Specific Heat Capacity of Water (H2O)

Introduction

The aim of this investigation is to determine the specific heat capacity of H2O (water). This is accomplished by measuring to which temperature a certain amount of energy takes the water. The independent variable of this experiment is the amount of current put into the water. The dependent variable is the waters temperature over time. The controlled variables are the initial temperature of the water and the amount of water.

Materials and Methods

The following materials were used:

  • scale
  • calorimeter
  • stirrer
  • thermometer
  • stopwatch
  • power supply
  • multimeter
  • 385 g H2O

The calorimeter was filled with 385 g of water, after which a measured current was applied to it the water. The temperature of the water was measured in increments of one minute, while the water was stirred with the stirrer.

The experiment was conducted only once, and thus the independent variable was not varied.

Results

Table 1: Measured current
Voltage (V) 15.81 V
Current (I) 3 A
Power (P) 47.43 W

P (the power) was derived with the following formula:

P = V * I

Table 2: Temperature over time
Time [m] Temperature [C] +/- 1
0 16
1 16
2 18
3 21
4 22
5 25
6 27
7 28
8 30
9 31
10 33
11 35
12 36
13 38
14 40
15 41
16 43
17 44
18 46
19 48
20 49
21 51
22 52

Since an accurate stopwatch was used, the error margin in the time-column is negligible. The thermometer used was difficult to measure with because of unclear display, which approximates to an error margin of 1 degree Celsius. There is a clear, linear trend visible in the data.

The specific heat capacity (c) was calculated using the following formula:

C = Q / (m*dT)

Where m is the mass of the water and dT is the change in temperature. Q is the heat added, which equals P times the time passed. Since the trend in the data is regular and lacks anomalous results, the total data can be used for this calculation. The calorimeters effect was neglected.

c = Q / ( m * dT ) = ( 47.43 * 60 * 22 ) / ( 0.350 * ( 52 – 16 ) ) = 4969 J(kgK)^-1

Conclusion

According to this experiment, the specific heat capacity of liquid water at 16 degrees Celsius is 4969 J(kgK)^-1. Comparing our result with literature values, Giancolis Physics (fifth edition) states that this value is 4186 J(kgK)^-1. Since our result was larger than Giancolis, we must have disregarded another loss of heat. This is because we assumed that the water absorbs all heat, which is not the case. Some heat is lost to the surroundings, e.g. the calorimeter and the air above it.

Note, also, that the current varied slightly during the investigation. It started as 3 A, but when the experiment had been completed, it had risen by approximately 0.1 A. From this, we can draw the conclusion that the resistance of water decreases by a small amount when it is heated between 16 and 52 degrees Celsius.

Evaluation

The method used is flawed on several points. The main weakness, though, is the fact that energy is lost to the environment. The calorimeter was a polystyrene cylinder with a mass of approximately 0.2 +/- 0.1 kg. Polystyrene has a specific heat capacity of 1.3 times 10^3 J(kgK)^-1 according to Wikipedia. The c*m factor of the formula for heat added (derived from the formula for specific heat capacity) is then for polystyrene 1.3 times 10^3 times 0.2 = 260 JK^-1, while the respective factor for water is 4.969 times 10^3 times 0.350 = 1739 JK^-1. Thus, the polystyrene calorimeter stands for a significant part of the heat lost. Heat was also lost to the heating apparatus as well as to the stirring stick and the thermometer, although these objects had low masses. By using an even more0 insulating calorimeter, with a lower specific heat capacity, we would decrease the heat lost to the environment.

The primitive method for stirring is another reason for heat-loss. A stick with a ring in attached to the end was used to manually stir the water, which led to exhausted arms and an inability to maintain stirring for a prolonged period of time. This could have been solved by using an automatic stirrer.

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