Instantly freezing something might sound impossible, for even after placing something directly into a freezer it still takes time to actually freeze. For this experiment, the instantaneous freezing is possible because the carbon dioxide, or CO2, in the club soda actually lowers the freezing point of the water. Now how exactly does the added CO2 do this? Well, whenever something is added and dissolved in a liquid it is called solute, and the liquid then is called the solvent. When solutes are added to a liquid, the freezing point of that liquid (solvent) changes based on the characteristics of the additive. Water freezes at zero degrees Celsius when pure; however, when a solute is dissolved in it, water can be cooled to temperatures below zero. This is referred to as a “freezing point depression.” Although it is a gas, carbon dioxide can dissolve in water and, as a result, become a solute and reduce the freezing point of water. The CO2 is only trapped within the bottle for as long as cap is on, so when the cap is removed the carbon dioxide is able to escape from the water and is no longer a solute. In this experiment, we cool the seltzer water to about -15 degrees Celsius, which is well below the freezing point of water (zero degrees Celsius). Once the CO2 is no longer a solute, the seltzer water is at a temperature below its freezing point, and as a result it instantly freezes.
In this experiment the soda is super cooled in an ice bath. In order for the ice bath to work it actually has to become much colder than ice while still being an ice bath, and not a large block of ice. To do this rock salt or ice cream salt, which is sodium chloride, is added to regular ice in order to bring down the temperature. Sodium chloride is also a solute and mixing it with water lowers the freezing point of the water. The freezing point of water and melting point of ice are the same temperature. When sodium chloride is added to ice (solid water), the melting point of ice is lowered and, as a result, the salt and ice mixture is much colder ( -10 to -15 degrees Celsius) than ice water (zero degrees Celsius). Using salt and ice, we can cool the club soda to a temperature that is much lower than the actual temperature of the ice without the added rock salt.
PK – grade 3 students
At this level the students can be asked about different physical states of matter. They can be asked to give examples of solids, liquids, and gasses. They can also be asked if they know how to change one state to another. You can discuss how water boils and how ice melts as examples. Then, they can be asked about what they observed during the experiment. What phase change took place? Why did it happen? Also, the students can be asked what makes the soda bubbly.
Grade 4 – 6 students
At this level the students will have already been introduced to, and have learned about the scientific method. Most of them will have also already conducted experiments in various science courses. So, ask them to go through the steps of the scientific method for this experiment. What happened to the soda when it was placed in the ice bath? How did the CO2 keep it from freezing long before the bottle was open? Have each student make their own observations and ask their own questions, and then allow them to draw their conclusions based on how they followed the steps of the scientific method. Students at this level are also aware of the properties of solid, liquid, and gaseous substances, that matter is made up of atoms, that elements are made of one kind of atom, and that the elements are organized in the periodic table by their chemical properties. They know that sodium chloride and carbon dioxide are compounds made each from a pair of elements.
Middle school students
At this level, students will have had some more science courses, and topics such how a phase change can occur can be discussed. A solid becomes a liquid when heat is added via melting, and a liquid becomes a gas in the presence of heat due to evaporation. A gas can also go back to being a liquid through condensation. To go a little further a solid can go straight to a gas through a process called sublimation, and a gas can become a solid via deposition. The last couple terms might be a little foreign to the students, but they should still be able to grasp the basic idea, and then they can be asked to explain which processes are occurring in this experiment, and how each component, such as the water and the carbon dioxide, plays a role. Students at this level know the structure of the atom and know it is composed of protons, neutrons, and electrons. They know that compounds are formed by combining two or more different elements and that compounds have properties that are different from their constituent element (e.g., sodium is a metal and chlorine is a gas while sodium chloride is a non-metallic solid. They also understand that the states of matter (solid, liquid, gas) depend on molecular motion.
College-level/ scientific description
At this level the students will have had physics and chemistry courses. At this point what is occurring at an atomic level can be addressed. What is the difference, molecularly, between each of the states of matter, and how does this affect their observable properties? For example, solids are tightly packed, liquids are more fluid and don’t keep their shape, meaning that they can spread out into their container/environments, and gases fill the entire volume of their container. How does knowing these properties help to explain what is occurring during the experiment? The ability of a substance (solute) to lower the freezing point/ melting point of a solvent as well as the ability of a solute to raise the boiling point of a solvent is referred to as “colligative properties.”