Science Buddies: "Ask an Expert"

Hello, my fourth grader is doing the pennies in a bowl of water to make a dome experiment. She has three bowls all the same size (4cups). One with water, one with two tablespoons of salt in the water and one with soda. The one with just water took 100 more pennies before the dome spilled over. Can you help me explain why? My reasons are pretty obvious but she's looking for the scientific explanation.
Thanks!

BrandiJoseph wrote:Hello, my fourth grader is doing the pennies in a bowl of water to make a dome experiment. She has three bowls all the same size (4cups). One with water, one with two tablespoons of salt in the water and one with soda. The one with just water took 100 more pennies before the dome spilled over. Can you help me explain why? My reasons are pretty obvious but she's looking for the scientific explanation.
Thanks!

Hi BrandiJoseph,

Welcome to SciBuddies! Is this a ScienceBuddies experiment, or an experiment from a different source? It would be very helpful if you linked the project in a reply so that SciBuddies Experts can see what the project is about / how the dome is made!

The closest link that I found that resembles the wording of this experiment is this: https://www.stevespanglerscience.com/la ... nny-drops/ in which the project tests the concepts of cohesion and the surface tension of water.

Please don't hesitate to give us a link, and we will definitely help you in the scientific explanation of the phenomenon!

-JSK

Hello JSK, thank you for helping us out. It's called the. Penny spill.

https://misdscience.wikispaces.com/file ... +Spill.pdf

Why the difference with salt water vs. just water? Does the salt create air or flotation that results in less pennies needed to spill?

Thank you!
Brandi

BrandiJoseph wrote:Hello JSK, thank you for helping us out. It's called the. Penny spill.

https://misdscience.wikispaces.com/file ... +Spill.pdf

Why the difference with salt water vs. just water? Does the salt create air or flotation that results in fewer pennies needed to spill?

Thank you!
Brandi

Hi BrandiJoseph,

My apologies that this took such a long time! It is probable that this is because the salt added to the water makes water denser; thus, it would spill over faster than water. Additionally, salt is an ionic compound, so it is also probable that it would attract the water (adhesion) and cause the dome to more easily break with less pennies. However, I couldn't find any scientifically-backed research papers specifically investigating how salt water affects cohesion except the tidbit of information below.

(Interestingly, contrary to observed results, there's a misconception that salt is a surfactant (a substance that lowers/breaks surface tension; in fact, salt increases surface tension in water, so it wouldn't be surprising if the dome held instead of collapsed. Link: https://socratic.org/questions/how-does ... n-of-water)

Hope this helped,

JSK

Thank you so much!