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.


Materials

  • Ice cubes
  • Three glasses
  • Cold water
  • Salt
  • Three strings, each about 20 centimeters long (Yarn works well.)
  • One sticky note
  • Watch or timer
  • Thermometer (optional)
     

Preparation

  • Gather all of your materials on a work surface that can tolerate spills.
  • Fill your three glasses with cold water


Procedure

  • Put a few ice cubes in the first glass of water. They float because ice is less dense than water.
  • Search for a cube whose surface is at about water level. Lay one end of a piece of string across that ice cube. Do you think the ice will stick to the string?
  • Lift the string. Does the ice cube stick to the string? Why do you think this is the case?
  • Do you have ideas on how you can lift the ice cube with a string, without touching the cube with your hands?
  • Try again, but now, sprinkle some salt over the string and ice cube. Do you think the ice will stick to the string if you lift it?
  • Lift the string. Does the ice cube stick to the string? After all, salt is not glue, right?
  • What do you think would happen if you left the ice cubes with the strings on top out for a few minutes?
  • Take the two other glasses filled with water, add a few fresh ice cubes to each glass.
  • For both glasses, lay one end of a piece of string across an ice cube whose surface is about water level; let the other end of the string hang over the edge off the glass.
  • Sprinkle salt over the string and ice cube in one of the glasses, and mark this glass with a sticky note. Wait for about two to three minutes. Do you think the ice will stick to the string after you give it some time? Would both, only one or none stick? Why would this happen?
  • After about two to three minutes, lift one string at a time. Can it pick up the ice cube? Can you explain what you see? If you cannot pick up any ice cube, try again but wait a little longer this time.
  • Extra: Try some other substances, such as sugar or food coloring. Can these make the ice cube stick to the string?
  • Extra: Make two identical water-and-ice baths. Add more salt to one bath, leave the other as is. Measuring the temperature of each water-and-ice bath every minute for the next five minutes. Graph your results. What do you observe? Is one colder than the other? Why would that be?
  • Extra: Can you re-create a mini iceberg in a saltwater ocean in a bowl at home? What temperatures can prevent your iceberg from melting and ocean from freezing?

Observations and results
Could you lift the ice cube you had sprinkled with salt and left untouched for few minutes? Did you fail to pick up the cube in all the other cases? Why does this happen? First, the ice around the string melts when you sprinkle it with salt, then the string freezes to the ice cube.

You probably wonder why it happens only when you sprinkle salt over the ice cube and string. When you sprinkle salt over ice it dissolves in the thin layer of water above the ice. Because saltwater freezes at a lower temperature than pure water adding the salt makes some ice melt and absorb heat in the process. The area just around it thereby cools and freezes water molecules to the ice cube, also freezing the string on. Without the salt, the water and ice are both at the same temperature and the string does not freeze to the ice. In both cases the ice cube gradually melts as it absorbs heat from the air around it, but without the salt the string cannot freeze to the cube.

If you used sugar, you would see the same effect. The cube sticks to the string. Dissolving other substances in water will also lower the freezing point and create the same effect.

Cleanup
Pour the water and ice cubes in the sink, and wash the glasses.

More to explore
Scrumptious Science: Making Ice Cream in a Bag, from Scientific American
High Seas: What Happens When the Glaciers Melt?, from Scientific American
Freezing-Point Depression, from CK–12
Freeze Your Fruit with Science, from Scientific American
Science Activities for All Ages!, from Science Buddies

This activity brought to you in partnership with Science Buddies

Science Buddies