Seeking easy to understand science facts behind simple experiments

Hi. I am writing preschool science activities and would like to verify what the scientific principles behind them are.

First example: when you put raisins in clear soda pop, exactly what causes the bubbles around the raisins to pop when they reach the top? Is it only the outside air pressure?

Second example: when you blow through a film of soap to create bubbles, is it surface tension that holds the soap together around the air? And what exactly happens when they pop?

Thank you.

marcusl
Gold Member
Hi Blouella, welcome to Physics Forums!
Hi. I am writing preschool science activities and would like to verify what the scientific principles behind them are.

First example: when you put raisins in clear soda pop, exactly what causes the bubbles around the raisins to pop when they reach the top? Is it only the outside air pressure?
Yes, this is correct. The pressure of the gas inside the bubble is greater than that of the air so they burst. Soda is bubbled by dissolving highly pressurized CO2 gas in it. The pressure is so high that some of the gas comes out of solution following the bottling (it lives in the little "air" space between the top of the liquid and the bottle cap); it escapes with a "whoosh" upon opening the bottle.
Second example: when you blow through a film of soap to create bubbles, is it surface tension that holds the soap together around the air? And what exactly happens when they pop?

Thank you.
You are right again, it is surface tension. If you look carefully at a bubble, you will see that fluid tends to distribute from the top of the bubble to the bottom over the space of a few seconds, due to the pull of gravity. Sometimes you even see the start of a drip formi9ng at the bottom as liquid collects there. Eventually the top becomes so thin that it can't support itself and the bubble comes apart--it pops.

Here's another interesting bubble phenomenon. The thickness of the soap film can get very small--comparable to the wavelength of light (of order 1/2 micron, or 5x10^(-7) meters). When this happens you see interference fringes, which look like wavy patterns of color. If the film becomes half the thickness of green light, say, or an integer number times that thickness, then green light reflecting off of the inner and outer surfaces of the film add in phase while other colors reflect out of phase and are suppressed. You see a green reflection. At a different thickness, a different color is favored. So as the bubble thins at top and thickens at bottom, you'll see a progression of color changes. They look wavy and multicolored both because illumination in a typical room is kind of random and because the film thickness is not uniform.

Thank you Marcus! This is exactly the information I needed.

Here are two more activities that I would like to verify:

What happens, scientifically, when you float two toothpicks in water then dip a straw with dish soap into the water? Or similarly with pepper in water and dish soap?

Also, an orange floats in (enough) water. So does the peel. But the peeled orange sinks. Is this due to air trapped in the peel making it lighter? Or is it a more complicated situation with the orange or the peel or the peeled orange being more or less dense than the water? And if so, what does "density" actually mean?

I hope these questions are not too basic, but these are the things I need to know.

Thanks again.

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Danger
Gold Member
Welcome from me as well.
I have nothing to contribute in the context of this thread, but I want to express how pleased I am that someone is introducing the youngsters to science in a manner appropriate for their age group. PF is all about education, and you will be a pleasing addition.

Thank you, Danger!

I am writing preschool science books that help children learn the science behind the activity, as you mentioned. It will also encourage them to think and guess (predict) instead of just being "handed" the answers. I believe this develops a measure of common sense thinking, which is sometimes in short supply! ;-)