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Determining the heat capacity of water with a kettle

(CHEM0009 Thermodynamics, Christoph Salzmann)

On page 35 of the lecture notes we have discussed how you could determine the heat capacity of water with an immersion heater. I have just found out that any old kettle, such as the one below, will do!

OK so this is the equation we need:

C  =       P . t

p        m(H2O) . T

The power of the kettle is easy to read off:

P ... power of kettle

t ... time it takes to heat from T1 to T2

m(H2O) ... mass of water in the kettle

T ... T2 – T1

My kettle doesn’t seem to be the best quality! We will take the average of 1850 and 2200 W which is 2025 W.    Remember that 1 W = 1 J s- 1 . Now we put say one litre of water into the kettle. The density of water is very close to 1 g mL- 1 . So, one litre of water weighs about 1000 g.

Strictly speaking we should use deionised water for this. But using just tap water is probably one of the smallest  errors we are introducing here. So the water goes into the kettle, I flick the switch and start the stopwatch on my smartphone. For my setup” it took 2 minutes and 56 seconds until the water boiled and the kettle switched itself off:

The water from the tap has about 16 degrees C. Quick check on the internet reveals that the air pressure today in London is 1005 mbar.

So it probably fair to say that the water boils more or less at 100 degrees which gives a T of 84 degrees. Putting all these quantities into the equation above finally gives a heat capacity of 4.2 J g- 1 K- 1. I don’t dare using more    than two significant figures! Now, let’s compare this value to the highly accurate values from the lecture notes:

4.21 4.20 4.19 4.18

280 300 320 340 360 380

T / K

Amazingly accurate I would say considering all the obvious deficiencies of this “experiment”!

How could we improve this setup? What do you think is introducing the largest error? Will using more or less water make the measurement more accurate?