PhysicsLAB Lab
Water Mixtures

The purpose of this lab is to experimentally determine the final temperature of a mixture of water samples. This will be done using calorimetry along with a LabPro temperature probe and the law of heat exchange.
 
Under the Start Menu go to Programs, Math, Logger Pro 3.1 to launch the program.
 
Logger Pro should automatically set up the graphs according to the connected sensor. With a temperature probe properly connected, the program should display a graph of temperature vs time. When you are ready to start collecting data, press Collect to start timing; then begin the experiment.
 
Equipment
 
Each group will need the following
  • LabPro with temperature probe
  • triple beam balance
  • 4 dry styrofoam cups - nested in sets of two
  • 50 ml of cold water
  • one beaker for boiling water
  • 50 ml of hot water
  • one hot plate
 
Data
 
Data Table I
  mass
description (kg)
empty nested styrofoam "cold" cups
cups with cold water
cold water only
empty nested styrofoam "hot" cups
 
 
Once the water in the beaker is boiling, use your probe to verify that the water is boiling at 100ºC. Next dry off the probe and set it in the cup of cold water and record its initial temerature. You can start and stop collecting temperature data arbitarily during this portion of the experiment. It is only critical that your record stable temperatures for both the cold and hot water.

Prepare to collect data for the final temperature by setting the duration to collect data for 2 minutes. When you are ready, transfer approximately half a cup (or less) of hot water into the "hot" nested cups. Then quickly, but carefully, pour the previously massed cold water into the HOT water cups. Note that no water should splash out of the containers. Use the temperature probe to gently stir the mixture - record the final equilibrium temperature. ALERT: The change occurs quickly - watch your screens for the PEAK temperature after the cold water has been added.
 
Data Table II
  temperature
description (ºC)
initial temperature of cold water
initial temperature of specimen
final temperature water mixture
 
 
Before we can complete our calculations, we have to determine how much hot water was used in the experiment.
 
Data Table III
  mass
description (kg)
empty nested styrofoam "hot" cups
cups with final water mixture
cold water only
hot water only
 
 
Calculations and Conclusions
 
When there is no phase change, the heat lost or gained by an object can be calculated with the formula: Q = mcΔT.
  • m is the mass of the sample in kg
  • c is the specific heat of the material in J/kgCº
  • ΔT is the change in temperature in Cº and is always expressed as Tf - To
The Law of Heat Exchange states that in a confined system, when thermal equilibrium has been reached, the heat lost by one object will equal the heat gained by the other.
 
ΣQlost + ΣQgained = 0.
 
Note that Qlost will be negative (since Tf < To) and Qgained will be positive (since Tf > To). In our experiment, the cold water sample gained heat and the hot water lost heat. The accepted value for the specific heat of water is 4186 J/kgCº. We are assuming that the styrofoam acts like a thermal insulator and does not gain or lose any heat.
 
Based on your mass and initial temperature data, calculate a theoretical value for the mixture's final temperature. 

Calculate a percent error between your theoretical value and the actual measured final temperature. 

Discuss what your group feels is this lab's greatest source of error. 

State a valid procedure that could minimize your error's negative consequences when the lab is completed next year. 



When you are finished with the experiment, close LoggerPro, dry off the probe, empty your water samples into the front sink and turn your empty cups upside down on a paper towel to dry out for next period.


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