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Saltwater Freezing Point
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Lesson: Salt and the Freezing Point of Water
Students will determine the temperature at which water freezes and learn that salt water freezes at a lower temperature than plain water and why. They will also gain experience in setting up experiments and interpreting results.
Glass, string, water, ice cube, salt, thermometers, graph paper
Each student will receive a cup with ice and a piece of string. Float the ice cube on water filled almost to the top of the cup. Place the string across the ice, and have students note what – if anything – happens. Next, give each student a teaspoon of salt; ask them to hypothesize what will happen if they pour the salt on the ice and string. They should write hypotheses. Let students pour on the salt and determine if the string freezes, sinks, or does nothing.
Next, give each student a cup containing about 2/3 ice and 1/3 water. Have them gently stir the ice water with a thermometer and record the temperature every two minutes for about 6 – 8 minutes. Students should record results. Then give them one teaspoon of salt to add and repeat the process. Finally, give them one more teaspoon of salt to add and repeat the process a final time.
Discuss results with students – what happened to the ice when salt was added? What was the freezing temperature of the ice water before salt was added? Discuss real-life applications involving salt and ice (roads, bridges, etc.). After discussing and evaluating the results, students should conclude that salt affects the freezing point of water, and that salt water freezes at lower temperatures than plain water. Have students graph their results.
Students will be assessed by their recorded observations, graphs, and discussion particpation
About this Document
Students will determine the temperature at which water freezes and learn that salt water freezes at a lower temperature than plain water and why. They will also gain experience in setting up experiments and interpreting results. For upper elementary-middle grades.