Stories

by Steve Davala

When things are combined together, they create a mixture. For example, a salad made with lettuce, tomatoes and croutons is a mixture. Another type of mixture is called a solution. This is where a solid, like salt, dissolves into a liquid, like water. As you can imagine, it is easier to take apart a mixture like the salad than a solution of salt and water. This month’s experiment is a challenge to see if you are able to think of ways to separate several different mixtures.

Before you get started, list as many different ways as you can think of to separate the mixtures. The point of these experiments is to try as many ways as possible—not necessarily just the first thing that comes to mind. Test as many ways as you can, and time them to see which is the quickest. Usually the first method thought of is not the fastest.

MATERIALS
–Rocks, gravel, sand
–Salt
–Water
–Plastic bowls (get permission to use some that will get dirty)
–Funnels
–2-liter soda bottles with the tops cut off
–Spoons
–Tweezers
–Sieves, coffee filters and anything else you can think of to help separate the rocks

PROCEDURES
Remember, you’re trying to find multiple solutions for each problem, and there is no right answer. Whatever works is what you’re going for. But if you are stuck for ideas, read the hints for more ideas.

Different sized rocks
In a big plastic bowl or a 2-liter soda bottle that has had its top cut off, mix several handsful of different sized rocks, gravel and sand. Separate them.

HINTS: The biggest rocks can be picked out easily, not so much for the tiny pieces. When you get there, use a sieve (like a spaghetti strainer). Only the smallest pieces will go through.  

An alternative example about natural separation is to mix them all in the soda bottle and just shake it gently. I won’t spoil the surprise.

Mud, gravel and water
This works best in a bottle with the top cut off. Mix a handful of dirt, gravel, sand, grass and water into the bottle. Separate them (including the water).

HINT: Wait for this to settle. Really small things will stay in the water (making it look muddy) for longer than the big chunks. Then slowly use the coffee filter and funnel to see if you can get the water to look clean.

Sand and salt
This calls for some tricky thinking. In a cup, mix together equal parts sand and salt. Separate them.

HINT: This is the toughest because the sand and salt particles are so similar. Picking them apart would take forever, and using a sieve won’t work because they’re the same size. If you go back to the beginning of the article, you can see a clue that involves something called “solutions.”

Not quite enough of a hint? If you mix the salt and sand with water, only one of them will dissolve: salt. At that point you can use the coffee filter again to filter out the sand. If you want, see what happens when you let this water sit for a day or two—or boil it if you get impatient. 

Experiment further
Can you make other types of mixtures or solutions and try to devise ways to separate them? Perhaps a mixture containing magnets and rocks? Be creative and think like a scientist.

Steve Davala is a middle school science teacher who likes to write. He’s got two kids of his own and subjects them to these science activities as guinea pigs.