Hot and Cold Water: Mixing

Tried this demonstration at home before I try it in the classroom. It shows how hot and cold water can both mix or not mix together.  I used blue food coloring for cold water and yellow food coloring for hot water (didn’t have red handy at home, but will use red at school).  It also demonstrates a great surface tension trick with the upside down jar of water and a playing card. I use playing cards b/c the waxy surface works best for either demo. (I used old glass jelly jars, we go through a  lot of jelly at my house. I run them in the dishwasher and save them for science experiments.)

For this demo, I am not going to tell the kids that I am using cold and hot water. Want to see if they can figure it out .

When the cold blue water is on the top, and the hot yellow water in on the bottom, as soon as you pull the card away, they mix and the water turns green in both jars. (purple if you use red). When the cold water is on the bottom, and the hot water is on the top, when you take the card away, it does NOT mix like before. there may be a small zone of mixing where the two meet. Great discussion about density and the effects of temperature on the movement of the water particles.

It is a pretty cool trick and I think the kids will love it when they see it, only b/c it is so unexpected for them .

For more information, check out this website: http://www.exploratorium.edu/science_explorer/watertrick.html

Density Bottles

As an introduction to density, I do a demonstration/discussion/group activity using density bottles. They are small sports drink bottles that I estimated to have a volume of approximately 400 mL.  There are 5 bottles, each  filled with a different item: cotton, air, sand, rice, and colored water.

These are some of the questions I used for our discussion:

• “Do these bottles have the same volume?” There is some uncertainty at first, but then they quickly say “yes”.
• “Do these bottles have the same mass?” No
• “Why don’t they have the same masses?” Variety of answers
• “Which one do you think is the heaviest?” We do a survey with a show of hands then have the kids give some reasons for their answers
• “Which one do you think has the most ‘stuff’ crammed into the bottle?” It’s interesting, there is a wide variety of answers and it usually doesn’t match the answer to the question of which is the heaviest

I tell them that they will find out the answers in a minute! We watch the BrainPOP movie for Measuring Matter. After the movie, I give the analogy of standing and waiting for an elevator.  Two identical elevators open up: one has 2 people in it and the other has 15 people in it.  “Which elevator would you choose and why?” Naturally, they say the one with only two people, there is more room in that one. I ask them, which elevator is denser? The one with 15 people, of course. We then discuss that there is less empty space available in the elevator with 15 people in it.  I then relate molecules to the people in the elevator, matter that has a lot of molecules, or atoms, crammed into a given space are denser than objects whose molecules or atoms have a lot of empty space between them.

I hand one bottle to each group and have them find the mass.  We collect the data and I write it on the board. I re-ask the following questions:

• Which one is the heaviest?
• Which one has the most ‘stuff’ crammed into the bottle? (Variety of responses)
• Which one is the densest? (Variety of responses)

Now that we have the mass and the volume, we calculate the densities for each bottle.  After we collect the data, I have the kids come over to the dunk tank.  One at a time, we predict which bottles will float. We do a survey and raise our hands if we think the bottle will float.  I have one student place the bottle into the tank and we see if it floats or not. We continue until all 5 are in the tank.

The cotton, rice, water, and air filled bottles floated, the bottle with the sand, sank to the bottom. I then ask the kids “Why did the bottle of sand sink?” They usually say it was the heaviest. I then say, “But a cruise ship is a lot heavier, and it doesn’t sink? Why?”  I give them a hint, “Look at our data, what do the bottles that floated have in common?” After a while, they figure out that the bottles that floated, all had numbers that were decimals, or less than one.  The sand was over 1, and sank. I tell them the density of water is 1, so objects with a density greater than 1 will sink.

We talked about how the bottle of sand is the densest b/c it has the most amount of “stuff” crammed into the same space, and that there is less empty space between the atoms.  I tell them that the density of gold is 19.3 g/cm3, and that if this bottle was filled with gold, it would be about 19 times denser, meaning that there would be 19 times more “stuff” crammed into the same space. The next day we talked about the story of Archimedes.  We calculated how much mass the same bottle would have if it was filled with pure gold - it would be 7,720 grams!! 

After the dunk tank, we did a small group acitivty using the graphic organizer from BrainPOP.  It shows a ring, balloon, yo-yo, and pillow.  We have to categorize them according to mass, volume, and density, from highest to lowest.  We do one category at a time and I give them a minute  for each, going over the answers between each category. I liked this graphic organizer b/c it really made them think about each item and their properties.

pg. 34 - BrainPOP – Archimedes

pg. 35 - BrainPOP – Mass, Volume, Density Graphic Organzier

Surface Tension Demonstration

I do this lesson as an observation with the students gathered around the table. We talk about surface tension and most kids have a general idea of what it is.

I fill up a glass with water, but not all the way to the top. I ask the kids “Can this paper clip float on top of the water?” Kids usually say no. I add a little drama and try really hard to get the paper clip to float on top of the water and act really disappointed when it drops to the bottom each time. Then I “remember” how to do it the right way.

I slowly add more water to the glass and the kids watch as the water rises over the top but does not run down the side of the glass. It forms a dome. We talk about surface tension again.

I then take a paper clip (the smaller ones work really well) and hold it horizontally. I place one edge on the lip of the glass and slowly slide the paper clip onto the dome of water. I give it a slight tap and the paper clip slides across the top of the dome to the other side. The kids think its such a neat trick. We then make observations of how the paperclip is slightly indented into the surface of the water and that the surface tension is holding it up.

I then add another paperclip and we make more observations. Sometimes the paper clips bump into each other and float around the top. We keep going until I can no longer place anymore on top. I think we had 15 floating at once as our highest count.

After we discuss this demo and wrap it up, I show the kids how to use a pipette and have them practice using it so they are ready for our surface tension lab the next class. Using a pipette is a fine motor skill and takes practice so all the water doesn’t gush out at once or come out in uneven large drops. I show them how to hold it with their thumb and first two fingers on the bulb end and to keep the pipette on a slight angle. You don’t want to hold it perfectly horizontal because you want the water to be near the opening and reduce air bubbles. Holding it vertically doesn’t give you as much control. You want to hold the pipette steadily and have good control.

One other key point is not to the have the tip of the pipette touch anything or submerge into the water. When they do the real lab, I remind them that the pipette tip should not touch the penny or any drops of water on the surface of the penny. I bring up that whenever I watch crime shows and they show some kind of testing liquid from a dropper touching the item they are testing it drives me nutty because they just contaminated the bottle they were using and its not using proper “CSI” techniques. =)

The kids then practice with different amounts of pressure and experiment on how to get a good even flow of drops of water, and to practice counting them. The kids really get into it and we see how many drops of water they can get in a row.

Using Publisher, I made a tri-fold brochure for this demonstration, here is the pdf.