Learning Lesson: We all Scream for Ice Cream


OBJECTIVE Observe how salinity effects the freezing point of water.
OVERVIEW The students will make homemade ice cream but the "freezing times" will vary using different amounts of salt to lower the freezing point of water.
TOTAL TIME 30 minutes
SUPPLIES For each pair of students:
One sandwich-size and one freezer-size zip-seal plastic baggy
4 ounces of milk, cream, or half-and-half
A dash of vanilla extract or 1 teaspoon of chocolate syrup
2 teaspoons of sugar
2 plastic spoons
For the classroom:
Several bags of ice
Rock salt
PRINTED/AV MATERIAL None
TEACHER PREPARATION None.
SAFETY FOCUS Cold Weather Safety


Background

Water freezes at 32°F (0°C). Adding salt to water lowers the freezing point. How low the freezing point goes depends upon the amount of salt in water.

Procedure
  1. For each pair of students, combine the milk, sugar, and vanilla/chocolate syrup into a sandwich-size baggy and seal closed.

  2. Have the students shake/squish their baggy for one minute to thoroughly mix the contents.

  3. Place the baggy inside the larger freezer-size zip-seal plastic baggy and fill that bag one half full with crushed ice.

  4. In 2 ounce increments, up to 10 ounces, place varying amounts of rock salt in each large baggy. Include a pair or two of students with no salt added.

  5. Have each pair estimate how long it will take for their mixture to freeze.

  6. With all students beginning at the same time, have them mix and churn their baggy at a fast pace until the contents have solidified.

  7. Record their times.

  8. When finished, dispose of the large bag and eat the ice cream.
Discussion

The baggies with the most salt should "freeze" first with the bags containing decreasing amounts of salt taking longer. The greater the salt content, the lower the freezing point of the water and therefore the colder saltwater/ice solution becomes generating ice cream quicker.

Body of Water Freezing point
Baltic Sea 31.3°F (-0.3°C)
Black Sea 30.2°F (-1.0°C)
The Oceans 28.5°F (-2.0°C)
Red Sea 27.9°F (-2.6°C)
Great Salt Lake 11.8°F (-11.2°C)
Dead Sea -6.0°F (-21.1°C)

A common misconceptions is salt makes the ice melt faster. Salt has nothing to do with how quickly the ice melts - it just determines what temperatures it will melt (or freeze). The table (right) provides the average freezing points of water for various bodies of water based on their salinity.

The lowest freezing point for a salt solution is -6.0°F (-21.1°C). At that temperature, the salt begins to crystallize out of solution, along with the ice, until the solution completely freezes. Below -6.0°F (-21.1°C), the frozen solution is a mixture of separate saltwater crystals and fresh water ice crystals, not a uniform mixture of saltwater crystals.

The baggies without any salt will not freeze. This is because nothing special about salt. Pretty much anything that dissolves in water (or milk) will lower the freezing point such as sugar. Salt is used on roads because it's inexpensive. Adding sugar to the milk lowered the freezing point to below that of the plain ice (32°F) and therefore will not freeze.

Live Weatherwise

Cold Weather Safety
Exposure to cold can cause frostbite or hypothermia and become life-threatening. What constitutes extreme cold varies in different parts of the country. In the South, near freezing temperatures are considered extreme cold. Freezing temperatures can cause severe damage to citrus fruit crops and other vegetation. Pipes may freeze and burst in homes that are poorly insulated or without heat. In the North, extreme cold means temperatures well below zero.

Frostbite is damage to body tissue caused by extreme cold. Frostbite causes a loss of feeling and a white or pale appearance in extremities, such as fingers, toes, ear lobes or the tip of the nose. If symptoms are detected, get medical help immediately! If you must wait for help, slowly rewarm affected areas. However, if the person is also showing signs of hypothermia, warm the body core before the extremities.

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