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Help with a new serial dilution

Posted: Mon Feb 13, 2012 6:40 pm
by scottr3
Hi. I'm working on the "How Salty Does the Sea Have to Be for an Egg to Float?" experiment with my 4th grader. We followed the instructions in step #3 to make a serial dilution without too much trouble and found our egg floats in a 50% solution.

I'm stuck at Step 7a - 7d though. The procedure says we should more accurately find how much salt it takes by starting with our 50% solution in the first cup and figure out a new serial dilution with smaller steps for each of the remaining cups. It suggests that each step should be 80% of the concentration that came before.

I'm stumped on how to mix a solution in the first cup and then dilute down the line to the remaining cups so that each cup is only a 20% step.

Any suggestions out there?

Re: Help with a new serial dilution

Posted: Thu Feb 16, 2012 7:33 am
by donnahardy2

Welcome to Science Buddies! I think you are doing this project, and it is an excellent one. ... p003.shtml

If you had metric measuring cups, it would be easy to make 20% dilutions by using a proportion of 80 ml of salt solution plus 20 ml of water for each dilution in the series. However, you probably have standard US measuring cups with ¼ and 1/3 cup increments, so doing a 20% dilution is not easy. I would recommend doing a 25% dilution instead. You can use a proportion of ¾ cup of salt solution plus ¼ cup water for each dilution in the series to obtain a more precise density measurement.

By the way, when you finish this project, please come back to the Science Buddies website and click on the “My Science Buddies,” tab and select the “I did this project,” and give us your feedback on this project. I have given advice on this project several times, and never realized the difficulty of doing the 20% dilution.

Donna Hardy

Re: Help with a new serial dilution

Posted: Sat Feb 18, 2012 11:04 am
by scottr3
Hi Donna;

Yeah, my daughter did this experiment several years ago from the Science Buddies website but the procedure was a bit different. In that experiment we added a teaspoon of salt at a time. This newer procedure seems like it will give us a more accurate answer but, we're still struggling with the 20% series dilutions.

I understand your description above - each cup should contain 20% water and 80% salt solution but, I'm having trouble getting the correct amount of liquid into each cup correct.
Either I have too much or too little!

Talking it out here, let me know if this sounds right...

Start with 200ml of salt solution in cup 1
Fill cup 2 with 80ml of tap water
And 40ml tap water to cups 3-5

Next I added 320ml of salt solution to cup 2
Then I pour 160ml of liquid from cup 2 to 3
Then 160ml from cup 3 to cup 4
Finally 160ml from cup 4 to cup 5

Well, that seemed correct as I wrote it but, in practice I end up with only 40ml of liquid in the middle cups. I need to have at least 100ml of liquid to float an egg in a 16oz cup.

Back to the drawing board. The 80% series solution sounds simple enough on paper but, it looks like I need much bigger cups and I need to use a lot more liquid!


Re: Help with a new serial dilution

Posted: Sat Feb 18, 2012 2:52 pm
by scottr3
I think I have Step 7a-7d figured out, however, the amount of liquid required is much greater than I expected!

First, I found I actually need 200ml of liquid to float an egg in a 16oz plastic cup

Then working backwards, to create an 80% series dilution I'll need 200ml of tap water to 800ml of salt solution.

For mixing the solutions I had to set aside the cups and pulled out 2 quart plastic containers!

We added 200ml of salt solution (this is the salt solution ratio we discovered in step #3 of the experiment) to Container #1
Then add 200ml of tap water to containers 2-5
Then add 800ml of salt solution to container #2 and stir
Add 800ml of the solution from container #2 and add it to container #3 and so on

We're mixing 1000ml or about twice as much liquid as a 16oz cup can hold to make our 80% serial dilution, however, after we pour off 800ml to mix with the next container, we're left with 200ml or about 7oz of solution - just enough to cover an egg to see if floats or not.

By the way, 200ml is about the same as 3/4 cup. Since we started the experiment using cups then 200ml becomes 3/4 cup and 800ml becomes 3 cups

Re: Help with a new serial dilution

Posted: Sat Feb 18, 2012 7:42 pm
by donnahardy2
Hi Scott,

Congratulations! You have figured it out correctly! Your plan for the 20% dilution is perfect, and you will be able to find a fairly precise concentration of salt that will float the egg. Let me know how the rest of the experiment goes.


Re: Help with a new serial dilution

Posted: Sun Feb 19, 2012 2:49 pm
by scottr3

The second series went well so we made a third series using 10% steps.
Now time to write some conclusions!

Thanks for your help!

Re: Help with a new serial dilution

Posted: Sun Feb 19, 2012 6:21 pm
by donnahardy2
Hi Scott,

Fantastic! This is going to be the most precise and quantitative project at the science fair. Great job!!!

You are welcome. Let us know if anything else comes up while you are completing the board.


Re: Help with a new serial dilution

Posted: Thu Apr 17, 2014 4:48 pm
by LeslieeS
Ive been working on this science project for awhile and alot of things confuse me! 1st of all i dont understand how to find relative salt concentrations and absolute salt concentrations. Whats the difference? 2nd of all I dont understand how to do another serial dilution. On the procedure it says to try diluting the solution by 25% with each step. How would I do that? im so cunfused. I have chemistry right now but I dont understand it much. so please try to put it as simple as possible. step by step perferably. Can anyone help me please? I would really appreciate it. :D