SUBJECT AREA: Chemistry

CONCEPTS COVERED: Melting Points

MATERIALS:

1. Ice cube floating in a cup of cold water.

2. Piece of thread or string

3. salt shaker

PROCEDURE:

1. Float the ice cube in the glass.

2. Show the audience the salt shaker and thread.

3. Dip the thread in the water, lay it over the ice cube and shake salt on it.

4. Wait a minute or so, lift the the thread slowly out of the water and the ice cube will be frozen to it.

QUESTIONS:

1. What does salt do to ice on roads in the winter?

2. At what temperature does ice melt?

3. What do you think salt does to the melting point of ice?

4. What must the melted ice above the string do in order to lift the cube?

5. Where else do we find this principle of lowering the melting point of ice applied in daily life?

RATIONALE:

As salt has a lower melting point or freezing point than pure water, the addition of salt to the ice cube makes it melt at the places where the salt is sprinkled. This means that the ice around the thread will melt, but the temperature of this water above the ice cube is still below zero degrees C. When the salt dissolves in more of the melted ice, the solution gets more dilute, increasing the freezing point close to zero degrees C. As this water still has a temperature of a few degrees below zero degrees C, it freezes again. This makes it possible to lift the ice cube out of the water. When making ice cream at home, crushed ice and salt are mixed in the ice cream maker to lower the temperature below the freezing point of water.

APPLICATION:

Salting streets and sidewalks

Home made ice cream

TAKEN FROM: Mr. Wizard's Supermarket Science