Science Buddies: "Ask an Expert"

Hi Science Buddy volunteer.
My 3rd grade daughter is doing the science project "What melts ice fastest?" She's tested table salt, black pepper, sugar, and sand. She's run the experiment twice now, and she's at tallying and analyzing her results. We followed the equations given with the project. 1. (Mass of melt water)/(initial mass of ice cube)x100. 2. (Remaining mass of ice cube)/(initial mass of ice cube)x 100. She found that there are times when pepper, sugar, and sand have over 100% remaining ice. This was consistent both times she did the experiment. All of the substances we used followed a descending pattern both times she did the experiment. For example in the first test pepper's %of ice remaining went 103.2, 103.2, and 93.54.

We are stuck trying to explain how something can be over 100%. Here are our questions
1. Is the higher percentage of ice remaining due to the weight of the substances used?
2. If question 1 is true, then why is table salt different?
3. Since sugar and salt dissolve in water, why doesn't the sugar react the way salt does?
4. What changes or modifications do we need to make to the equations that will justify we're getting the correct measurements for the ice. OR does that not matter and we simply need to note what's affecting the higher percentage rate?

Thank you so much for your help!

AHerr_13,
Thank you for your questions, and welcome to the forums. It can always be a bit frustrating to try and explain the unexpected result when performing science. However, you and your daughter have done a good job of performing multiple runs of the experiment to verify your results. I will try to answer your questions below.

AHerr_13 wrote:1. Is the higher percentage of ice remaining due to the weight of the substances used?

The short answer is yes. For example, if you started with 1000g of ice and added 50g of test substance and then after some time you found that 30g had melted then you would find the ice now weighs somewhere on the order of 1020g (1000g+50g-30g=1020). Did you happen to weigh the test substances prior to adding them to the ice? Even the 30g of melt water isn’t just water. Some unknown percentage of the test substance will have dissolved and be present in the water while the rest will remain on/in the ice.

AHerr_13 wrote:2. If question 1 is true, then why is table salt different?

Salt isn’t different. However, salt is much more efficient at lowering the freezing point of water such that the effect is much more pronounced. If you added the weight of the remaining ice and the melt water, I expect you’d find that you have more mass than you started out with. But given the example above at a much higher melt rate for salt, say 80g of melt water, then 1000g+50g-80g=970. If you divide that by the initial weight of the ice then you have (970g/1000g)x100 = 97% showing a reduction in the initial mass of the ice.

AHerr_13 wrote:3. Since sugar and salt dissolve in water, why doesn't the sugar react the way salt does?

Salt and sugar react similarly, but salt is just more efficient at it. The answer can be complicated, but the simple answer is that when a molecule of sugar dissolves in water you end up with a single molecule in solution. This extra molecule prevents water molecules from combining with ice crystals and freezing normally, thereby depressing the freezing point. Salt (and all substances) also do this, but when a single molecule of salt dissolves, it breaks into two ions, a sodium ion and a chloride ion, thereby effectively doubling the “freeze blocking” effect.

AHerr_13 wrote:4. What changes or modifications do we need to make to the equations that will justify we're getting the correct measurements for the ice. OR does that not matter and we simply need to note what's affecting the higher percentage rate?

If you happened to weigh the test substances prior to adding them to the ice, you could then account for the additional mass in your report, and not change anything, or you could rerun your numbers with the added mass. For example, instead of starting with an initial weight of 1000g as used above, your initial weight would be 1000g+50g = 1050g of initial ice, but in the report include that the weight includes the test substance. As long as you explain your results in a clear manner I think you can do it either way.

Thank you so much for helping us understand what's happening. We did not weigh the ice cube again once we put the substances on it at the beginning of the test.

Just to make sure I get this correct, if we ran the experiment again and subtracted the weight of the container and the weight of the substance each time, we would get a more accurate picture of the percentage of remaining ice?

Oops, I need to clarify the 3rd question I asked. I've pasted it below, and my follow-up question is below that.
AHerr_13 wrote:
3. Since sugar and salt dissolve in water, why doesn't the sugar react the way salt does?


Salt and sugar react similarly, but salt is just more efficient at it. The answer can be complicated, but the simple answer is that when a molecule of sugar dissolves in water you end up with a single molecule in solution. This extra molecule prevents water molecules from combining with ice crystals and freezing normally, thereby depressing the freezing point. Salt (and all substances) also do this, but when a single molecule of salt dissolves, it breaks into two ions, a sodium ion and a chloride ion, thereby effectively doubling the “freeze blocking” effect.

Follow-up question: I think what I'm trying to figure out here is why salt didn't affect the weight of the ice cube like the sugar did since both substances dissolve. Is sugar heavier than salt or does it have something to do with the changes sugar goes through when it dissolves with water? I hope that makes sense!

Thank you again!

AHerr_13,

AHerr_13 wrote:Just to make sure I get this correct, if we ran the experiment again and subtracted the weight of the container and the weight of the substance each time, we would get a more accurate picture of the percentage of remaining ice?

I don't think I was as clear as I could have been with respect to this part. If you were to redo the experiment:

1) I would obtain the weight of the container (clean and dry) and subtract that value from any measurements where the container is used. You don't need that as any part of your calculations, it is just the vehicle used to hold your test samples.

2) I would weigh the substances before placing it on the ice and ADD it to the initial weight of the ice and that would be the number to use as your initial value. Keeping the same example as above: If you had 1000g of ice and 50g of substance then your initial weight value would be 1050g. This would be the number I'd use to obtain the percentages. This will prevent the situation where it looks like you have more than 100% ice remaining after some of the ice melts. Just be sure you note this in your procedures. The reason I would do it this way instead of subtracting the weight of the substance (i.e. 50g) from the final weight is that you don't really know how much of the substance dissolved and went into the water vs. how much remained on the block of ice.

AHerr_13 wrote:Follow-up question: I think what I'm trying to figure out here is why salt didn't affect the weight of the ice cube like the sugar did since both substances dissolve. Is sugar heavier than salt or does it have something to do with the changes sugar goes through when it dissolves with water?

Without truly knowing your test procedures, my guess is that it did affect it the same way. It just isn't as apparent because the melting effect salt has is so much more pronounced than with the other substances that the melting result masked the fact that there may have been a significant difference in initial weight. Again following my example numbers. Initial weight of ice = 1000g and weight of salt = 50g. If after say 5min after you apply the salt to the ice you measure that the ice weighs 900g. If you neglect the added mass of the salt and use the initial weight of ice only, then: (900g/1000g)*100 = 90%. It looks like you have a 10% reduction in the weight of the ice. However, the results are somewhat skewed because the 900g includes some unknown amount (close to 50g, but some will have dissolved and gone into the water as well) of salt material in it and the 1000g doesn't. But if you don't neglect the added mass of the salt and use the initial weight of the ice and salt as your initial weight, then: (900g/1050g)*100 = 85.7%, almost a 15% reduction in weight of the ice. I would expect, given these numbers in the example, that the weight of melt water would be around 150g. This would include some unknown amount of dissolved substance that you can’t just subtract out.

One additional note: strive to use the same amount of ice and test substances in all cases. So, for example, try to use a 1000g ice cube each time and apply 50g of substance each time. Again, my numbers are made up to illustrate the point only. Use whatever amounts are convenient for your experiment, just be consistent. That way you don’t have to worry about one substance being heavier than another. 50g of salt = 50g of sugar = 50g of pepper, etc…

I really hope this makes some sense and actually clarifies things some. Please post back with any other questions, particularly if I made things worse and confused you further.