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Name: Unknown
Status: student
Age: 7th Grade
Location: N/A
Country: N/A
Date: 1999-2001


Question:
My son is in 7th grade and is doing a science experiment on "Does salt water retain heat better than tap water?" His experiment was to heat tap water to 90c and then measure the falling temperature every 5 minutes for 60 minutes. He then repeated the experiment with various quantities of salt water. His experimental data seems to support that salt water loses its heat slower than tap water. We have scanned the site archive looking for supporting reasons. We have reviewed the answers relating to "Boiling Point Elevation", but unfortunately these don't seem to address his specific question.

Can you provide some reasons (or point us in the right direction) why salt water appears to retain its heat better than tap water?


Replies:
Hello,

Without looking up the numbers here is some information to get your started.

The property of a substance determining how much its temperature rises as a result of a certain amount of heat input is called specific heat, commonly denoted by C. Specific heat is defined as the amount of heat it takes to raise the temperature of 1 kg of a substance by 1 K. If I remember correctly, water has the highest specific heat of any substance. Thus, adding salt to water can only reduce its specific heat. This means that salt water temperature rises/falls more than pure water for the same amount of heat addition/subtraction.

It takes 4.18 kJ (or the heat generated by a 1000 W (W= Joules per second) stove for just over 4 seconds) to raise the temperature of of 1 kg of pure water by 1 K. If you add some salt to pure water and take one kg of the solution, you will find that it takes less than 4.18 kJ to rise its temperature by 1 K.

You can estimate, look up, or measure the specific heat of the solution. Please look at thermodynamic books for more information. If you are doing it experimentally:

1- Measure the weight of pure water

2- Add a measured about of salt to it (say, 1/20 of the weight of water)

3- Take a fixed amount of the solution and heat it while measuring its temperature rise over time.

4- Repeat this experiment (keep the experimental conditions unchanged, e.g., same container, stove, etc.) but with different amount of salt.

5- If we assume no heat loss (i.e., all the heat goes into heating up the water), then, we can write:

q'= m C (delta T/ delta t) where:
q' = heat input per unit time
m = mass of water mixture
C = specific heat of the mixture
T = measured temperature at time t (t=0 when you turn on the heat)
t = time

A plot of T vs. t should give a straight line whose slope is q'/mC. If q' and m are constant, as we assumed, then we should see an increase in the slope of the curves (i.e., decrease in C) as the salinity of water is increased.

If you cannot estimate your q, you still should be able to correlate changes in C with salt fraction. You can then compare your results with published data. If you find data and did the comparison, please send us your results and references and we can put them on the web for others' benefit.

Ali Khounsary, Ph.D.
Advanced Photon Source
Argonne National Laboratory



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