Friday, October 14, 2011


This is such a simple demonstration that really blew Squirt's mind. She'd had a bit of a rough day so a part of me wanted to skip this and just have her play in her room - but I realized that we both needed it. Again, we did something quick and easy that I used in my classroom. The concept is density and it can be done using supplies from your kitchen.

Kids (and many adults) grasp density as being an idea of weight, even though in reality that isn't quite right.

We started by dropping an egg into a glass of water. I asked her what she thought would happen, she said it would sink, and then I had her test that.

She was right, of course.

So then I had her add salt (a lot) to the other glass and asked her what she thought would happen. Since I'd had to convince her that this would be a good idea I'd already mentioned that we were going to make the egg float. She guessed that adding the salt would make the egg float.

It takes a lot of stirring to dissolve the salt. If you have more time, you can heat the water to make the salt dissolve more quickly - but you'll want it to cool to room temperature again before little hands are playing with it.

Even though she *knew* what would happen, I'm not so sure she believed it.

Even I didn't expect it to be so exciting! She loved that the egg bobbed up to the surface after she dropped it in the glass.

2 eggs
2 glasses of water, filled to the same height
salt - a lot

Drop one egg into a glass of water. What happened to it? Do you think anything will be different if we add salt to the water?

Pour in a good amount of salt into the second glass and stir it to dissolve. Place the egg into the glass. If it doesn't float, use the spoon to pull the egg out (Squirt tried to reach in and spilled the salt water all over the kitchen), add more salt, stir to dissolve, and try again. The amount of salt you need to add will vary based on the size of your glass so just go for the test and try again approach. The egg should float in the salty water.

**In class I'd then add food coloring to the plain water, gently and slowly pour it into the container holding the salt water (down the side of the container or into a spoon to slow it's fall) and the kids could see the layering that forms because of different densities of water. Then if you add the egg again you will see it float between the two layers. We attempted this and had major food coloring issues and had a hard time pouring it slowly enough to keep the layers from mixing. I will save that one for when she's a little older and we can talk about layering in the Ocean.**

As a mom this is how I explain it: What is heavier, the water or the egg? {the egg} That's why the egg sinks. When we add the salt to the water and mix it up we make the water heavier and the egg floats.

As a teacher this is how I explain it: Density is a measure of how much "stuff" is packed into a certain amount of space. Water is less dense than the egg so the egg sinks. When we add salt to the water and the salt dissolves, you will notice that the level of the water hasn't really changed. Now, instead of having just the water in that space we've got the salt too. We've packed a whole lot more "stuff" into the same amount of space. This increased the density of the liquid. If we increase the density of the water to more than the egg, then the egg will float. This is the same reason that a can of Coke will sink to the bottom of cooler when the ice is melted but a can of Diet Coke will float. Coke has a lot of sugar dissolved into it which increases the density of the liquid. Diet Coke has a lot less mass of artificial sweetener so the liquid is less dense. Next time you're at a barbecue, check it out!

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