provide a greater surface area on which salt can be dissolved, resulting in faster melting.
Using more pieces of crushed ice in your ice cream maker, for example, will be more effective than using fewer large
cubes.
Boiling Salt Water
While salt will lower the freezing point of water, it does not lower the boiling point. In fact, salty water will boil at a
higher temperature than non-salty water. Again, adding salt to water does not lower the temperature.