Freezing point depression,Eutectic point,Freezing rate & Types of freezers

Freezing point depression is a colligative property of water.

Colligative properties of solutions are properties that depend upon the concentration of solute molecules or ions, but not upon the identity of the solute.

Freezing point depression occurs when the freezing point of a liquid is lowered by adding another compound to it. The solution has a lower freezing point than that of the pure solvent.

When you add salt to water, you introduce dissolved foreign particles into the water. The freezing point of water becomes lower as more particles are added until the point where the salt stops dissolving or temp remain const. And then decrease called eutectic point.

For a solution of table salt (sodium chloride, NaCl) in water, this temperature is -21°C (-6°F) under controlled lab conditions. On a real sodium chloride can melt ice only down to about -9°C (15°F).

A compound that yields more ions into a water solution would lower the freezing point of water more than salt.

Sodium chloride dissolves into 2 types of particles: one sodium ion and one chloride ion per sodium chloride 'molecule'.

For example, calcium chloride (CaCl2) dissolves into three ions (one calcium and two chloride ion) and lowers the freezing point of water more than sodium chloride.

Calculate the molar mass of the dissolved compound.

Molar mass is calculated as the sum of mass of all atoms in the molecule. Atomic weights of corresponding elements are given in the periodic table of the chemical elements.

example: Molar mass (NaCl)=M(Na)+M(Cl)=23+35.5=58.5 g/mole.

Divide the mass of the dissolved compound by its molar mass to calculate its amount in moles.

Amount (in moles) =mass (compound)/molar mass (compound).

Example - amount (Nacl)=15g /58.5 g/mole= 0.256 moles.

Divide number of moles (Step 2) by the mass (in kg) of the solvent to calculate the concentration of the dissolved compound. Note this concentration is called molality.

Molality(compound)=number of moles (compound)/mass of solvent in kg.

Example, molality (Nacl)=0.256 moles/0.1 kg of water=2.56 moles/kg.

Consider a possible dissociation of the dissolved compound and calculate the total number of ions (referred to as the van't Hoff factor) resulted from the dissociation.

The salt Na2SO4 dissociates as Na2SO4=2Na(+)+SO4(2-) i.e. produces 3 ions.

Sodium chloride produced two ions (Na+ and Cl-) i.e. its van't Hoff factor equals 2.

Navigate to solvent properties table or consult references and find the cryoscopic constant (Kf) and the freezing point for a particular solvent. Example- Kf of water is -1.86 and its freezing point is 0 degree celsius.

Solvent Formula Melting

Point (°C) Boiling

Point (°C) Kf(°C/m)

Water H2O 0.000 100.000 -1.858

Acetic acid HC2H3O2 -16.60 118.5 -3.59

Benzene C6H6 -5.455 80.2 -5.065

Camphor C10H16O -179.5 ... -40

Carbon disulfide

CS2 ... 46.3 ...

Cyclohexane C6H12 -6.55 80.74 -20.0

Ethanol C2H5OH ... 78.3 ...

Multiply molarity (Step 3), the cryoscopic constant (Step 5) and van't Hoff factor (Step 4) to calculate the freezing point depression (dT).

dT(degrees Celsius )=molality x cryoscopic constant (Kf) x van't Hoff factor.

Example - dT = 2.56 x (-1.86) x 2=-9.5 degrees Celsius.

Add the freezing point depression to the freezing point of the solvent to calculate the freezing point of the solution.Example - freezing point of the NaCl solution=0+(-9.5)=-9.5 degrees Celsius.

Name Formula Lowest Practical Temp.

Ammonium sulphate (NH4)2SO4 -7°C (20°F)

Calcium chloride CaCl2 -29°C (20°F)

Calcium magnesium acetate (CMA)

Calcium carbonate CaCO3, magnesium carbonate MgCO3, and acetic acid CH3COOH

-9°C (15°F)

Magnesium chloride MgCl2 -15°C (5°F)

Potassium acetate CH3COOK -9°C (15°F)

Potassium chloride KCl -7°C (20°F)

Sodium chloride (rock salt, halite)

NaCl -9°C (15°F)

Urea NH2CONH2 -7°C (20°F)

Def - Amount of heat extracted from the product per unit time called freezing rate.

Factors which affect freezing time:- Freezer type. Freezer operating temperature. Air speed in an air blast freezer.

Product temperature Product thickness Product shape Product contact area and

density Product packaging Species of fish

Plank's equation for calculating freezing time.

L = Heat to be extracted between the initial freezing point and final temperature (kcal/kg)

V = Specific volume of fish (m3/kg) D = Thickness of product (m) k = Thermal conductivity of frozen fish

(kcal/h m °C) P and R = Constants which depend on

shape

Freezing times for fish products :-

Air blast freezers: Fluidized and semi-fluidized freezers. Plate freezers :• Horizontal plate freezers.• Vertical plate freezers.• Automatic plate freezers. Liquid nitrogen freezer. Carbon dioxide freezer. Immersion freezers.

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