Experiment 6. Accuracy and precision in the measurement of temperature

Introduction

        In many of the experiments we do in Chemistry it is required that we measure the actual temperature of a liquid. On some occasions it is the temperature change which accompanies a reaction, that is required. The accuracy and precision of the thermometer are probably the main factors which affect such measurement, but good technique is also important to ensure that the true temperature is measured.
        Most thermometers are calibrated so that they read correctly when the entire column of mercury is immersed in the solution whose temperature is being measured. It is not practical to do this every time a measurement is made, but we should at least have all the bulb covered, whenever measurements are made. The liquid should be well mixed to ensure a uniform temperature throughout its volume, and the thermometer should remain long enough in the liquid to ensure that it comes to thermal equilibrium at the temperature which is being measured.
        When temperature changes are being measured, one can reduce the error by taking both temperatures with the same thermometer.
        In this experiment the accuracy of two thermometers is investigated by observing their readings at the ice point and at the boiling point of water. We also investigate how some factors which are part of the measurement technique, can affect the reading obtained.

Please note that thermometers are expensive and need to be handled with care.

Materials
        Deep funnel with rubber tubing and clamp, two 250-cm3 beakers, 250-cm3 flask, 2 thermometers with different colour scales and reading in the range -10 to 110 °C, timer (not necessary if the person has a digital watch), ring stand with 2 clamps, thread to suspend thermometers.

Procedure
  1. Leave the thermometers in a beaker of tap water (200 cm3) with their bulbs completely immersed.
  2. Obtain a glass funnel with a piece of rubber tubing and a clip attached to its stem. With the clip closed, fill the funnel with crushed ice. Clamp the funnel onto a ring stand and leave for a few minutes.
  3. Open the clip and allow the water formed to run off. Now add a small amount of distilled water and leave again for about five minutes.
  4. Read and record the temperature shown by the two thermometers in the room temperature water, and transfer the two to the ice bath, again, with their bulbs completely immersed. They should be suspended from a clamp so that the bulbs are about mid-way in the depth of the ice bath.
  5. Start reading and recording the two temperatures (of thermometer A & B) at 1 min intervals, and continue to do this until the temperature has stopped decreasing for at least four readings.
  6. Remove the thermometers and let them come to room temperature in the beaker with distilled water. Repeat the procedure in 5, but this time suspend the thermometers so that only about of the bulb is immersed in the ice bath. Again record temperatures at 1 min intervals until the temperature has stopped decreasing for four readings.
  7. Remove the thermometers from the ice bath and leave them in the room temperature water bath for a while. Set some distilled water to boil in a conical flask (add 3 or 4 boiling chips). and clamp at the neck to ensure that you do not spill hot water over yourself or your partner).
  8. Plot the four sets of readings from parts 5 and 6, putting the two curves for the same thermometer on the same graph (temperature vs. time).
  9. Insert the two thermometers into the boiling water with the column of mercury immersed as far as possible without the bulb being too close to the bottom of the flask, and take readings until the temperature has stopped increasing for at least four readings. (Be careful not to spill hot water on anyone or get your hand scorched in the steam).
  10. Return the thermometer to the beaker of water at room temperature, leave for about 10 min and note the final temperature of the water in this beaker.
Results
  1. Tabulate the temperatures recorded for the ice point and the boiling point of water for the two thermometers, and include the “standard” values in the table. Our experience is that the procedure used, gives a temperature for the ice point of 0.0 ± 0.1 °C. The “standard” boiling for water will be given each day, taking the atmospheric pressure of the day into account. Give an estimate of the precision of the readings in the heading of each column of your table.
  2. How long do the thermometers take to come to equilibrium with the ice bath in 5 and 6 above?
  3. Are the temperatures between the ice-bath temperatures in 5 and 6 significant? Comment.
  4. What change in temperature has occurred in the water which has been sitting in the beaker at room temperature? Take the difference using the same thermometer reading before and after, then take the difference using readings from different thermometers. Compare the results.
  5. Comment on the accuracy of the thermometers according to the results you tabulated in 1.
  6. Comment on the accuracy of the temperature difference obtained in 4.
  7. Estimate (in Joules) the heat lost or absorbed by the water in the beaker at room temperature during the course of the day, and determine the error in this heat, which is due to the uncertainty in the temperature readings. Assume that you have 200.0 (± 0.1) g of water.
Q= m c ΔT
where Q = heat change, m = mass, c = specific heat capacity and ΔT = change in temperature.

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