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Freezing and Thawing of Water and Salt

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Author: Marc Rosner

In winter salt is sprinkled on highways and sidewalks to melt sleet and snow, but in summer salt and water are mixed to freeze ice cream! The reason for this paradox is that salt lowers the melting point of water. There are several experiments you can conduct to demonstrate this effect.

Salt, or sodium chloride, is an ionic solid (a substance created when two or more elements react to form a fixed crystalline structure). The sodium and chlorine that make up sodium chloride are positive and negative ions. These charged particles grab water, which is polar (having a pair of charges that are equal and opposite). Because water has a positive and a negative side (hydrogen and oxygen), it is harder for the water molecules to bind to each other during freezing.

Materials

  • ice cubes
  • glass
  • water
  • table salt
  • book of matches
  • mortar and pestle (optional)
  • test tube or other small glass vial
  • stirring rod
  • thermometer (range at least –20° C to +120°C)
  • notebook and pencil
  • small bowl
  • uninsulated copper wire

Freezing and Thawing of Water and Salt

Procedure

  1. Try this parlor trick: Float an ice cube in a glass of water. Have a shaker of salt within arm's reach. Hand a friend a paper match and challenge him or her to remove the ice cube from the water without lifting it with any implement other than the match. When your friend gives up, bend the head of the match at a right angle, place the body of the match flat on top of the ice, and cover the match with a thin layer of salt. The match will promptly freeze to the cube. Lift the cube from the glass by the head of the match.
  2. The salt melted some of the ice all around the edge of the match. Ions of sodium and chlorine gained freedom of motion when they dissolved in the film of water on the surface of the cube, and motion was induced by heat drawn from the ice. As a result, the temperature of the film of water in contact with the lower surface of the match dropped below the freezing point and turned into ice that cemented the cube to the match.
  3. Crush some ice in a mortar and pestle (or by another method). Put it in a test tube or other small glass vial. Add a little water. Gently mix with a stirring rod. Take the temperature with the thermometer. It should be very close to Doc, the melting point of pure water. Now add some salt and mix well. Record the new temperature. It should be lower. How low can you make the temperature go? Temperatures of –5°C or lower are possible in an amateur science lab. Quickly dip the bottom of the vial in some water in a bowl, then take it out. Frost will form on the outside as liquid water freezes to the colder surface.
  4. The lowest temperature Gabriel Fahrenheit (1686–1736) could obtain by mixing salt and ice was –17.8°(, or O°F.
  5. Take a thin copper wire, about 15 to 20 cm long, from which the insulation, if any, has been removed. Place an ice cube on a table. Grasp each end of the wire so that your thumbs are about 5 cm apart. Push down on the ice cube with the wire. As you hold the wire, watch as it magically melts into the ice cube and the water freezes behind it. This refreezing after the removal of pressure is called regelation. You can actually pass the wire through the cube this way, leaving the cube intact.
Pressure (force per unit area) also decreases the melting point of water.

Freezing and Thawing of Water and Salt

References

Adventures with Atoms and Molecules: Chemistry Experiments for Young People (Adventure with Science, Nos.I–5) by Robert C. Mebane and Thomas R. Rybolt (Berkeley Heights, N.J.: Enslow, 1998).

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