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Phase Changes: Determine the Freezing and Boiling Points of Water

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Author: Janice VanCleave

Phase changes, such as melting, freezing, evaporation, and condensation, occur whenever the physical phase (gas, liquid, or solid) of a substance changes. Solutes may play a significant role in these changes. Salt, for example, is used to melt ice on sidewalks and to make ice colder in an ice-cream maker.

In this project, you will determine the freezing and boiling points of water and the effect of salt and other solutes on these temperatures. Colligative properties, which depend only on the number of particles in a solution, will be studied and used to calculate the freezing-point depression and boiling-point elevation of solutions.

Getting Started

Purpose: To determine the boiling point of water and plot a time-temperature graph of the phase change.

Materials

  • 1 quart (1 liter) of distilled
  • water
  • 2-quart (2-liter) cooking pot
  • fondue fork (or heavy wire)
  • candy thermometer
  • stove

Procedure

Phase Changes Effects of Solutes

  1. Pour the distilled water into the pot.
  2. Place the fondue fork (or heavy wire) across the pot and clip the thermometer to the fork so that the bulb is suspended in the middle of the water (see Figure 13.1)
  3. Read and record the temperature of the water in a data table such as the one shown on the next page.
  4. Place the pot of water on the stove.
  5. Heat at a medium temperature.
  6. Every 15 seconds, read and record the temperature until the water stops getting hotter and three readings of the same temperature are recorded. Note: This should take about six minutes.
  7. Observe and make note of the appearance of the water each time the temperature is recorded.
  8. Plot the results on a graph with the temperature on the vertical axis and the time on the horizontal axis (see Figure 13.2).

Results

As the temperature increases, tiny bubbles appear on the bottom and around the sides of the pot. The bubbles increase in size and begin to break at the surface of the water as the water temperature reaches (exactly or approximately) 212°F (100°C). The bubbles continue to break at the surface, but the temperature remains constant.

Phase Changes Effects of Solutes

Why?

Certain conditions are required to effect a phase change of matter from one physical phase (gas, liquid, or solid) to another. Heating the water in the pot causes vapor bubbles to form on the bottom and around the sides of the pot where the water is the hottest. As the bubbles rise through the cooler water, they not only cool but also are pressed from all sides by the water molecules, causing them to collapse (see Figure 13.3).

As the water temperature increases, the pressure inside the vapor bubbles increases. Eventually, the boiling point is reached (a temperature at which the pressure inside the bubbles equals the atmospheric pressure outside). At the boiling point, the bubbles do not collapse but escape from the surface of the water.

During boiling, the temperature of the vapor is the same as that of the liquid. The kinetic energy (energy of motion) of liquids is less than that of vapor. Because of the lesser amount of kinetic energy, liquid molecules move less and are more closely bonded together. It takes extra energy to physically break the bonding between liquid molecules and change them into gas. At the boiling point of a liquid, the energy applied by the heating source does not increase the water's temperature but is used in the phase change from liquid to gas.

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