Homework Help: How to measure specific heat capacity of tap water using a microwave oven

1. Oct 21, 2009

chem_is_lovex

Hi,
Can someone please give me some suggestions on how to use a microwave oven to determine the specific heat capacity of tap water? This is not a homework question, but an assignment and I have absolutely no idea how I can do this.This experiment will be done at home, so please no immersion heaters or anything else that won't be used at home.

thanks :)

Last edited: Oct 21, 2009
2. Oct 21, 2009

I hope you're not asking people here to plan the assignment for you,that would defeat a major part of the exercise.It is your assignment and has been set for you to do some research from which you can learn.Get to your books and find out what you can about specific heat capacity and how it can be measured.When and if you get stuck report back here,showing what you have worked out so far and then people(including myself if I am around) are likely to help.Best wishes and try to enjoy the challenge of assignments.

3. Oct 22, 2009

chem_is_lovex

oh, no... sorry if I gave you that impression, but I just wanted some suggestions of what I could do.
I'm going to place a glass of water in the microwave and microwave it for a certain amount of time and measure the temp.
I am thinking of using the equation P=wd/t, to find the work done in joules. Then use that in the Q=mc$$\Delta$$T. I'm not too sure whether to use time or temperature as the variable though. or whether any of this can help me find the SHC. If not, you have any other suggestions?

thanks :)

4. Oct 22, 2009

Hello chem is loveX.Using your method we can write Pt=mcdT(t=time in secs,dT = temperature change).Your method is good.
This is a suggestion,using a fixed mass of water heat it for say 10 seconds then 20 seconds etc.Use fresh water each time and allow the measuring jug plus water to return to room temperature before each experiment.You can draw up a table of results and plot a graph of t against dT.Within experimental error you should get a straight line through the origin but as you approach boiling point the temperature will level off.The gradient of the rising part of your graph should equal mc/P and by measuring this gradient you will be able to calculate c.There are other variations you can try using such as not using fresh water each time or heating different masses of water for a fixed time.Your experiment sounds so good that I am tempted to try it for myself.

5. Oct 22, 2009

mgb_phys

A couple of practical points.
A larger volume of water might work better since you can go to longer times before it boils.
A microwave heats water unevenly (it mostly heats the outside) so you need to stir it before measuring the temperature.

WARNING - it's possible for a microwave to heat water past it's boiling point (superheating) and then have the water boil suddenly when it's disturbed - like by putting a thermometer in it. When you have nearly reached boiling point be very careful about handling the container of water.

6. Oct 22, 2009

I have seen a film on the sudden boiling of coffee that has been superheated and then stirred,it went up like a mini volcano.Thanks for reminding mgb and take care chem.

7. Oct 27, 2009

chem_is_lovex

Hi, thanks so much for the advice mgb and for the help dadface. I have done what you suggested and I'm sorry to be a pain...but I'm lost. I did the experiment and got these results:

time | temp
10 | 22
20 | 25
30 | 29
40 | 33
50 | 35
60 | 38
70 | 39
80 | 43
90 | 47

I graphed them like you said, and when I tried to find the gradient (m=(y2-y1)/(x2-x1)), but I ended up with a very small number (0.something) and this cannot be right! I am panicking now because I have no idea what to do. Do you know what I'm doing wrong?
Also because the time is varied in this experiment, which time do I use for the equation Q=Pt (to manually find the c value)? Do I just average the time and temperature values? I tried that and got a value of 7785.47 for c. I'm really sorry, and THANK YOU so much for helping me :) I really appreciate it!

8. Oct 27, 2009

Without plotting the graph it looks like you were getting a gradient(temperature rise per second) of about 0.3degrees per second.What is the power of your microwave and what mass of water did you use?

9. Oct 27, 2009

arithmetix

While I have not experimented in this way into the required energy to heat water, I have noticed that the results quoted for the specific heat capacity of water always specify the temperature from which the water is to be raised by (usually) 1 degree.
From what I recall the temperature at start of heating is usually about 20 or 25 degrees Centigrade.
Perhaps convection within the water is important. I suggest that it might be interesting to vary the amount of water being heated, while keeping heating time constant.
Yet I have a strong feeling that once again the results won't be easy to interpret (because I know that hot water can freeze more quickly than cold, I fear to make predictions).

P.S. I wonder whether more easily interpreted results might be obtained if the water were to be thoroughly stirred first, in one direction or the other depending on which hemisphere you are in, and putting the still-rotating fluid into the heater. I would try to have the water still moving under the influence of the stirring at the end of the heating. In this way the influence of convection might possibly be minimised or at least made similar over several experiments.

Last edited: Oct 27, 2009
10. Oct 27, 2009

chem_is_lovex

The mass of water used is 500g and the power of the microwave is 900w. The initial temp of water was 20 degrees Celsius.

11. Oct 27, 2009

Pt=mcdT.Get t/dT from your graph and plug in the numbers to find c.

12. Oct 28, 2009

chem_is_lovex

haii, it's k, I figured it out :) thanks for the help, I never would've been able to do it without your help~~

lets hope I get a decent mark in the assignment ^_____^

xx