Saturday, February 21, 2009

Science Saturday Challenge #4

We've been learning about properties of matter, and this week's experiment demonstrates a few of them. We read pages 10 - 19 in the Usborne Science Encyclopedia. Search for keyword "properties of matter" at your library for relevant titles.

First you'll need to make a super saturated salt solution (a.k.a. brine) by boiling 2cups of water and then adding salt in batches, stirring in between, until the salt no longer dissolves. Let it cool.


Water (hot and cold)
Food coloring
3 small glass containers


Put equal amounts of brine, ice water, and hot tap water into each of the three containers (we used special plastic test tubes, the ones from which 2L bottles are made, with 2 tablespoons of liquid in each.)

Add a drop of food coloring into each at approximately the same time and watch what happens. In this video, red is hot, yellow/green is cold, and blue is brine.

The red and the yellow/green drops sank at about the same rate, but the red diffused throughout the water much faster. You can see this better with purple dye better (purple is cold, red is hot water.)

Density is mass (g) per volume (ml) or
D = m/v
For a liquid this is easy to calculate. For a solid you can figure out the volume by how much water it displaces (see pg. 17 of the encyclopedia to set this up.)

Other Resources:

Chem1 Virtual Textbook: Density and Buoyancy (advanced)

I Love Density has a complete science project on density (intermediate)

We found several Gizmos that demonstrated density. One had objects on a shelf that you could put on a scale to get weight (mass), then in a graduated cylinder to get volume. I did the calculation for the kids (they were fractions that I converted to a decimal.) You can then put the object in a liquid in which you can adjust the density. The kids were impressed when I started out with the object floating and as soon as I adjusted the fluid density to a number less than the object, the object sank.


Jimmie said...

That is neat. I've bookmarked this as we'll be beginning chemistry soon. I don't really understand the concepts here; I guess I'll have to study your links and such.

I love your cute test tube set! I want that!

Kris said...

I demonstrated two concepts, which is why it may be a little confusing. The first is density. Brine is more dense than water; a neat way to show this is to fill a measuring cup to, say, the 1/2 c mark with hot water and then add a tablespoon of salt. Ask Sprite to see how the volume changes when the salt is added--it doesn't! That's because the liquid is becoming more dense (more mass, same volume.) That's why the blue dye just floats on top of the brine, because it is less dense than brine. Compare that to how the dye sinks into the hot and cold water.

The hot and cold test tubes demonstrate how heat energy makes molecules move faster. The dye diffused throughout the hot water much faster than the cold water where the dye just sat at the bottom.

You can get the test tubes and rack from Steve Spangler Science for $6, though I don't know what the shipping will be to you!

dstb said...

Thanks Kris,

We did this experiment last night. My younger son was not convinced that the dye in the hot water spread out faster than the cold, even though we did it multiple times. We used baby food jars, so maybe test tubes would have made things easier to see.

We are taking turns setting up the experiments and being the "scribe". My husband set this one up and at the end he showed, by using the hydrometer from an old beer making kit, how the density varied. VERY noticable difference between the salt and regular water!

Great experiment. Thanks!

Kris said...

Neat--I gotta get me one of those hydrometers ;o)

The hot and cold difference is better appreciated with different color dyes--you can really see dark dye just sitting around the bottom. I also held the test tubes under very cold or very hot water before filling them to make a really large temperature difference.

I'm glad you had fun with it!

Evelyn Saenz said...

We just had a wonderful time trying out your experiment. One of the kids thought that the difference could be in the color of the dye rather than the temperature of the water so we tried it again using the same color in each tube.

This fit in well with our Unit Study on King Bidgood's in the Bathtub.

Kris said...

He, he, he I can see how that would fit in--very cute!

susan said...

Hi Kris - I'm from Steve Spangler Science. Thank you for the link to our test tube set. I know I am partial, but it is a really great, sturdy set.

You have a lot of fun science experiments on your site. I want to check them out for my own Science Saturdays.

Did you know you can get a widget from Steve Spangler Science for your blog that shows our experiment of the week? You can get it at: