Did you ever wonder why they use salt to de-ice roads? Did you know that snow more readily sticks to pavement treated with salt? Why would this be the case? In this activity you will use the same principles to pick up ice cubes with a string. Is it possible to do this—without getting your hands cold? Do the activity and see what a pinch of salt can do!
Background
Water is made up of tiny building blocks called molecules. These water molecules don’t sit still—they wiggle and move around all the time. The water’s temperature is a measure of how much these molecules move. When you cool water you lower its temperature, and the molecules slow down. Eventually after you cool the water enough they move so little they can form strong connections and the water freezes, turning into ice, which is a solid. For pure water this transition happens around 0 degrees Celsius (32 degrees Fahrenheit). Conversely, if you add heat to a block of ice, its molecules will start wiggling more; eventually they move around too much to stay stuck together, and the ice melts, turning back into a liquid. These transitions, however, do not happen instantaneously—water and ice can coexist. (That is, a whole ice cube does not turn to liquid water all at once.) To visualize this process you can think of a group of excited children—like molecules at higher temperature, they move and wiggle a lot. The higher their excitement, the more they move. Calm them and they will slow down just like molecules slow down when you reduce the temperature. Eventually you might get these children to hold hands and stand nicely in line—so they behave like a solid.
When the water is not pure, the water molecules cannot connect as readily to form a solid; other particles get in the way. That is why salty, sugary or other water solutions’ freezing points—temperatures at which they turn solid—are lower than 0 degrees C. This explains why treating roads with salt in winter can prevent them from icing over. The salt dissolves in the water, lowering its freezing point, which will only turn to ice at temperatures well below 0 degrees C. In this activity you will use this characteristic in a clever way to pick up an ice cube.
Answers & Comments
Answer:
Key concepts
Chemistry
Freezing
Melting
Freezing-point depression
Introduction
Did you ever wonder why they use salt to de-ice roads? Did you know that snow more readily sticks to pavement treated with salt? Why would this be the case? In this activity you will use the same principles to pick up ice cubes with a string. Is it possible to do this—without getting your hands cold? Do the activity and see what a pinch of salt can do!
Background
Water is made up of tiny building blocks called molecules. These water molecules don’t sit still—they wiggle and move around all the time. The water’s temperature is a measure of how much these molecules move. When you cool water you lower its temperature, and the molecules slow down. Eventually after you cool the water enough they move so little they can form strong connections and the water freezes, turning into ice, which is a solid. For pure water this transition happens around 0 degrees Celsius (32 degrees Fahrenheit). Conversely, if you add heat to a block of ice, its molecules will start wiggling more; eventually they move around too much to stay stuck together, and the ice melts, turning back into a liquid. These transitions, however, do not happen instantaneously—water and ice can coexist. (That is, a whole ice cube does not turn to liquid water all at once.) To visualize this process you can think of a group of excited children—like molecules at higher temperature, they move and wiggle a lot. The higher their excitement, the more they move. Calm them and they will slow down just like molecules slow down when you reduce the temperature. Eventually you might get these children to hold hands and stand nicely in line—so they behave like a solid.
When the water is not pure, the water molecules cannot connect as readily to form a solid; other particles get in the way. That is why salty, sugary or other water solutions’ freezing points—temperatures at which they turn solid—are lower than 0 degrees C. This explains why treating roads with salt in winter can prevent them from icing over. The salt dissolves in the water, lowering its freezing point, which will only turn to ice at temperatures well below 0 degrees C. In this activity you will use this characteristic in a clever way to pick up an ice cube.