Separating a Mixture

We have explored the concept of separating mixtures before.

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

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

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."

 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.

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 my middle son had 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 separated 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?

Sources:

GEMS: Sifting Through Science

Kindergarten- 2nd grade

4 Activities

104 pages

Three free-exploration learning stations build to a whole-class activity where students directly apply what they’ve learned. At the stations students investigate properties of objects, including: Materials that sink or float, Magnetic and non-magnetic objects, A sand and bean mixture, with tools to sift and separate the elements in the mixture. Then each student is given a “mini-garbage dump” and challenged to separate the “garbage” using the knowledge, tools, and techniques from the previous activities. Mathematics is integrated throughout; graphing and other data analysis deepen the learning.