Melting Point Depression

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Page ID: 131409

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Required Training	Required PPE
UC Lab Safety Fundamentals	Lab coat, safety glasses/goggles, nitrile gloves

Equipment	Chemicals
Glass baking dish	Ice
String, 20 cm long	10 g Sodium Chloride, NaCl
200 mL Water
250 ml Beaker

Procedure:

I. Method 1

Make a large block of ice a few days ahead of time
Transfer ice to a glass baking dish right before the demo
Pour some NaCl onto one area of the ice surface
The ice will start to melt and form a puddle/hole where the ice was poured

II. Method 2

Place an ice cube of any size in a beaker of water
Wet the string and lay across the floating ice cube
Pour some NaCl on top of the string
After a few seconds the string will freeze to the ice cube and you can lift the ice cube by holding both ends of the string.

Discussion:

The melting point of pure water is 0ᵒ C. As the salt dissolves into the ice, the melting point is lowered below 0ᵒ C causing it to melt faster. Because the phase change from solid to liquid can only occur at the melting point, the temperature on the ice surface drops below 0ᵒ C. This causes the water in the string to freeze to the ice cube slowing the cube to be lifted.

Hazards:

N/A

SOP

N/A

Disposal (by Storeroom)

The ice, water, and ice can all be flushed down the drain.