Hi all,
The ISEE education team recently participated in CGI U (Clinton Global Initiative University - http://www.cgiu.org/Default.asp) and was able to share their commitment to action to San Diego schools at the CGI U exchange. During this event we challenged CGI U attendees to one of my favorite activities - the magic cup of ice water.
To do this activity all you need is a cup, ice, water, and a permanent marker. Put the water and ice in the cup and mark the water level with the marker. Then make some predictions. What will happen to the water level when the ice melts? Will it rise? Fall? Or stay the same?
At CGI U 15% guessed above, 55% said below, and 30% same. After participants guessed, we asked them to explain their answers.
Those who said above argued that the ice would add more water to the cup upon melting because when the ice melted it would leave behind more water.
Those who said below argued that ice expands as a solid and therefore would leave behind less water when it melted.
Those who said the level will stay the same pointed to the d=m/v equations and many of them said they had done this experiment in a class before.
Who is right? Well we didn't tell anyone at the conference, instead they had to come here and let the question stew in their brain for a few days.
So now for the amazing solution check out the video below from youtube user onefivefour:
Not convinced? Here's another version (shorter) with a measuring cup:
So why does this happen? While its true that H2O is larger in its solid form than in its liquid form, when we think back to displacement and density equal to mass divided by volume, the solution becomes clear.
Lets say we have an ice cube mass m floating in water, in order to do so it must displace some of the water in the cup to hold itself up. This displacement is key because it pushes the water up slightly increasing the volume as apposed to a cup without any ice in it at all. When the ice melts, its density decreases and it fills the spaces it originally displaced, except this time with water so it smooths over.
Or if you're more of a number person...
density of water = 9x10^5 grams per cubic meter
density of ice = 1x10^6 grams per cubic meter
mass of water in cup = 1000 grams
mass of ice = 10 grams
v = m/d
vinitial = 10/9x10^5 + 1000/1x10^6
= 1 x 10^9 liters
vfinal = 1010/1x10^6
= 1 x 10^9 liters
Don't you just love it when math and science work?
Still don't believe us? Try it yourself! I've had students test this with once ice cubes, ten ice cubes, water level starting very low and water level starting at the brim of the cup. Just make sure to hypothesize, test, and understand.
Thanks to Bill Clinton, CGI U staff, UCSD staff, and all the wonderful attendees!
