Freezing Point Depression (Jackie Recinos)

Title: Discovering the impact solutes have on solvents through ice cream making.

Principle(s) Investigated: Students will observe the colligative properties in an ice water- solute solution and determine which solute is better effective for real world applications.

Standards: MS-PS1-2. Analyze and interpret data on the properties of substances before and after the substances interact to determine if a chemical reaction occurred.

HS-PS1-3. Plan and conduct an investigation to gather evidence to compare the structure of substances at the bulk scale to infer the strengths of electrical forces between particles.

HS-PS2-6. Communicate scientific and technical information about why the molecular level structure is important in the functioning of designated material.

HS-PS3-4. Plan and conduct an investigation to provide evidence that the transfer of thermal energy when two components of different temperatures are combines within a closed system results in a more uniform energy distribution among the components in the system (Second Law of Thermodynamics).

Materials:

• 2 Freezer (large) Ziploc bags
• 2 small Ziploc bags.
• 2 Ziploc bags of cream solution (1 per group)
• 2 pairs of oven mitts
• 8 cups of Ice
• Table salt
• Sugar
• Simulation K-12 interactive
  • 2 Thermometers
• Collaborative document

Procedure:

1. Review

A. Questions will be asked to the class:

• What is a solution?
• What are solvents and solutes?

B. Students will then open collaborative document to asses their knowledge on solutes and solvents.

2. Phenomenon

A. Class will separate into two groups.

B. Groups will be provided:

• 1 Freezer Ziploc bag
• 1 Small Ziploc bag with cream solution
• 1 pair of oven mitts
• 4 cups of Ice
• Thermometer
• Unknown solute A or B

C. Students will add ice and small Ziploc bag with cream solution into Freezer Ziploc bag (Large). They will then combine the unknown solvent into the ice and measure the initial temperature and record into Group Collab document.

D. Each student in the groups will have 2-3 minutes to shake the Ziploc bag with oven mitts for a total of 8-10 minutes.

Note: Groups may or may not get ice formation in cream solution.

E. After ten minutes, the groups will record the final temperature of the solvent.

F. Once complete, students will analyze their data collected and figure out their unknown solute.

3. Explanation

A.Teacher will explain the differences in temperature and the product of the cream solution

B. Teacher will show photographs of the molecular structure of water when frozen and attempt to demonstrate what is happening when the solutes "dissolves" with the ice with:

• simulation
  • water molecule physical demonstration
    • 6 students need. Students will group up and use their right hand and place it on their neighbor's shoulder until they form a hexagonal shape. two other volunteers will act as solutes and disturb the groups interaction
  • ΔTf=kf⋅m⋅i
• Temperature change is dependent on the number of moles/ kg of solute (molality).
• teacher will demonstrate dimensional analysis of 1 gram of salt and sugar to show amount of molecules interacting.

(1 g of sugar) x((1 mol sugar) / (342g of sugar)) = 0.0029 moles of sugar (sucrose) present

(1 g of NaCl) x ((1 mol NaCl) / (58g of NaCl)) x ((1 mol of Na⁺) / (1 mole of NaCl)) = 0.017 moles of Na⁺ ions

(1 g of NaCl) x ((1 mol NaCl) / (58g of NaCl)) x ((1 mol of Cl^-) / (1 mole of NaCl)) = 0.017 moles of Cl^- ions

(0.017 moles of Na⁺ ions) x ( 0.017 moles of Cl^- ions) = 0.034 moles of combined Na⁺ and Cl^- ions

• Temperature change is also dependent on the properties of the solvent (K_{F constant}) and the number of compounds the solute dissociates into (i).

3. Applications to real life and nature

A. salt on the roads and sidewalks

B. Arctic ocean doesn't freeze over.

C. Arctic fish that don't freeze.

D. Frozen frogs can come back from the dead? YouTube

.

Student prior knowledge:

Students need to know the inter molecular forces between ionic and covalent bonds and the interactions between water molecules. They need to have a comprehension of moles and compounds.

Students need to understand the concepts of the second law of thermal dynamics and that pure water freezes at 0 degrees Celsius and will not decrease its temperature further.

Explanation:

Dissolving a non-volatile solute (sucrose or table salt) to a pure solvent, such as water, will reduce its freezing point. Reducing the freezing point (F_p) will stimulate a phase change from solid to liquid because the newly reduced F_p is not reached. This means that the water and solute solution will decrease in temperature past 0 degrees Celsius, absorbing more energy. The ice in the Ziploc bag gains energy from the cream mixture, water and environment in order to phase change from solid to liquid. With time, the water and solute (sugar or table salt) solution will removing most of the energy ( heat ) from the cream solution. The cream solution will form ice crystals and become ice cream.

Freezing point depression is dependent on the properties of the solvent and how well it interacts with the solute. The temperature change is also dependent on the solute's amount of moles/kg and the number of compounds the solute dissociates into. The water interacts well with both solutes tested, table salt and sucrose. However, due to the molecular structure of sucrose, it gets solvated. The water molecules will surround each individual sucrose molecule uniformly without destroying the structure of sucrose (See figure below). Sucrose is significantly a larger molecule, than table salt. In 1 kg of sucrose, there are 2.92x10^-6 moles; while 1 kg of table salt contains 3.4x10^-5 moles. Also, table salt molecules dissociate into two ions. Na⁺ and Cl^- interact more efficiently with water molecules to disrupt their hexagonal ice formations. As a result, more energy is required for the water molecules to organize once again into their hexagonal formations.

(1 g of sugar) x((1 mol sugar) / (342g of sugar)) = 0.0029 moles of sugar (sucrose) present

(1 g of NaCl) x ((1 mol NaCl) / (58g of NaCl)) x ((1 mol of Na⁺) / (1 mole of NaCl)) = 0.017 moles of Na⁺ ions

(1 g of NaCl) x ((1 mol NaCl) / (58g of NaCl)) x ((1 mol of Cl^-) / (1 mole of NaCl)) = 0.017 moles of Cl^- ions

(0.017 moles of Na⁺ ions) x ( 0.017 moles of Cl^- ions) = 0.034 moles of combined Na⁺ and Cl^- ions

Questions & Answers:

1. Are there solutes other than salts that are as or more effective on lowering the freezing point of water?

Salts are the ideal for effective colligative properties. Salts are made up of ions and can easily be separated due to their weak ionic bonds. However, since the freezing point depression is directly proportional to the molality (mol/kg) of the solute in a solvent, 5 moles of a carbohydrate can be just as effective as 0.5 moles of a salt.

2. How are the water molecules interacting with the solute and the temperature continues to drop?

In the case of a solution of water and salt, the salt dissolves and the ions separate to further interact individually with water molecules. The electrogenative Oxygen, in a water molecule, is also attracted to a positively charged ion, such as sodium, and the electronegative Hydrogens are attracted to the negatively charges ions of chlorine. Therefore, the hexagonal shape of solid water molecules are now being disrupted. This disruption makes the water gain energy, causing the temperature to drop. With the right ambient temperature, the water can continue to gain energy until the ions have no affect and the water recrystallizes again.

3. What if I added more salt to my ice water (solvent), could this make the ice cream process faster?

It can depend on how much water you have and how much solute will you put into the solvent. If there is not enough solvent to dissolve the solute, then you will not observe a freezing point depression, causing supersaturation. The freezing point depression is proportional to the molality (# moles of solute/ kilograms of solvent).

Applications to Everyday Life:

Salting Roads: Cities that experience extreme cold weather deal with dangerously frozen roads/pavement. With the use of salts such as CaCl₂ and MgCl_2, they contain more ions that can interact with the water molecules and lower water's freezing point more effectively. Additionally,engineers are finding ways to add carbohydrates such as cane sugar and acetone to the deicing solution to have less corrosive effects and be a little more environmentally friendly.

Frogsicles: Certain species of frog, such as the North American wood frog, can become frozen during the winter and enter a state of suspended animation (most organs cease to function without killing the organism). Once the days become cold enough, urine is stored into the blood and the liver starts converting its stored glycogen into glucose. Urine and blood are dispersed into the circulatory system. The frog creates its own solution of antifreeze, which prevents the cells from loosing too much water. This is enough to keep tissue alive for the spring thaw.

Car's antifreeze or coolant for extreme weather: A car's engines can easily freeze over or over heat, but with a help of the radiator, it can help with the heat exchange in a car. However, there are cases when the radiator is not working properly, due to extreme temperatures. By adding antifreeze/ coolant made up of a base of Ethylene Glycol, it can lower the freezing point in your radiators water and also increase the boiling point .

Photographs: