In this lab, students will investigate how dissolving salt (sodium chloride) in water changes the freezing point of the solution. While investigating this, they will make ice cream from small coffee creamer cups.
By the end of this lab, students should be able to
- Understand that dissolving chemicals in water changes the freezing point of the solution.
- Identify phase changes, such as melting and freezing.
- Describe the meaning of a solution, and identify the solute and solvent in a situation.
This lab supports students’ understanding of
- Freezing-Point Depression
- Properties of Matter
- Phase change
Teacher Preparation: 1 hour
Lesson: 45 minutes
- 1 Individual, sealed cup, of Liquid Non-Dairy Creamer for each student
- Empty Wide Mouthed Bottle with lid (squeeze bottle, as shown in photo work best), 1 bottle for every 2 students.
- Quart sized zip-lock bags can replace bottles (do not recommend for young students)
- Rock salt (table salt will work also)
- Measuring spoons:
- ¼ Tablespoons if using a small bottle
- ½ cup if using quart bag or large bottle
- Ice ( Approximately 15 pounds for 20 students)
- Ice chest
- Thermometer 1 for each group
- Make sure to have plenty of ice available, as it melts, student scan add more to their container. I used about 15 pounds for 20 students.
- There are a lot of options for brands, and flavors of creamers to use. I used the Stone Cold Vanilla flavor! It tasted just like Ice Cream.
- If bottles with lids are not available, Quart sized zippered bags will work, but I don’t recommend using the bags with young students.
- Make sure to have excess salt on hand, it is necessary for the process to work correctly.
- The salt is added to ice and lowers the freezing point of water from 0°C to –10°C. Packing the creamer cup in ice, water, and salt and then swirling to lower temperature evenly through the cup, results in an iced creamer cup.
- Always wear safety goggles.
- Students should not drink the salt water solution
- Students should wash their hands thoroughly before leaving the lab.
- When students complete the lab, have students dispose of the salt water into a sink.
- Do not consume lab solutions, even if they are otherwise edible products.
- Food in the lab should be considered a chemical not for consumption
- I recommend that you do not do this lab in the science lab setting. Students should not eat in a science lab. For the least amount of mess/clean-up, it works best outside. The cups will sweat quite a bit and will get very messy. Some of the lids did pop off during the process.
- Most things on earth are liquids, solids, or gases, which are the three states of matter. States of matter can change from one to others through melting (solid to liquid), freezing (liquid to solid), boiling (liquid to gas), sublimation (solid to gas), and deposition (gas to solid). Substances have a temperature point where each of these phase changes occur. For this experiment, freezing point is a crucial concept.
- Freezing point is the point where a liquid becomes a solid. The freezing point of water is 0°C. The process of freezing deals with how compact the particles are. Imagine a solid composed of many molecules tightly packed together. In contrast, liquids are more fluid because their molecules are not as tightly packed together. When you freeze a substance, the liquid molecules become more compact and become solid. Pure liquids are not the only liquids that can be frozen. Solutions can be frozen as well.
- Solutions consist of solutes and solvents. A solute is the substance dissolved, while a solvent is the substance that the solute dissolves in. For example, salt dissolves in water. Salt is the solute, and water is the solvent. Imagine freezing pure water. Water molecules will become more tightly compact and become solid when they are frozen at zero degrees. Now imagine freezing a solution mixture, such as salt in water. The salt molecules will get in the way of crystal formation, and therefore the solution will have a harder time freezing. Because the salt molecules are in the way, the ice will melt faster. This is because the water molecules can easily be separated from one another since the salt molecules prevent them from being as close to each other as they are when pure water is frozen.
- This means that when salt is added to ice, it makes the ice melt faster, resulting in a slushy mixture at zero degrees, rather than solid ice at zero degrees. If at zero degrees the salt in water is a slushy mixture, then the point where this solution will freeze will be a lower temperature. This is known as the lowering of the freezing point, also called freezing point depression.
- For this lab, it is important to lower the freezing point to make ice cream. Ice cream forms at temperatures lower than 0 degrees Celsius Making ice cream without a freezer is difficult to do with ice formed by pure water because the solid ice is at about zero degrees Celsius. By lowering the freezing point by putting salt on ice, a water-salt-ice solution is formed which is lower than zero degrees and will allow the ice cream to freeze.
- Additional background information
For the Student
The melting and freezing point of water is 0⁰C. When you add a substance to the water it will change the melting, freezing, and boiling point.
- Did you know that people put salt on icy roads to help melt the ice? Why does the work?
- Why don’t the oceans freeze?
In this experiment, you will investigate how dissolving chemicals in water changes the freezing point of a solution.
- Liquid Non-Dairy Creamer
- Empty Wide Mouthed Bottle With Lid
- Sodium Chloride (salt),
- Safety goggles should be worn during this activity
- Do not drink the salt water solution
- Wash your hands thoroughly before leaving the lab.
- When you complete the activity, dispose of the salt water into a sink.
- Place your creamer cup in the bottle.
- Fill your bottle or bag ½ full with ice.
- Pour the ¼ cup of sodium chloride (salt) into your bottle.
- Fill the bottle with ice.
- Add water about ½ full.
- Measure the temperature of the mixture in your bottle.
- Record the temperature in your data table below.
- Place the lid on your container.
- Shake your bottle for 1 minute to mix everything together.
- Measure the temperature again and record in your data table.
- Shake your bottle for 3 minutes.
- Measure the temperature again and record in your data table.
- Shake for another 2 minutes.
- Measure the temperature again and record in your data table
- Remove the creamer and, without opening it, check for firmness either by squeezing the sides or shaking the cup near your ear. If you feel or hear liquid sloshing, place the creamer back in the bottle for 2 more minutes.
- Open and enjoy!
Starting Temperature (⁰C)
Temperature After 1 minute (⁰C)
Temperature After 4 minutes (⁰C)
Temperature After 6 minutes (⁰C)
Total Temperature change (⁰C)
How much did the temperature change during this experiment? Subtract your final temperature from your starting temperature.
- What happened to your creamer cup when you added the salt and ice to your bottle/bag?
- Why did this happen?
Write a few sentences summarizing what you learned from this activity.
- What did you like the best? Why?
- What did you not like? Why?