This week in Science, in honor of Yom Ha’atzmaut, we explored floating and sinking.
First we filled up a container with tap water and talked about what the words ‘float’ and ‘sink’ mean. Then we each picked an object out of a box. Everyone had a turn to guess whether his/her item would sink or float when we put it in the water. We tested our hypotheses. Depending on our result, we chose a card that said ‘sink’ or ‘float’ and placed it on a chart in the first column next to our object.
Here’s what we came up with:
Good scientists make sure that their experiments can be repeated with the same results. This time, we used a container filled with water from Morah Elaine’s house and we tested all of the objects again. We recorded our results on the chart in the second column next to the objects.
As we would expect, all of the objects that floated in the school water floated in the water from Morah Elaine’s house. Many of the objects that sank the first time sank the second time…but not all of them. Some of them sank the first time but floated the second time (the objects that are pulled off of the chart onto the table, above). That was surprising!
Why did some of the objects float in the second container but not in the first? The objects didn’t change and the containers were the same…
It had to be the water that was different. We knew that the water in the first container came from the classroom sink, but what exactly was in the second container? We weren’t really sure. Everyone got a drop of water from the second container to taste.
It tasted awful!
That’s because the water in the second container was completely saturated with salt. Note that while this water did come from my house, it did not come out of my faucet this way. 🙂 I added lots and lots of salt to it before bringing it to school.
How salty was the water?
Let’s pretend that Zac’s hands are a pot full of tap water and we add some salt (aka white pompoms) to it. We heat the water, and then stir in the salt until it dissolves. We add more and more salt until the water can’t possibly hold any more. Then the salt starts coming out of the water and onto the table.
That’s pretty much what happened with the salt solution in the second container. I kept adding salt until the water couldn’t hold any more. That’s called a ‘super saturated salt solution.’
Next, we looked at a photo of the Earth taken from space.
All of the blue parts are water, but not all water is the same. Water in lakes, ponds, and streams is fresh water (like the water in container number 1). Water in the oceans and seas is salt water (sort of like the water in container number 2). As we discovered today, some things that sink in fresh water can float on salt water. If the water is salty enough, people can float on it without even trying. The best example of this is the Dead Sea (Salt Sea) in Israel. The Dead Sea has the saltiest water of anyplace on the whole Earth. We looked at some pictures of people reading newspapers and books while floating on the Dead Sea:
Finally, we learned a really big word, ‘buoyancy,’ which has to do with an object’s ability to float on the water.
As we discovered today, changing the amount of salt in a given amount of water, changes the buoyancy of objects that are placed in it. Adding salt to the water makes objects more buoyant (making it easier for them to float).
That’s a lot to think about!
See you next week,