Do hands-on science experiments to show how freezing and boiling points can change.
Interactive science experiments, because they are by nature hands-on, allow students to experience learning rather than simply observe. Introduce these experiments by asking them if they know the temperatures at which water boils and freezes, then ask if these temperatures can be changed in any way. Conduct the experiments, then follow up to see if their predictions were correct.
Lower the Freezing Point of Water
Have each child place an ice cube on a plate, and try to pick it up with a piece of string (it won't work). Then place the string on the ice cube and sprinkle some salt over top. Wait about 15 seconds, and pull on the string. You will now be able to pick up the ice cube. This happens because salt lowers the freezing point of water, so the ice cube starts to melt when the salt is on it. As the water dilutes the salt, the ice cube refreezes with the string in place.
Freezing Point of Ice Cream
Instruct the students to fill a coffee can half full with crushed ice and mix in 6 tablespoons of rock salt. Shake the can to thoroughly mix the ice and salt. In a resealable plastic bag, mix 1/2 cup of cream, 1 tablespoon of sugar and 1/2 teaspoon of vanilla. Seal it, removing as much air as possible, and place it in another bag and seal. Place it in the ice mixture, put the lid on the can and, wearing gloves, shake for 15 to 20 minutes. Because the salt lowers the temperature of the ice, the cream will turn into ice cream at a temperature below 32 F.
Salt and Boiling Point of Water
Have students boil a quart of water on the stove and carefully measure the temperature of the water while it is boiling. Next add 1 tablespoon of salt to the water and return to a boil, measuring the temperature. Repeat, adding another tablespoon of salt. Students will find that adding salt raises the boiling point of water by about a few degrees for each tablespoon of salt.
Additives and Boiling Point of Water
After doing the experiment with salt and boiling water, have students start with a few of clean pots, one quart of fresh water in each one. Boil each pot of plain water as a control, then add a different substance to each one to see how they affect the boiling point of the water. Try pepper, sugar and baking soda to see the effects. In general, sugar raises the boiling point slightly, baking slightly less, and pepper barely at all.
Tags: tablespoon salt, Boiling Point, boiling point, Boiling Point Water, Freezing Point, point water