The Magic of the Freezing Point of Water
Light Skins
The City College Of New York
March 26, 2024 – April 19, 2024
Funding Requested: 28.25$
Abstract
Have you ever tried making ice cream on your own? Ice cream needs to be cold but the question is how cold does it need to get to turn into the delight of a treat we know it to be? It is very rare that you meet someone who doesn’t like ice cream so learning how to make it could be a fun activity to do with friends or even a loved one. For that reason we decided to look into the science behind how cold ice cream needs to freeze, ice cubes won’t be enough, and even sometimes putting it in our home freezers isn’t enough. We will be using a styrofoam cup to create an iced salt water bath to get the water to -10 degree celsius in order to see if we can make ice cream at that temperature. We expect a soft serve type of ice cream to come out with this temperature instead of a hard style of ice cream.
Introduction (Alexis)
If you’ve ever churned ice cream with an old-fashioned hand-crank machine, you’ve likely packed the cream container with a mix of ice and rock salt. This allows the ice-salt mixture to get colder than pure water ice. This then freezes the cream within the ice cream machine.
Similarly, during the winter you may also have seen trucks or city workers spreading salt or sand on the sidewalks to prevent roads from getting icy after a snowfall. This is because the salt lowers the temperature at which water freezes. This makes the ice melt even when the temperature is below the normal freezing point of water.
In both cases, we see salt being used to lower the freezing point of something. This phenomenon, known as freezing point depression, occurs when a solute like salt is added to a solvent like water, causing the freezing point to drop below 0°C. This causes cases where pure water freezes at less than 0° Celsius (C) due to a solute like salt or sand.
This principle applies to all solutions, where the extent of freezing point depression depends on factors like the solution’s molality, the Van ‘t Hoff factor of the solute, and the molal freezing-point-depression constant of the solvent.
For this project, we’ll utilize water as the solvent and sodium chloride (table salt) and sucrose (granulated sugar) as the solutes. As we form the solution, we’ll closely examine each of the aforementioned factors.
Literature Review (All)
Project Narrative
There will be seven different test liquid solutions used in the experiment: three salt solutions, three sucrose (sugar) solutions, and a pure water control. The pure water control will determine the freezing point of the water in the experiment which may or may not be exactly 0℃.
- Test liquid #1 = 2.9 g NaCl in 100 mL water
- Test liquid #2 = 5.8 g NaCl in 100 mL water
- Test liquid #3 = 11.7 g NaCl in 100 mL water
- Test liquid #4 = 17.1 g sucrose in 100 mL water
- Test liquid #5 = 34.2 g sucrose in 100 mL water
- Test liquid #6 = 68.5 g sucrose in 100 mL water
- Test liquid #7 = 100 mL of water
The ice bath will be the area to freeze the test liquid solutions. The ice bath will be kept constant during the experiment, made with the equal amount of ice and salt every time.
Preparing the Ice Bath:
The ice bath will be used to freeze several test liquid solutions. The ice bath will be created after all test liquid solutions have been prepared.
- The styrofoam cup will be filled ¾ full with ice and ¼ – ½ inches of table or sea salt will be added on top of the ice.
- The ice-salt mixture will be stirred with a spoon or stirring rod to evenly distribute the salt with the ice.
- A thermometer will be used to check the temperature of the ice-salt mixture and wait until it drops to -10℃ before placing the test liquid solutions in.
- During the course of the experiment, the ice-salt mixture will melt into salt water. When this occurs, the salt water will be poured out of the styrofoam cup and will be replenished with ice and salt. Steps 1-4 will be conducted again to create the ice bath as many times as necessary.
Important Note: The temperature of the ice bath will always be waited until it drops to about -10℃ before continuing the experiment. The thermometer will always be carefully rinsed and dried before used to measure the freezing point of the test liquid to prevent carrying salt water into the test liquid solutions and vice versa.
Determining the Freezing Points of Test Liquids:
- First, one of the test tubes and one of the 250 mL beakers will be labeled a number corresponding to the test liquid, eg. “#1” for test liquid #1, with a permanent marker.
- To prepare test liquid #1:
- The beaker labeled “#1” will be placed on top of the digital scale and 2.9 g of table or sea salt (NaCl) will be weighed into the beaker.
- A graduated cylinder will be used to measure 100 mL of water, and the water will be poured into the beaker with the salt.
- The salt solution will be stirred with a stirring rod until all the crystals are dissolved.
- The stirring rod will be rinsed with water after each mix to prevent contamination to other test liquid solutions.
- The test tube labeled “#1” will be filled ⅓ full with the solution created in beaker #1 and placed into the ice bath in the styrofoam cup.
- The liquid in the test tube will be placed below the level of the ice and salt in the cup.
- Any ice or salt in the cup will not be allowed to get into the test tube.
- The test liquid in the test tube will be gently stirred with a thermometer while keeping track of the temperature.
- The test tube will be checked frequently during the first 5 minutes for appearing crystals, then checked every minute after that for at least 45 minutes.
- When the first ice crystals appear on the inside wall of the test tube, the temperature will be recorded in the lab notebook. This temperature will be the freezing point of the test liquid, Tn.
- After recording the temperature down, the content of the test tube will be emptied down the drain by rolling the test tube in the hand and melting it. Then, the same test tube will be refilled with fresh test liquid #1. Steps 3-6 will be repeated with a fresh sample from beaker #1. The test tube will be emptied and refilled again, repeating steps 3-6 for a third time. A total of three replicates for test liquid #1 will be obtained, all carried out in the same test tube.
- Steps 1-7 will be repeated for each of the remaining test liquids, ensuring the use of a different beaker and test tube for each test liquid to prevent contamination.
Analyzing the Data:
- The freezing point depression, T, will be calculated for all three replicates of each of the NaCl and sucrose solutions. These values for T will be recorded in the lab notebook.
- First, the freezing point temperatures observed for plain water, test liquid #7, will be averaged. This averaged temperature will be the solvent freezing temperature, Tf.
- Then T will be calculated using the equation below.
T=Tf-Tn
where Tn is the freezing point of the solutions recorded previously in the lab notebook.
- The freezing point depression, T, will be averaged for each of the NaCl and sucrose solutions. These averages will be recorded in the lab notebook.
- The molalities of the NaCl and sucrose solutions will be calculated.
- Molality, defined as the number of moles of a substance divided by the weight (in kg) of the solvent, as shown in the equation below, will be calculated.
Molality (moleskg)=Moles of Solute (mol)Kilograms of Solvent (kg)
- The number of moles of a substance will be defined as the weight of the substance (in grams), the amount added to create test liquids, divided by the gram molecular weight of the substance.
- The gram molecular weight of NaCl is 58.443 g.
- The gram molecular weight of sucrose is 342.3 g.
- 100 mL of water will weigh 0.1 kg.
- A data table of the results will be made, following the example shown in Table 1.
Table 1. Data to calculate the molality of the solutions.
| Solution | Grams of the Substance | Molecular Weight of the Substance (g) | Amount of Water (kg) | Molality (mol/kg) |
| Test Liquid #1 | 2.9 g NaCl | … | … | … |
| … | … | … | … | … |
| Test Liquid #7 | … | … | … | … |
- The expected freezing point depression for each of the NaCl and sucrose solutions will be calculated using the equation below:
T=Kf mi
where T is the freezing point depression, Kf is the molal freezing point depression constant, m is the molality of the solution and i is the Van’t Hoff factor of the solute. A second data table will be created to help with the calculations, following the example shown in Table 2.
Table 2. Data to calculate expected freezing point depression.
| Solution | Substance | Van’t Hoff Factor | Molality (M) | Kf for Water (℃/m) | Expected Freezing Point Depression (℃) |
| Test Liquid #1 | NaCl | 2 | … | … | … |
| Test Liquid #1 | NaCl | 2 | … | … | … |
| Test Liquid #1 | NaCl | 2 | … | … | … |
| … | … | … | … | … | … |
| Test Liquid #6 | sucrose | 1 | … | … | … |
| Test Liquid #6 | sucrose | 1 | … | … | … |
| Test Liquid #6 | sucrose | 1 | … | … | … |
- The experimentally measured freezing point depression will be compared to the calculated freezing point depression for each of the NaCl and sucrose solutions.
Personnel
All members of the [group name], Joshua Valle, Alexis Martinex, and Yuki Li, are undergraduate students at the City College of New York majoring in Computer Science.
In this proposal, every member was responsible for the annotated bibliography, Joshua was responsible for the abstract and budget, Alexis was responsible for the introduction, and Yuki was responsible for the project narrative, personnel, and timeframe.
Budget (Joshua)
| Item | Quantity | Cost per Unit | Total Cost |
| Gram Scale | 1 | $39.99 | $39.99 |
| Thermometer | 1 | $14.99 | $14.99 |
| Test Tubes (16 mm by 150 mm) | 7 | $1.50 | $10.50 |
| Test Tube Rack | 1 | $12.99 | $12.99 |
| 250 mL Beakers | 7 | $4.99 | $34.93 |
| 100 mL Graduated Cylinder | 1 | $9.99 | $9.99 |
| Glass Stir Rods | 1 | $1.99 | $1.99 |
| 12 oz. Styrofoam Cup | 2 | $0.99 | $1.98 |
| 20 lbs of Ice | 1 | $4.99 | $4.99 |
| Black Permanent Marker | 1 | $1.99 | $1.99 |
| Table or Sea Salt (500 g) | 1 | $3.99 | $3.99 |
| Granulated Sugar (360 g) | 1 | $3.99 | $3.99 |
| Plastic Spoon | 2 | $0.99 | $1.98 |
| Notebook | 1 | $0.99 | $0.99 |
| Total | $145.29 | ||
Timeframe
