Chemists use cooling curves to help determine the freezing points of liquids or melting points of solids. Freezing-point determinations typically involve placing a liquid in a glass test tube with a thermometer, then immersing the test tube into a bath at a temperature below the sample's freezing point. You then monitors the temperature of the liquid in the test tube with time. You can then create a "cooling curve" by plotting the temperature and time data as x-y coordinates and drawing a curve through the data points.
- Skill level:
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Things you need
- Test tube
- Distilled water
- Thermometer or temperature probe
- Large beaker or bowl
- Graph paper
Fill a test tube approximately half full with distilled water and place a thermometer or temperature probe into the water.
Prepare a cooling bath by filling a beaker or bowl half full with ice. Add water until the beaker or bowl is about three-fourths full, then sprinkle 3 or 4 tablespoons of table salt over the ice-water mixture.
Set up a data table on a piece of paper with two columns. Label the first column "Time" and the second column "Temperature."
Take an initial temperature reading of the water in the test tube and write down this value as the temperature at time zero. Thus, if the temperature was 25.5 degrees Celsius, then the first entry in the data table would read "0, 25.5." If your thermometer only reads Fahrenheit, then record the data in Fahrenheit.
Immerse the test tube containing the water and thermometer into the ice-water bath and start the stopwatch. Take a temperature reading and record the elapsed time along with the temperature in the data table every 15 seconds. Use the thermometer to stir the water in the test tube between temperature readings. Continue recording data and stirring the water until you record four consecutive data points with no change in temperature.
Set up x- and y-axes on a piece of graph paper to plot time in seconds on the x-axis and temperature in degrees Celsius on the y-axis. If you recorded temperature data in degrees Fahrenheit, then convert the values to degrees Celsius using on online conversion tool, such as the one provided in the Resources section. Plot all of your data points on the graph.
Connect the dots on the graph, but do not draw straight lines between the data points. Draw the smoothest curve possible through the data points, even if this means the curve you draw doesn't touch every data point. The graph now represents a cooling curve for water.
Tips and warnings
- For faster data processing and construction of the graph, consider using a software program such as Microsoft Excel for generating plots of your time and temperature data.
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