Goals:
(1) Convert Celsius temperatures to Fahrenheit and Kelvin temperatures.
(2) Graph a heating curve to show the change in temperature with time.
(3) Identify changes of state on the heating curve.

Thermometers
The laboratory thermometers are different than thermometers you may be used to using to take your temperature. The liquid in a laboratory thermometer responds quickly to the surroundings, therefore, you should NEVER shake down a laboratory thermometer. When you measure the temperature of a substance, always read the thermometer while it is immersed in the substance. The entire bulb of the thermometer should be immersed in order to get an accurate reading.

Temperature Scales
Common temperature measurements that we encounter everyday are usually given in degrees Fahrenheit (° F). For example, the human body temperature is 98.6 ° F and the weather person reports that wet roads may freeze if the temperature dips below 32 ° F. However, in science, temperature is more commonly reported in degrees Celsius (° C) or Kelvin (K). The following equations are used to convert these temperature scales:
 

TF = 1.8(TC) + 32	(Celsius to Fahrenheit)
TC = (TF - 32)/1.8	(Fahrenheit to Celsius)
TK = TC + 273.15	(Celsius to Kelvin)

Heating Curves
When a substance is heated, the temperature of that substance rises until a phase change occurs. An example of a phase change is when ice melts (solid to liquid) or when water boils (liquid to gas). A heating curve is a graph of time vs. temperature. When a substance is heated over a certain period of time, a heating curve can be plotted. A change of state is observable on a heating curve when a horizontal line (plateau) appears on the graph where temperature is constant over time.
 
 

Procedures

A. Measuring Temperature

 Measure the temperature of each of the following to the nearest 0.1 degree and record your data in the table below: (Remember, when measuring the temperature of a liquid, place the bulb of the thermometer in the center of the solution)

a. Room temperature: Place the thermometer on the lab bench.

b. Tap Water: Fill a 250-mL beaker about 1/3 full of tap water.

c. Ice water mixture: Add enough ice to the water in part b to approximately double the volume.

d. Ice water/salt mixture: Add a layer of salt approximately 1 cm thick to your ice water mixture from part c and mix.
    Allow ~5 minutes for the temperature to change.

Record your measurements in the table below and complete the table by converting the Celsius temperatures to their corresponding Fahrenheit and Kelvin temperatures.

Temperature Data Table
 

		° C	° F	K
a.	Room
b.	Tap Water
c.	Ice water mixture
d.	Salt ice water mixture

B. Preparing a Heating Curve
Make a chart in your notebook to record temperature and time. You will probably have 30-35 readings so be sure to leave enough space.

Record the mass of a clean, dry 400-mL beaker.  Fill the beaker to the 100 mL mark with ice and add 20.0 mL of distilled water. Quickly record the mass of the beaker with the ice water mixture.  Stir the ice-water mixture with your scoopula until the temperature is constant. Record this temperature measurement at a time of 0 min. Place the beaker of water on a hotplate and begin heating with the setting at 5. Use a timer or a watch(clock) with a second hand and record the temperature (to 0.1 ° C) of the water each minute. Note the temperature at which the last piece of ice disappears. When the recorded temperature reaches 20 ° C move the setting on the hotplate to 7.

Eventually the water will come to a full boil. (The appearance of small bubbles of escaping gas does not indicate a full boil. During boiling, bubbles will be seen beneath the surface of the water). After boiling begins and the temperature has become constant, record five more readings.

Use the data you have collected to prepare a heating curve for water (see discussion above). You can use any computer plotting program to do this. On the graph, label the areas of solid, liquid and boiling on the graph.

The plateau (flat part of the graph) indicates the boiling point of the water. Record the boiling point you observed below. Determine the temperature change for the water from the temperature at which the last piece of ice melts to the temperature of the boiling water (plateau).
 

a.	Boiling Point of water:	___________
b.	Temp at which last piece of ice melted:	___________
c.	Temperature change (a - b)	___________

Using the mass, the temperature change, and the specific heat (1.00 cal/g.° C) as shown in the equation below, calculate the calories absorbed by the water to increase its temperature to the boiling point.
 

calories = (mass)  (DT ) (1.00 cal/g ·° C)

 

d.	Mass of empty dry beaker	___________
e.	Mass of beaker + ice water	___________
f.	Mass of Water (e-d)	___________
g.	Temperature change (c. from above)	___________
h.	Calories absorbed by water	___________

Issues to be addressed in your conclusion...

When water is heated, the temperature eventually reaches a constant value and forms a plateau on the graph of the heating curve. What does the plateau indicate?

What happens to the energy that is added when the temperature of the boiling water is in the plateau region on the graph?

You determined the number of calories absorbed by the water when you heated it. Using that value, determine how many kilojoules (kJ) were absorbed to heat the water to boiling?

Water has one of the largest specific heats of any substance. Why is this important for the human body?