Freeze Your Fruit With Science!
Pop Science Quiz: What happens to water when it reaches 0°C (32°F)?
Answer: It freezes!
But does water always freeze when it reaches 0°C (32°F)?
Believe it or not, water can sometimes be cooled to temperatures below its freezing point and still remain liquid. In this state the water is supercool.
How can this happen? In this activity you will create your own supercool water, and initiate its transition from liquid to solid. All while making a tasty snack!
When water is warm or even room temperature, the water molecules are moving around, bumping into each other. The warmer the water, the higher the energy of the molecules and the more they collide with one another. In contrast, as water cools, the molecules move slower until they eventually stop moving altogether. In a frozen state, water molecules are locked together in a crystalline array. These crystals are thought to form around ‘impurities’ in the water, such as dust and minerals. Without the presence of these impurities, the water can be supercooled past its freezing point.
In this activity you will create a ‘bath’ using salt and ice. Adding the salt results in a bath with a temperature below water’s freezing point, a phenomenon known as freezing point depression. You will use this bath to supercool the pure water it surrounds. Then you will perform some supercool experiments on this water!
- Fresh fruit, cut into large chunks (at least 3 pieces)
- Wooden skewers (at least 3)
- 3 tall, clear plastic cups
- 2 Large metal or glass bowls
- 2 kitchen thermometers (or one that you will switch between your two cups)
- A freezer
- Distilled water
- A clock or timer
- 4-5 tablespoons of salt
- A cardboard box (any size)
- Put one piece of fruit on each skewer.
- Place the skewers in the freezer for 2 hours.
- Cut two circles out of the cardboard box, big enough to act as lids for the plastic cups.
- Fill each small glass jar 1/3 full of filtered water.
- Place 1 cup in the center of each large metal bowl.
- Cover the plastic cups with your cardboard lids.
- Surround each cup with ice. Make sure the ice is well-packed around the cup, and comes up higher than the height of the water in the cup. Don’t get any ice inside the cup.
- Sprinkle at least 2 tablespoons of salt over the ice. Don’t get any salt inside the cup!
- Remove the cardboard lids from the cups.
- Monitor the temperature of the water in your cups using your thermometer. If you only have one thermometer, keep it in one cup until the water in that cup cools to below 30F. At that point you can remove the thermometer and check the temperature in the other cup.
- Once the water has reached a temperature of less than 30° F (or between -1 and -3° C) your water is supercooled! This should take approximately 45 minutes.
- Carefully remove the thermometer and set it aside.
- Remove the frozen fruit skewers from the freezer. What do you notice about the surface of your fruit? Does it have ice on it?
- Gently dip a piece of fruit (still on the skewer) into the first cup of supercooled water. Watch what happens to the fruit when it touches the water. What do you notice about the water surrounding the fruit? What about the fruit itself?
- Carefully remove the fruit from the water. Look at the fruit. What do you notice about the surface of the fruit?
- If there is any water left in the first cup, test the temperature of the remaining water. Wait for it to become supercool again, then repeat this experiment with your remaining fruit skewers!
- For the second cup, take a fresh piece of ice (without any salt on it) and carefully drop it into your cup. What happens to the water in the cup when the ice reaches it? How does this compare with what happened to the fruit?
Observations and Results
In this activity you made water REALLY cold – in fact, you made it supercool! To understand what this means, imagine the molecules that make up the water in your cup. When water is warm or room temperature, those molecules move around, bouncing off each other and the side of the cup.
However, when you remove the heat energy from the cup, those molecules slow down and eventually stop moving. When they stop moving, the molecules form crystals, like those you see creating frost on your windows, or drifting from the sky as a snowflake. Crystals don’t just form on their own. Before you have a crystal, you need a ‘seed’, something for the water molecules to hold onto as they arrange themselves. The water you used in your cups was distilled water, and therefore generally free of contaminants such as dust and microbes. As a result, there wasn’t anything in the water for the molecules to crystallize around. Therefore, instead of crystalizing when it reached its freezing point, the water remained liquid.
This changed when you added your fruit, and your ice cube, to the water! Both the fruit and ice acted as the ‘seed’ in your supercool water, and gave the water molecules something to crystalize around. In the case of the fruit, the ice crystals you noticed on the surface of the fruit acted as the seed. And in the second cup, the ice itself was the seed!
Ask an Expert
- Supercooling Water and Snap Freezing, from Science Buddies
- Chemistry of Ice-Cream Making: Lowering the Freezing Point of Water, from Science Buddies
- Science Activities for All Ages!, from Science Buddies