Science Buddies: "Ask an Expert"

Hi, I am going to do the simming in acid project. My question concerns the mussel shells. Do you have to crush them up or can you keep intact and use half a shell for each trial. I would weigh the 1/2 before and after. My teacher requires a minimum of 5 trials and to save time and space I was wondering if this is possible. Also would doing this alter the results. I was going to put apporx. 5 - 1/2 shells in each jar and mark each one-five per jar, in both the contral and experimental. Then weigh and compare each # 1 #2, etc.

Would this still work. I's appreciate any help and guidence you can offer.

Thanks.

Hi
I didnt realy understand what your project is. please give a little more info.
thanks kamran

Hi,
You actually lightly crush the shells to the size of a US quarter coin.

I hope this helps.

CLKline

Hi,

I think you are doing this project on the science buddies website:

https://www.sciencebuddies.org/science- ... p013.shtml

This is a great project that measures the dissolution of calcium carbonate (the mussel shells) in acid. Christina is right; you do need to crush the shells as this will increase the surface area exposed to the acid. The important parameters to control in this experiment are the size of the shell pieces in each sample (they should be as uniform in size as possible, from sample to sample) and the ratio of shells to the water volume (identical for each sample). The temperature should also be the same for each sample. Smaller pieces of shell will have more surface area, so will dissolve more quickly.

Please let us know if you have any other questions.

Donna Hardy

Hi Ms. Hardy,
Thank you for your reply. I still have a question however. When you said, "Christina is right; you do need to crush the shells as this will increase the surface area exposed to the acid." I was wondering if you kept the shells the original size would the experiment not work? Also, by crushing the shells are we actually speeding up the process; in other words would it take much longer for the acid to dissolve the full shell in the ocean? In this experiment we are suppose to keep the shells in the solution for one month. In real life would this process take longer and if so how much longer. I guess I am asking if the process is speeded up how "real life" is this experiment and how do you compare this to what will actually happen in the ocean?

I appreciate any help you can offer.
Thanks

Hi,

This experiment will work if you keep the shells whole, however, the rate of dissolution depends on the surface area. The following website explains how surface area is related to rates of reaction:

http://www.chemguide.co.uk/physical/bas ... earea.html

If you left the shells whole, they would dissolve much more slowly compared to crushed shells, and you might not get a measurable result within a month. The project is designed to allow you to obtain results within a reasonable amount of time (before the project due date). You would have a controlled experiment if all of the shells were left whole, but you might not see a measurable loss of weight within a month. The measure the effect of pH on the dissolution of calcium carbonate shells, you would need to keep the shell pieces all the same size.

You could change this experiment and set up several samples using water with one pH, and use shells with different particle sizes (you would need to measure the surface area), and then measure the effect of surface area on the rate of dissolution.

If you do the project with crushed shells, you can extrapolate what would happen in the ocean with whole shells by comparing the surface area of your sample with the surface area of a whole shell.

I hope this helps.

Donna Hardy

Ms Hardy,

Thank you very much for your reply, it was most helpful

I did this experiment and nothing happened. It was a complete failure. I did everything to a t: hammered the shells to quarter size pieces, i kept the pieces in the jar over a month, and measured the pieces before and after and NOTHING changed. I now have an experiment that didn't work. I spent a lot of money on the supplies and shipping and handling. I have to turn my results in soon and I have none. What results was I supposed to get? Does anyone know of an experiment that works and that can be done in a short amount of time? I am extremely disappointed. I completely followed the directions and nothing happened?!

I would appreciate any help you can offer.

Thank you

Hi Mo,

I'm sorry you think that you did not get any results. However, your project is absolutely not a failure! You did a controlled experiment and you just did not observe dissolution of the calcium carbonate. However, you did an experiment and you have data to show that the shells did not dissolve, so you have a complete project to turn in. You are going to turn in a wonderful project and explain the science of why you didn't observe the results that you expected. This is why scientists do experiments; to find out what will happen. A project with unexpected results can be the start of a great discovery, so please do not be discouraged. If you still have the water you used, please don't discard the samples; we may need it for further investigation.

Please post the details of your materials and method and your procedure and all of your results. And please answer the following questions:

1. Did you measure the pH of your water samples? If so, how did you do this measurement?
2. If you still have the water sample you used, please measure the final pH.
3. Did you change the water during the experiment?
4. What was the ratio of shells to water (grams to volume)?
5. What was the accuracy of the balance you used to weigh the shells?
6. Did you dry the shells before you weighed them?
7. What type of controls did you use, if any?


I'll help you with an explanation of the results. Please let me know when you will turn in your project.

Donna Hardy

Ms. Hardy,

1. Yes I did measure the pH of the water. I bought the pH meter listed on the sight from Edmund Scientific.
2. I threw out the water so i do not have the final pH.
3. I did not change the water in the jars during the experiment. I left them undisturbed.
4. I used the Bell mason jars filled to the top like the directions indicated and used one oz of shells.
5. I purchased a kitchen scale from Target that went to the tenths place. I realize this might cause some inaccuracies because it did not go to many decimal places.
6. I let the shells sit over night so they would be fully dryed.
7. My controls had a pH of salted water of 8.1 for both the room temp. and refridgerated jars.

I used Instant Ocean® salt purchased in a pet store and Heinz White Vinegar.
I kept the shells in the jars for 35 days.
I bought fresh mussels and discarded the meat.
My results are due January 5 and my report is due January 15.
In my results, both controls (the six jars in cold temp. and the six jars in room temp.) had a weight 1 oz. In the jars being tested, 1 out of 6 of the jars in room temp. had a weight of .9 oz and in the tested jars for cold temp., 2 out of 6 jars had a weight of .9 oz.

Thank you for all your help.

Hi,

Thanks for the details; please send the detailed data as soon as possible. What was the starting pH of each container? Did you take any other pH measurements during the experiment? What was the weight of the shells that you added to each(send all data) and what was the ending weight? What was the volume of water in each container?

Send me the data as soon as possible.

Then, while you are waiting for me to reply, take one batch of shells and reweigh them; then put the shells in a slightly warm oven (200-250degrees F) for 1-2 hours. Cool the shells and reweigh them. It's possible that an overnight drying did not remove all of the moisture from the pores of the shells. If the shells lose weight, then redry them for another 1-2 hours to make sure all the moisture was removed. Maybe you are going to see a difference in results afterall.

Donna Hardy

Hi,

I will give you a little more explanation of why I am asking the questions. The plan for your project now is explain why the acid did not apparently dissolve the shells. Here is an outline of your strategy:

1. What quantity of shells would have been expected to dissolve in each jar?
You used 1 ounce, or 28 grams of sea shells. Calcium carbonate (CaCO3) has a molecular weight of 100, so you used:
28 grams x 1 M/100 grams = .36 moles calcium carbonate

With the pH of your samples, we can calculate the concentration of hydrogen ions in the water samples and estimate what the dissolution potential was. For example, a pH 6 water sample would have 10 to the minus 6 moles, or .000001 M of hydrogen ions. Since 2 hydrogen ions will dissolve 1 molecule of calcium carbonate:

CaCO3(s) + 2 HCl(aq) → CaCl2(aq) + CO2(g) + H2O

http://en.wikipedia.org/wiki/Calcium_carbonate

How much vinegar did you add to each sample? What was the starting pH?

2. Next, you need to consider the solubility constant or Ksp of calcium carbonate, which is .99 x 10 to the minus 8 at 25 degrees C. This number will give you the quantity of calcium and carbonate ions that would be present in the actual volume of water you used. Since you used a closed system, the calcium carbonate would dissolve until the solution was saturated [Ca++][CO3--] = 0.99 x 10 to the minus 8.

http://en.wikipedia.org/wiki/Solubility_equilibrium

3. Next, you need to determine the accuracy of the balance you used. What is the smallest unit of weight you can measure? One ounce of shells is plus or minus what unit of measure? The problem may be due to the balance you used. Do you have access to a scientific balance that would weigh within 0.1 or 0.01 gram? It would be helpful to reweigh your samples even though you can't get a time zero value. Don't worry if you don't have this; I'll think of something else.

4. I have already suggested oven drying a sample of the shells to see if residual water is causing a problem. Do you have any new shells that have never been exposed to water? It sounds like you used fresh shells, which would have contained some moisture. It would be interesting to dry a sample of these shells in order to estimate the error in the initial weight measurement.

I don't know how much chemistry you have had, so please ask questions if this is not making sense to you. Please send the pH and the volume of the water samples, the volume of the jars, and the smallest

Also, since you did not get the expected results, you need to do a spectacular job with other sections of your science board. You can go ahead and write up your question, hypothesis, background information, materials, and procedures sections. How about references?


Donna Hardy

Ms. Hardy,

I apologize for the late response.

The starting pH was 8.1 for the control jars and 7.5 for the jars that were being tested. I did not take any pH measures during the experiment because the instructions said not to open the jars once they were sealed. The volume of the container was 1 pint. In every jar there was 1 oz of shells. The volume of the water in the jars was 15 oz.

I am going to try the oven drying now. I haven't had that much experience with chemistry so I am a little bit confused about the formulas. I do not have a scientific balance but I my teacher does. I asked her if I could use it for my experiment and she said no (but that was a no for taking it home) so maybe if I bring the shells in she will let me use the balance. The only problem is that I will not have access to it until January 5th which is the day the results are due.

Again thank you for your help. It is much appreicated.

Hi,

Thanks. Now we have the essential information to explain why your science fair project did not work. And, I’ll try to explain one step at a time so you can understand the chemistry.

1. Here is the composition of your samples:
28 Grams or .36 M calcium carbonate (CaCO3)(mussel shells).
15 ounce or .44 L ocean water
pH 8.1 or pH 7.5
http://www.metric-conversions.org/volum ... rs.htm?arg

2. Here is what happened during your experiment; this is the quantity of calcium carbonate that could have dissolved during the month at 25 degrees Centigrade in pure water.

During the month that the shells were immersed in the ocean water, some molecules of calcium carbonate would dissolve and come to equilibrium with the solid calcium carbonate. The solubility constant (Ksp) of calcium carbonate in the website I found today is given as 8.7 x 10-9, so we’ll use this number for the calculations.

http://en.wikipedia.org/wiki/Calcium_carbonate

The solubility constant of a compound is the maximum concentration of ions that can dissolve:

Ksp calcium carbonate = [Ca+2] [CO3-2] = 8.7 x 10-9

So the concentration of calcium and carbonate ions that could dissolve in water would be 9.3 x 10-5 M (moles per liter) of both calcium and carbonate ions.
http://www.math.com/students/calculator ... e-root.htm

Now, adjusting for the volume of your water, the amount of calcium and carbonate in your sample would be:

9.3 x 10-5 M/ L * .44 L = 4.2 x 10 -5 moles per jar

And converting this to grams

4.2 x 10 -5 moles/liter * 100 grams/mole = 4.2 x 10 -3 grams, or .0042 grams

So, you should have been able to detect a loss of 0.0042 grams of calcium carbonate during the month, if you had used pure water. If your scale is not accurate to the thousands of a gram level, then this could explain why you didn’t see any change in the weight of the mussel shells. I think that the scale your teacher has doesn’t have the accuracy required for this experiment either, so this is the primary reason for lack of results.

2. Effect of the pH difference.

Here is an explanation of the difference in the concentration of hydrogen ions between pH 7.5 and pH 8.1. You should include this explanation in your write up since you were doing a pH experiment and you will want the science fair judges to know that you understand pH.

http://en.wikipedia.org/wiki/PH
Here is the formula for calculating pH, which is the negative log of the hydrogen ion concentration:


Remember from yesterday’s post, 2 hydrogen ions are required to dissolve one molecule of calcium carbonate.

CaCO3(s) + 2 HCl(aq) → CaCl2(aq) + CO2(g) + H2O

The concentrations of hydrogen ions in your two samples were:

pH 8.1 = 7.9432823472e-9 M=concentration of hydrogen ions
pH 7.5 = 3.1622776602e- 8M= concentration of hydrogen ions

http://www.1728.com/logrithm.htm

Subtracting these numbers, you will see that you had 2.37 x 10 -8 more moles/L of hydrogen in the pH 7.5 sample compared to the pH 8.1 control. Adjusting for your volume (.44 liters), you would have had 1.04 x 10 -8 more moles in the pH 7.5 jar, so this would potentially dissolve 0.5 x 10 -9 more moles of calcium compared to the pH 8.1 jar. Converting this to weight, if every extra hydrogen ion had been used to dissolve the calcium carbonate, you would have seen the following difference in weight between your two jars.

0.5 x 10 -9 x 10 g/M = 0.5 x 10-7 grams, or 0.0000005 more grams of calcium carbonate dissolved in the pH 7.5 jar compared to the pH 8.1 jar.

3. Effect of water matrix.

There’s one more consideration that you need to account for. You used instant ocean, which contains some salts, including about 400 ppm calcium. What effect would that have on your results?

http://www.aquacraft.net/w0014.html

400 ppm calcium is equal to 0.4 g Ca/liter x M Ca/40 g= 0.01 M (moles per liter)

Since there are already calcium ions in solution, this will limit the amount of calcium carbonate that can dissolve. Using the solubility constant again, you can find the concentration of carbonate that could dissolve in 0.44 liters of instant ocean is:

[Ca+2] [CO3-2] = 8.7 x 10-9 =[0.01][CO3]

Solving for concentration of carbonate:

[CO3] = 8.7 x 10-9 / 0.01 = 8.7 x 10-7 moles of carbonate.

Since only 8.7 x 10-7 moles of carbonate could dissolve from the shells, then only the same concentration of calcium could have also dissolved. Converting this to grams,
8.7 x 10-7 moles of calcium carbonate x 100 grams/mole = 8.7 x 10-5 grams of calcium carbonate or 0.000087 grams. So if you had a balance with an accuracy to the nearest 10,000th of a gram, you would have been able to measure a loss of weight in your control sample, and if you had a balance with an accuracy to the nearest 1,000,000th of a gram, you would have been able to detect a difference between the control and the experimental sample.

So, there is lots of science in this project after all. And, I’m sure you did get some results, but you just couldn’t measure them. Be sure to write up the complete project; you should go ahead and graph your results, even though it’s going to be a flat line or flat bar graph, depending on what you decide to use. If it appeals to you, you can use some humor to explain your results. The above information should be used in the discussion section, and you should include a final paragraph describing what you would do differently if you had time to repeat the experiment (e.g. put 25 grams of shells in a container the size of a swimming pool).

And, please do let me know if you have any questions on the chemistry. This is a study in solubility constants and pH, and you need to really understand what is happening so you can explain your results.

http://en.wikipedia.org/wiki/Solubility_equilibrium

Donna Hardy

Hi,

How are you doing on writing up the boards for your science project? Do you need any additional explanation about the chemistry of why the mussel shells did not dissolve?

Donna Hardy

Hi,

Thank you for the information. I think I am good for now. I will let you know if I need any more help.

Thanks

Hello,

I would crush the shells ever so slightly so as for the acid to seep through.

All the best!

Mrs. Hardy,

Thank you for all the information. I am working on the report now and it is coming around well.

Thanks again,
JMo