Effects Of Solutes (page 2)

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Author: Janice VanCleave

Try New Approaches

  1. Does increasing the heat source raise the temperature of the water? Repeat the experiment using a higher heat setting on the stove.
    1. Does adding a solute (substance dissolved in a solution) to the water affect the boiling-point temperature of the water? Repeat the original experiment adding ½ cup (125 ml) of sodium chloride (table salt) to the water.
    2. Does using a different solute change the results? Repeat the original experiment adding ½ cup (125 ml) of sucrose (table sugar) to the water. Science Fair Hint: Use neatly labeled graphs as part of a display.

Design Your Own Experiment

    1. At what temperature does the freezing (the physical change from a liquid to a solid) of water occur? The temperature of a mixture of ice and water will change until the freezing point (or the melting point) is reached. At the freezing point, the temperature of the solution remains constant as energy is used in the phase change. Demonstrate this by filling an ice-cube tray with distilled water and placing it in a freezer overnight. Fill a drinking glass half full with these pure ice cubes (made with distilled water) and cover the ice with distilled water. Insert a thermometer into the glass of ice water. Set the glass in a large can. Fill the can with ice (this does not have to be pure ice) mixed with 1 cup (250 ml) of table salt. Gently stir with the thermometer. Read and record the temperature every 15 seconds. Add pure ice as the cubes in the glass melt. Continue stirring and recording the temperature until a constant temperature is reached.
    2. Does adding a solute to the water affect the temperature of the ice water? Repeat this experiment adding 4 tablespoons (60 ml) of table salt to ice water in the glass.
    3. Rock salt is added to ice in an ice-cream maker to lower the temperature. Is rock salt more effective at lowering the temperature than table salt is? Repeat this experiment using rock salt instead of table salt. Use information about colligative properties (properties that depend only on the number of particles in solution) to explain the results.
  2. Do solutes affect the freezing of water? Fill two 5-ounce (150-ml) cups with distilled water. Dissolve 1 tablespoon (15 ml) of salt in one of the cups of water. Label the cup containing the salt with the letter S. Set both cups in a freezer. Check the cups every hour for one day; then leave the cups in the freezer overnight.
  3. Place the frayed end of a 12-inch (30-cm) cotton string on top of an ice cube. Rub the ice as you press the string against it. The string should cover as much of the surface of the ice as it can and lay flat against the ice. Sprinkle 1 teaspoon (5 ml) of salt over the string. Wait for one minute and gently lift the string. The ice cube should be stuck to the string and it should be possible to suspend the cube by holding the string up. A brief explanation is that rubbing the ice as you press the string onto it melts the ice and that water is absorbed by the string. The salt also melts the ice. The salt then dissolves in the water, producing a salty solution that freezes at a lower temperature than the freezing temperature of water, which is 32°F (0°C). The temperature of the salty water surrounding the string is lower than the freezing temperature of any unsalted water absorbed by the string; thus, the pure water in the string freezes and sticks to the ice cube. Improve upon this explanation and include information about freezing-point depression and the fact that some of the lower-concentration salty water freezes.

Get the Facts

  1. A solution has a different boiling point and freezing point than a pure solvent because of the colligative properties of the solution. Colligative properties depend only on the number of particles dissolved in the solvent and not on the nature of the solute or solvent. Find out more about the effects of colligative properties on solutions, including vapor-pressure lowering, boiling-point elevation, and freezing-point depression. How much of a pressure and temperature change can solutes make? Why does salty water have a lower temperature than ice water without the salt? Why are icy sidewalks sprinkled with salt or sand?
  2. The freezing and boiling points of a solution can be predetermined with the molal freezing-point and boiling-point constants for the solvent. Use a chemistry text to find out about these constants for water. Use the boiling-point constant to calculate the accepted boiling point of solutions containing sugar and salt. Compare the accepted values with your experimental values.
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