Overview
Water freezes at 32°F (0°C). Dissolving salt in water lowers the freezing point (the temperature at which water turns to ice) and also lowers the temperature. How low depends upon the amount of salt in water. The students will make homemade ice cream, but the freezing times will vary due to different amounts of salt in the ice bath.
| TOTAL TIME | 30 minutes |
|---|---|
| SUPPLIES | For each pair of students: One sandwich-size and one gallon-size zip-seal plastic freezer 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; gloves or a towel. For the classroom: Several bags of ice; Rock salt |
| PRINTED/AV MATERIAL | None |
| TEACHERPREPARATION | None |
| SAFETY FOCUS | Cold Weather Safety |
Procedure
- For each pair of students, combine the milk, sugar, and vanilla/chocolate syrup into a sandwich-size baggy and seal it closed.
- Have the students shake/squish their baggy for one minute to thoroughly mix the contents.
- Place the baggy inside the larger zip-seal plastic baggy and fill the larger bag one half way with crushed ice.
- In 2-ounce increments, up to 10 ounces, place varying amounts of rock salt in each large baggy. One or two pairs of students should have no salt added.
- Have each pair estimate how long it will take for their mixture to freeze.
- With all students beginning at the same time, have them mix and churn their baggy at a fast pace until the contents have solidified. Students should wear gloves or wrap the baggy in a towel to protect their hands.
- Record their times.
- When finished, dispose of the large bag and eat the ice cream (Have extra or share with the no-salt group as theirs likely will not freeze.)
Discussion
The baggies with the most salt should freeze first while the bags containing decreasing amounts of salt take longer. The greater the salt content, the lower the freezing point of the water. At the same time, dissolving salt in the liquid water causes the water temperature to drop, generating ice cream quicker.
A common misconception is that salt makes the ice melt faster, but salt has nothing to do with how quickly the ice melts – it just determines what temperatures it will melt (or freeze). By lowering the freezing point, the ice will melt into liquid water without becoming warmer (instead, it actually becomes slightly colder). 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.
Pretty much anything that dissolves in water will lower the freezing point. Milk is naturally made up of water with a variety of proteins, sugars, and other dissolved substances; therefore, its freezing point is below that of the plain ice (32°F). As a result, the ice cream in the plain ice baggies will not freeze, especially with the extra sugar added to the milk.
| Body of Water | Freezing point |
|---|---|
| Baltic Sea | 31.3°F (-0.4°C) |
| Black Sea | 30.2°F (-1.0°C) |
| The Oceans | 28.5°F (-1.9°C) |
| Red Sea | 27.9°F (-2.3°C) |
| Great Salt Lake | 11.8°F (-11.2°C) |
| Dead Sea | -6.0°F (-21.1°C) |
Building a Weather-Ready Nation
Exposure to cold can cause frostbite or hypothermia, which can be life-threatening. What constitutes extreme cold varies in different parts of the country. In the Deep South, near freezing temperatures can be considered extreme cold. Far north, extreme cold may mean temperatures well below zero.
Freezing temperatures can cause severe damage to living organisms and to buildings. Pipes may freeze and burst in homes that are poorly insulated or without heat. Many crops and other vegetation often can not survive freezing and thawing.
Frostbite occurs when body tissues begin to freeze. It 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 of frostbite are detected, get medical help immediately! If you must wait for help, slowly rewarm with layers of blankets and dry clothing, and place frostbite areas in warm-to-the-touch but not hot water (do not rub the skin or use hot water, heating pads, or a fireplace or radiator for frostbite).
However, if the person is also showing signs of hypothermia (which may include shivering, exhaustion, confusion, fumbling hands, memory loss, slurred speech, or drowsiness), warm the body core before the extremities. Hypothermia occurs when the core body temperature drops too low.
