Saltwater Battery Concentration

[deleted-847417]

I am doing a saltwater battery experiment (zinc and copper) for my Science Fair, and I am researching how the concentration of the saltwater affects the voltage of electricity produced. My prediction was that the voltage will increase as the concentration increases, but in my actual experiment, the voltage only decreased as I added more salt (tried a multiple times, but the same result). Can anyone explain what is going on???
Used two bottles of 300ml tap water for the saltwater part
Salt Used: Table salt
Increments: No salt, 2 teaspoons of salt, 4 teaspoons of salt

Please reply asap...

[norman40]

Hi XteVVen,

I'm assuming that you're working on the project described here:

https://www.sciencebuddies.org/science- ... -saltwater

The battery's maximum voltage is determined by the chemical reactions shown in the project background. You should see the lowest voltage with tap water. Adding electrolyte should increase the voltage to some value less than the theoretical maximum.

Are you sure that the salt is dissolved in your solutions? If so you might try cleaning (or replacing) your electrodes between trials with different salt concentrations.

I hope this helps. Please ask again if you have more questions.

A. Norman

[deleted-847417]

I tried again as you told me (cleaning the plates after each increment) but still got the same result. Respectfully, the voltage got higher when I added 1 teaspoon of salt, but after that, it kept decreasing. Maybe that is what's supposed to happen?

[deleted-847417]

Perhaps adding too much salt decreases the voltage?

[norman40]

Hi XteVVen,

Too much salt might result in decreased voltage. But you mentioned adding as much as 4 teaspoons of salt (roughly 24 g). The project procedure specifies 25 g of salt in 300 ml of tap water so you've added no more than the recommended amount.

It's possible that once you have “enough” electrolyte the voltage won't increase with the addition of salt. Perhaps your tap water has high electrolyte concentrations and adding a small amount of salt makes little difference. If possible you might try the experiment with distilled water instead of tap water. Adding salt to the distilled water may have a bigger effect on the voltage.

I hope this helps. Please ask again if you have more questions.

A. Norman

[deleted-847417]

Hi Norman,

Actually, I changed my experiment a bit to 200ml of tap water instead of 300 and 0,1, 2, 4 teaspoons of salt. I got 1.57V, 1.62V, 1.60V, and 1.55V each. As you can see, it got higher at first but then kept decreasing. Also, because of the Coronavirus, getting distilled water for my experiment would be almost impossible.

Considering the deadline, I'm going to stop doing my experiment. Now, I need to "discuss the reliability of the data using scientific reasoning," and I cannot find any sources that talk about this. It would really help me if you could explain why - as you said - "too much salt results in a decreased voltage."

[deleted-847417]

I also have to write about the strengths and weaknesses of my experiment and about how to improve (I have to provide another experiment that is an extension of this one). Could you tell me more specifically about how and why I should have used distilled water? And how should my extended experiment be like?

XteVVen

[norman40]

Hi XteVVen,

Adding an electrolyte (like salt) makes the solution more conductive. Greater solution conductivity improves the flow of electricity between the battery electrodes. As a result higher voltage is expected with more electrolyte in the solution.

Distilled water has very little conductivity because it has very low electrolyte content. Because of its low conductivity, using distilled water in your battery should result in a low voltage. Adding salt to the distilled water will increase its conductivity and the battery voltage.

Tap water can contain several electrolytes. As I mentioned in my previous post, it's possible that your tap water is sufficiently conductive for your battery to operate properly. If so, adding salt might not have much effect on the voltage. The voltages you posted for your tests are not very different. Observing bigger differences in an experiment with distilled water would suggest that your test results were due to high conductivity of your tap water.

At very high salt concentrations the solution could become viscous and reduce the conductivity. But you did not add enough salt to approach this condition.

Regarding your discussion of data reliability, my suggestion is to address the repeatability of your results. Based on information you posted it seems that you've found the same trend in your voltage data in repeated experimental trials. And you may have repeated voltage measurements for each of your salt concentrations within experiments. You could compare the consistency of your repeated trials with the voltage differences for the various salt concentrations. If differences within repeat trials are smaller than the voltage variation for the salt concentrations you have some confidence that your observed voltage trend was not due to chance.

I hope this helps. Please ask again if you have more questions.

A. Norman

[deleted-847417]

Hi Norman,


Btw I can't test with distilled water right now because of quarantine and no shops are open. Plus, I don't have much time so I have ended my experiment and now I am working on my Evaluation.

"At very high salt concentrations the solution could become viscous and reduce the conductivity. But you did not add enough salt to approach this condition." So I guess you mean my data IS at least a BIT weird. Would it count as an anomaly? (Because I have to identify anomalies and trends of my experiment.)

Explain the repeatability... yes, I have done like 7 trials because the results never matched my prediction. I think that's a really good idea! I will say my data is reliable because I tested multiple times. Thanks!

And maybe for the part where I have to write negatives, I will say I used tap water, and for the improvements, I will say I should have used distilled water?


XteVVen

[deleted-847417]

Wait, it actually says I have to "discuss the reliability of my data using SCIENTIFIC REASONING."
Sorry for this noob question, but am I fine or does repeatability not count as scientific reasoning?

[norman40]

Hi XteVVen,

I understand that you are not able to pursue an experiment with distilled water. You could list that experiment as an extension to your project. The results might help to explain your tap water test data as I outlined in my previous post.

Scientific reasoning involves systematic use of observations to form conclusions. A discussion of the repeatability of your data to draw conclusions about the reliability qualifies.

I hope this helps. Please ask again if you have more questions.

A. Norman

[deleted-847417]

I asked the question from my previous post to my teacher and she said, "I would not suggest you to call data reliable even if your data is consistent as you can`t discuss it using scientific reasoning," and "it`s important that your hypothesis based on science and if you collected contradictory data it does not mean that science is wrong." What should I do... any more ideas for the reliability of my data?
Please REPLY ASAP!!! will be appreciated...

[norman40]

Hi XteVVen,

I'm not sure that I understand the comment from your teacher about data reliability and scientific reasoning. But you should make your discussion of data reliability fit with her expectations. And you should discuss this further with your teacher if you need clarification.

Based on the information you've provided about your experiments, it appears that you have data that partly contradicts your hypothesis. When results don't fit the hypothesis, try to explain why things didn't go as expected. And you could describe additional experiments that might help explain your results. The distilled water experiment mentioned in a previous post is one example.

I hope this helps. Please ask again if you have more questions.

A. Norman