Crosslinking Polymers

Sam, 2008

James, 2008

Chemistry is probably the most fun, especially for boys, when chemicals are mixed together to make gooey substances to play with. Sam  has mixed  together Guar gum and polyvinyl alcohol, which are long molecules joined together in long chains which slip and slide over each other very easily, so they are very slick and runny. But then, if you add sodium tetraborate it causes little bridges or crosslinks to connect the long molecules together, making a stretchy web of molecules.....or slime! These little crosslinks are fairly weak, so they break easily when the slime is stretched, so the slime flattens out on a flat surface. We added a bit of green food coloring just to make it visually interesting.

Sam, 2008

But you don't need chemicals such as these, which are a little harder to obtain, to have fun with making polymers; all you need is glue and liquid laundry starch.
I prepare plastic cups in advance to make the experiment go smoother. First put 4 tablespoon of water into a clear plastic disposable cup and mark where the level of the water comes to with a permanent marker on the outside of the cup. Add 4 more tablespoon of water and mark the level again. Pour out the water. Now you are ready for your students.
Have your students look carefully at the texture and consistency of white Elmer's glue and/or Elmer's Blue Glue, and liquid laundry starch before you start mixing them. My boys described the white glue was thick and sticky, the blue glue was thick and gooey and the starch was thin, slippery and white.

James, 2010

Have your students put either glue in the cup up to the level of the line. 
Now add laundry starch up to the second line.
When laundry starch is added to the glues the consistency changes immediately.



Quentin (2010) with Elmer's white glue and laundry starch



Quentin (2008), Elmer's blue glue and laundry starch

James (2008) with Elmer's blue glue and laundry starch

 As you knead the laundry starch into the glues, the glue-starch becomes a ball that you can take from the cup. It is now bouncy, stretchy and somewhat elastic.

Sam (2010) with Elmer's white glue and laundry starch

Alex (2010) with Elmer's white glue and laundry starch

Quentin (2008) with Elmer's blue glue and laundry starch

Katie (2008) with Elmer's white glue homemade Gak

 The blue glue-starch ball becomes more hard and lumpy than the white glue-starch ball.

Quentin (2008) with Elmer's blue glue homemade slime

Sam (2008) with Elmer's white glue homemade Gak

The different glues might be made of different molecules, my boys supposed. I reminded them of what they had learned about polymers (long chains of molecules hooked together), and illustrated what had just happened in the experiment by using 3 chains of paper clips laying side by side. I showed them that the paper clip strands slide by each other easily, and this is how the glue alone acts. I then hooked two from one chain to two from another chain, making cross-links. I showed them how the chains cannot easily slide back in forth now, illustrating the changes that occur when the laundry starch is added to the glue.

More fun with polymers at Paul's Treehouse , here at Science Kids at Home and here at Macrogalleria.



Separating a Mixture

We have explored the concept of separating mixtures before.

In 2006 I gave Sam a mixture of beads, salt and water and challenged him to separate them. First he separates the salt and beads by using a filter.

Now to separate the water and salt.
"Add heat," says Sam.

He boils the mixture until the water evaporates and all that is left is the salt.

"Too bad I don't have a way to collect the steam."



James, 2008

 Then, in 2008, gave each of the boys a cup which contained a mixture of salt and pepper and challenged them to figure out how to un-mix this mixture. They decided that the pepper might float to the top if they poured water into the cups and then they could skim it off the top.


Quentin, 2008



Sam, 2008

After they poured the water in, they could see that, although some of the pepper might have floated to the top, there was far too much just mixed in what was now a solution. They decided that we could filter the solution and that perhaps one would filter out, leaving the other behind. We made a filter out of a 2-liter bottle and a coffee filter. They were right and the pepper, which did not dissolve in the water, stayed behind in the filter, leaving the salt, which had dissolved in the water in the filtered water. Thinking back to a previous activity in which Sam separated salt and beads, he suggested that we heat the mixture to evaporate the water. I suggested that instead we could take the time to let the water evaporate on its own, and encouraged them to pour the solution onto squares of black paper. It took about a week for us to notice salt crystals emerging on the black paper, creating an impression of a "starry night." The crystals are large because they look a long time in forming.

This time I gave them a more complicated mixture. I mixed together wax shavings, iron filings, sand and salt and challenged them again to separate them as best as they could. It didn't have to be a perfect separation, but basically separating them back into four piles.

Their first idea was to add water to it. They saw immediately that the wax shavings floated to the top and they the could easily skim them off the top with a spoon.
Now we were down to three components in our mixture.

They then decided to filter out the water, hoping that the salt had dissolved in the water and that it would come out with the water, leaving the sand and iron filings behind. It did indeed work out as they had planned, but then they had to separate the water from the salt.

They had remembered our previous experiments and wanted to leave it out to evaporate, but I suggest that this time we could get it out a faster way and they suggest that we could get it out by using the stove. We were indeed able to boil the water away, leaving the salt behind.
We had now seperated out the salt and the wax. What about the sand and iron filings?

 They decided to try extracting the iron filings from the sand with a magnet.

Which is what I had planned all along. The sand, however, was still wet and with the water on it, stayed with the iron filings when they were attracted to the magnet, so was not successful in separating them.
They left the sand-iron filings mixture to dry (by evaporation, of course) and then the magnet picked up (mostly) just the iron filings.
All-in-all, I was very pleased with their ideas on how to separate mixtures as they used
filtration
evaporation
skimming (or physical removal)
and using the properties of the substance, in this case, magnetism.

How would you separate a mixture of sand, salt, wax shavings and iron filings?

Chemistry: Making Things Un-Mix: Chromotography

Chromotography is the un-mixing or separating of chemicals. The easy way to do a chromotography experiment is to have your students add lots of different colors of food coloring to water which, of course, made the water black. Quentin, in 2008, adding food coloring to water.

You can then ask your students if they can think of any way of getting those colors separated again. They should dismiss the things we had tried in the past to seperate mixtures such as filters or evaporation, deciding that they wouldn't work. James, in 2008, having set up his paper chromotography experiment.

Then provide them with sticks ( or pencils), tape and coffee filter strips. Have them tape the strips to the pencils and put them over the cups so that the filters touched the water. Quentin, in 2008, observing the paper strips.

The water will begin to migrate up the paper strips. You should see bands of blue, green, yellow and red on the paper strips. 

James, in 2008, observing the colors separate.

A more difficult way of doing a chromotography experiment is with colored markers. The markers become the test substances, and are used to draw lines onto strips of paper coffee filter, which are the medium. The strips of paper are then taped to sticks, in this case pencils, so that they can be suspended in water, which is the solvent.

Sam (2008) setting up paper chromotography experiment.

Water is put into the trough so that it can pass through the paper. As in all chromotography, the solvent passes through the test substance, and as it does so some of the test substance may be attracted to the solvant and follow it up the medium. Different types of molecules are transported different distances, causing them to separate. In paper chromotography, when the inks separate, they make little rainbow-like patterns. The green, for example, separated into various shades of yellow and blue.

This year we tested some new materials including brown ink. Katie had some brown ink from art projects that she let us use. You could also break open an ink pen. Black also breaks down into several colors.

We also tried soaking the brown coating off M&M's and letting it soak up water.

Sam also tried a more complicated form of chromotography with plant pigments. First he gathered three leaves. Two were from trees we know (and we know what colors two of them turn into in the autumn) and one we are not familiar with.

Taking each one seperately, he cut the leaves into small pieces.

Then he put a little sand in the mortar and pestle and ground the leaf into a paste.

He then put each leaf in a separate jar and poured Isopropyl alcohol over the leaf paste. He let this soak for about a half hour.

He then made strips of paper towel.

He put drops of the soaked leaf paste on the paper towel strips.

Then these were suspended in more (plain) isopropyl alcohol by dowels. This was left overnight.

I also put some paper towel strips to soak in the leaf paste jars.

The Results?

 These are the leaf strips that worked the best, which were the ones I soaked in the leaf paste. You can see bands of yellow (carotene), perhaps orange? (xanthophyll) and the green chlorphyll in the first two examples. It is interesting that the different shades of green are called chlorophyll A and chlorophyll B.

The one on the right is from the Sassafras leaf, the middle one is from the Norway Maple and the third is the unknown sample. I know that the Sassafras leaves vary from yellow to orange. The Norway Maple turns yellow.

 These are the samples Sam did, which did not break down into bands.

This is a sample of one of the brown M & M's. You can see that the brown is made of many colors.

The marker didn't seem to break down with just water.

The ink seem to break down into bands of orange.

 The varied results can be for many reasons. Sometimes there needs to be more of the sample on the paper. Different papers will vary the results, sometimes one type of paper will work better on one type of sample and not on another. The solvent can also make a difference. Alcohol works better on some samples, whereas water or salt water(1/8 teaspoon to 3 cups water) might work better on another. Goo Gone is another good solvant as well.