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Hey, /sci/.
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You are currently reading a thread in /sci/ - Science & Math

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Hey, /sci/.

Been conducting some experiments in my shed out of interest, looking at how the surface tension of water changes against temperature. This is done by method of a propane torch to warm a beaker of water which is then moved into a squidgy pipette, dripped drop by drop into a cup on some scales (20 times), and then the resulting mass is weighed. I'm using the drop weight method, basically.

Looking online, it seems that the surface tension is supposed to decrease as temperature increases however my results obtained via the aforementioned method show no such correlation (pic related).

I suspect this is something to do with the changing density of the water as temperature is increased and the constant circumference where it is in contact with the pipette, but trying to work it out from first principles is just showing me what I'd expect.

Does /sci/ have any idea what could be causing this effect?

(Sorry for the shitty labelling of the graph - x axis is temp, y is surface tension, title should read "Surface Tension $Nm^{-1}$ against Temperature (Degrees C)"
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Ignore the result at 0,0 by the way, just me getting excel to show me the full y axis.
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>Does /sci/ have any idea what could be causing this effect?

A very wrong procedure. You're not accounting for heat transfer, I assume?
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>>7797064
Temperature is monitored in the heated beaker via a thermometer. I'm doing the pipette transfer pretty quickly so I'm taking losses due to heat to be negligible.
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>>7796590
>surface tension of water
>changes against temperature
This requires a delicate measurement in a carefully-controlled environment of temperature and humidity.
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>>7797153
I've got a shed.

Would the method with the needle and the weights be better?
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>>7797195
>no error bars
You could be right with big error bars. Try to evaluate the uncertainty of your method.
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>>7797208
Just ran the whole lot again, and the results are looking very similar. Will combine the graphs in a bit to get a proper set of results.

Got an average of 73 dyn/cm for surface tensions, with a very slight downwards trend (y=-0.1174x + 79.611).

Any idea how I could go about working out my error? The only error I can think of (instrumentally) is the scales and the thermometer. There's losses due to heat exchange but I'm not sure how to call that error.
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>>7797208
Standard deviation is 3.619262
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>>7797222
surface tension of water at 0°C: 75.64
at 100°C: 58.85

-uncertainty on the size of the pipette
-uncertainty of the scale
-uncertainty on the thermometer
-uncertainty on the lost heat

In the end, you have uncertainty about the radius, the temperature and the mass.

I can't remember how this is called, but you can use this method for the total uncertainty $\frac{ \Delta \gamma }{\gamma} = \sqrt{\frac{ ( \Delta r }{r} )^2 + ( \frac{ \Delta m}{m} )^2 + ( \frac{ \Delta T }{T} )^2 }$
(hope the latex works)
for example if you have a scale precise to 1mg, and you measure 20 drops to be 1g, then you have $\frac{ \Delta m}{m} = 0.001$
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Do you account for other things that change with temperature and affect the measurement? Like viscosities and densities?
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>>7797237
Right, here's my errors -

Scales = +- 0.005g
Calipers = +- 0.005mm
Thermometer = +- 0.5 deg C

>>7797291
No.
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File: file.png (14 KB, 532x289) Image search: [iqdb] [SauceNao] [Google]
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Most recent graph.
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install R, construct linear regression
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>>7798139
what
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>>7797237
How can I work out uncertainty on lost heat?