Why are the limestone rocks weighing more in vinegar water

[deleted-349499]

I started my science project last night...now you see now, now you don't know...how acidic water makes rocks disappear. I had the limestone rocks soaking for the 24hrs in the solutions. The rocks appear smaller but weigh more. The scale was zerod so I dont know what I'm doing wrong. Please help as the project is due in two weeks and I can!t start over. The other thing is the pH of 100% i have as a 1 from my pH paper. The project says a 2. What one do I go of off?

[donnahardy2]

Hi,

I think you are doing this excellent project:

https://www.sciencebuddies.org/science- ... ml#summary

I'm not sure why the mass of your rocks increased over time. According to the directions, all of the rocks should be submerged briefly in distilled water so that the mass measurement of each rock includes adsorbed water. After the initial weighing, each rock is weighed and the pH of the solution is measured.

What did you soak the rocks in for 24 hours? Did it contain anything that might have precipitated with calcium or carbonate (the limestone)? Did you zero or tare your balance before you weighed anything? Have you tried weighing an object three times in a row to determine the reproducibility of the measurement? Are you sure you have limestone? If you can't think of anything else, you may need to attribute the increase in weight to experimental error.

Don't worry about the pH reading discrepancies. Just read the pH as measured by your pH paper. There is some variation in pH paper, so if you record results consistently, your results should make sense. Acetic acid ionizes and will give a pH of 2, but you should record a pH of 1 if that is what your pH paper is reading. You can include a discussion on the accuracy of pH paper in your discussion section.

Go ahead and proceed with your project and weigh each rock and measure the pH according to the project protocol. If you can borrow a different balance to use or a different brand of pH paper, you can use those to compare results with your current materials. Your results are empirical so continue with your measurements and observations and don't think about starting over.

Please answer the questions from the paragraph that starts with "What did you soak the rocks in," and do let us know what happens in the next two or three days.


Donna Hardy

[deleted-349499]

Thank you very much for the response and I will certainly forward to my daughter. This has been very confusing for her. She said she did not soak the rocks for teh 24hrs as she understood the directions to dunk, dry and weigh after placing in the distilled water. Thinking about the process, it would make sense to soak in the water for 24hrs then weigh. Will this change the results much that she can continue on with her project? It's something that she can certainly note in her findings the possible differences.

[donnahardy2]

You are welcome.

I think you are right. If the rocks were briefly soaked in water, as suggested in the instructions, then it is likely that there were some pores that were not saturated with water at the beginning of the experiment and the rocks continued to absorb water after they were submersed. . So the weight graph will have a weight increase on during the first 24 hours. Your daughter can easily make a note to explain these results.

For the discrepancy on the pH reading, here is an abstract of a scientific reference comparing pH meter, reagent strips, and pH paper used to measure the pH of cat urine.

http://www.ncbi.nlm.nih.gov/pubmed/12040484

The authors found that the pH meter gave the most accurate results, and that the pH paper gave the least accurate results, especially at lower pH readings. This paper can be used to make another interesting discussion point for your daughter's paper. She can point out that the pH paper may not give an accurate reading and that she would try to use a pH meter if one were available.

However, if your daughter uses the same pH paper for the entire project, the results should be consistent. If the pH paper is giving a pH 1 reading, then it should give a consistent reading throughout the experiment.

Here is a website that lists the pH of different concentrations of acetic acid. Vinegar, which is about 5% acetic acid, has a pH of about 2.4. As the calcium carbonate of the limestone dissolves in the acetic acid, the pH of the solution will increase.

https://depts.washington.edu/chem/facil ... istry.html

Please let us know if there are more questions.


Donna