The density experiment – part II

(Last year’s experiment re-printed here).

So we did work on the density experiment last night. I must say the girls are really excited about doing experiments. We first placed some objects to see what would sink or float and in which layer they would end up. We had the girls each predict where the layers would end up when we poured them in the jar, then recorded our results. Then we added small objects, predicted about which layer they would end up when we added them to the jar; and then we recorded our observations. Pretty straightforward stuff. Though I must say my kindergartener guessed correctly almost all of the time and also was able to say why.

But then we made something really cool happen with an ice cube. I have two pictures showing something really interesting. The layers are (from bottom to top) honey, colored water, and oil. Check out the ice cube though. This first picture shows the ice cube on the interface of the water and oil layer. This was we got when we put the ice cube in and let it sit awhile. The ice cube stayed by the oil/water interface. (oh, in case you were wondering, that’s cheerios and popcorn floating on the top).

Photobucket

Then, we added some hot water to the jar, and to our surprise (yes, mine too, I am learning right along with them), the ice cube floated to the top of the oil. This was the most visually exciting part of the experiment and I had to post pictures of it. Changing the temperature of the fluids seemed to release the cube from the interface and the ice cube floated! (I can’t tell who’s more excited, me or them).

I’m not quite sure what is going on there, because that’s not what the website said was going to happen. I really wasn’t sure it was supposed to happen. According to the density page we got this from, we really didn’t do the experiment quite right. We were supposed to add very hot water of a different color (which we did the first time), to see if the layers of hot and cold water would be separate (and we must have messed it up because they just turned the water green). Instead, we got the funky ice cube effect. I repeated it because I wasn’t sure if that’s what was supposed to happen (but this time I skipped the coloring the hot water, I wanted to just focus on the ice cube phenomenon). And for the second time, we had that the ice cube do the same thing – stay on the interface until the hot water caused the ice cube to float.

Photobucket

What was going on with that ice cube? Was it something like surface tension at the interface holding the cube at the bottom and we broke it by adding the hot water? Was the ice cube denser than the oil when the oil was cold, but when it was warmer, the ice cube less dense and therefore floated? I don’t know.

I’m so excited to bring science to my children. It’s so very cool.Dd wanted to do another experiment today. It’s also another density type experiment. She wanted to see what other kinds of things sank or float, just in water though. I have a few surprises for her. I have been told by a teacher friend of mine that certain fruits will float (like watermelon – though I’m not sure if I can come by a watermelon in the middle of winter, but I’ll try). I also have been saving a pumice stone for a while just for this very experiment. (Why, you might ask? Shhh, don’t tell my girls, but it’s a stone that floats!)

If we can get a small watermelon, we’ll be trying that experiment today or this weekend.

2/9/08

Interesting…

I went back to another part of the diyscience website, and found another experiment called dancing ice that may help explain what’s going on. Apparently the density of ice and vegetable oil is about the same…920 kg/cubic meter. But the density of ice cube changes when it melts. Water is unique in that it is the only substance that is less dense when solid. So it floats in other liquids. In the process of melting though, liquid water will adhere to the surface of the ice cube for a bit before dropping off. If there is enough water on the surface of the ice, it will be sink because the density of the ice and the water will be greater than that of the oil. When the water falls off the cube, the ice cube will float. Come to think of it, I had my 4 year old hold the ice cube for a bit before we placed it into the oil. By the time we were ready to put it in, it already was starting to melt pretty well. So it was dense enough to sink in the oil. The ice cube was going to float on the oil eventually, it was just a matter of time.

There we go, mystery solved!

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