Key concepts
Chemistry
Food science
Freezing
Solution

Introduction
Do you enjoy ice-cold drinks? A slushy is about as close as you can get to liquid ice: colder than water but more drinkable than ice! Using some common household items, a little bit of patience and the help of science, you can make this delicious, sweet drink at home. Try this activity, and you will be rewarded with a delightful treat!

Background
A slushy is a dense, ice-cold beverage that resembles the thickness of melting snow. Slushies can be made in two ways: You can flavor finely crushed ice or you can alter the freezing process of flavored water so that loose, icy crystals form. Shaking the liquid while keeping it at the freezing point does just that.

The freezing point of a substance is the temperature at which the substance changes from a liquid to a solid. For pure water (H2O), this is 0 degrees Celsius or 32 degrees Fahrenheit. At this temperature water particles start to stick together and form a lattice, or a crystalline hard structure.

Dissolving salt in pure water lowers its freezing point. This happens because the salt particles are in the way, making it harder for the water particles to stick together. This phenomenon is called freezing point depression. Salt is not the only substance that lowers water’s freezing point but it is a very effective one that is commonly available. Dissolving other particles such as sugar has a similar but smaller effect. In general the more particles that are floating around in a fixed volume of water, the lower its freezing point will be.

Curious to know how freezing point depression can help us make slushies at home? Do this activity to find out!

Materials

  • Water
  • Table salt
  • Measuring cups
  • Teaspoon
  • Small mixing bowl
  • Four zipper-lock sandwich bags
  • Freezer
  • Gloves or a towel
  • Two smoothie shakers or 32-ounce food containers with lid, preferably translucent or transparent
  • Fruit juice—for example, orange, apple or grape juice (Do not use a sugar-free version.)
  • Food coloring (optional)
  • Thermometer that can go to –12 degrees C or 10 degrees F (optional)
  • Soda or fruit-flavored syrup (optional)


Preparation

  • Pour half a cup of water into a bowl. Add one teaspoon of salt and stir until it is dissolved. If you have food coloring, you can mix in one or two drops. This will give your special ice a nice color—and makes it easier to detect leaked saltwater solution in your slushy.
  • Pour the contents in a zipper-lock bag. Close the bag and set it aside.
  • Repeat the previous two steps three more times until you have four zipper-lock bags of saltwater solution.
  • Store the bags for at least five hours or overnight in the freezer. Make sure the bags do not touch one another so they do not freeze together.
  • Store at least one cup of juice and one cup of water in the refrigerator.


Procedure

  • After five hours check if the four saltwater solution bags in the freezer are frozen. If not, wait a few hours longer before proceeding.
  • Take the frozen bags of saltwater solution from the freezer using gloves or a towel to protect your hands. For each bag open the zipper, push the air from the bags and close the zipper again.
  • Touch the bag with ice briefly with your fingers. If you have a thermometer, measure how cold this ice is. Regular ice cubes are 0 degrees C (32 degrees F). Does this ice feel colder than regular ice cubes? Did the thermometer confirm what you felt? Why would you need this special ice to prepare a slushy?
  • Take the juice and water from the refrigerator. Pour one cup of juice in a shaker or food container. Rinse the measuring cup and pour one cup of water in the other shaker or food container.
  • Put two of the sealed zipper-lock bags with frozen saltwater solution into each container before closing the lid.
  • Shake one container a couple of times. Look through the sides of the container. Repeat with the other container. How do the liquids drip down from the sides of the container?
  • Place your containers down and count slowly to 20 before repeating the previous step. Do this for several minutes. Does the way the liquids drip from the sides of the containers change? Why would this happen? Does it happen at the same time in both containers?
  • Open the container after about three minutes. Do you see hints of slush? Do you see a similar amount in both containers?
  • Close the containers and continue shaking intermittently.
  • Once you see a thick mass sliding down the sides of a container, open the container again and look inside. Do you have a slushy? How did the special ice in the bags change? Why would it have changed?
  • If the special bag of ice still has solid pieces of ice inside, you can close the container and continue shaking intermittently to make a denser slushy. Why, however, might it not be a good idea to continue if the special ice has melted?
  • If the special ice has melted, take out the bags of saltwater and set them aside. If saltwater has leaked in your slushy, discard the slushy, freeze the zipper-lock bags with saltwater solution after rinsing them and try again.
  • There is a possibility the juice never turned into a slushy. If your juice did not freeze, refreeze the zipper-lock bags with saltwater solution after rinsing the outside and dissolving an additional half teaspoon of salt in each. Why do you think your juice did not turn into a slushy? Do you think you can solve by adding more salt to the solution in the bags? Why or why not?
  • Pour the slushy content into a glass. If you have diluted syrup or soda, pour some over the slushy made with just water, mix well and drink. Drink your juice slushy as is. Is one slushy better than the other?
  • Extra: Find out if it is easier to first mix syrup in with the water and make a slushy of the mixture or to mix in the syrup after you have created slush from the water alone. Why would this be the case? Which way tastes best?
  • Extra: Try other juices. Which juice is easiest to transform into a slushy?
  • Extra: Can you find ways to make a larger slushy?
  • Extra: Find out what happens if you mix more salt into the water to create the special ice. What is the coldest special ice you can make?
  • Extra: Test and see what happens if you do not shake the containers while making the slush.

Observations and results
Did water turn into slush faster than juice did? Juice freezes at a lower temperature compared with water, so it needs to cool over a longer period before ice crystals appear.

Pure water freezes at 0 degrees C (32 degrees F). Adding particles to water makes it freeze at a lower temperature. That is why the saltwater you made in this activity felt so cold when you took it out of the freezer. While making the slushy, it absorbed heat from the liquids in the containers. This process melted the saltwater ice as it cooled the liquids.

Fruit juice is made of fruit sugars dissolved in water together with some other particles, such as vitamins and minerals. Because fruit juice contains pure water with particles dissolved in it, it freezes at a lower temperature than pure water does. Most often its freezing point is not as low as that of the saltwater you created, so the frozen saltwater could still freeze the juice, although it did so more slowly than it could freeze water.

Cleanup
If you would like to make more slushies later, you can reuse he zipper-lock bags with saltwater solution after rinsing and refreezing them. To discard them, you can dispose of them in the sink.

More to explore
Make Ice Cream in a Bag, from Science Buddies
Freeze Your Fruit with Science!, from Scientific American
Chemistry of Ice Cream-Making: Lowering the Freezing Point of Water, from Science Buddies
What Makes Ice Melt Faster?, from Scientific American
Science Activities for All Ages!, from Science Buddies

This activity brought to you in partnership with Science Buddies

Science Buddies