We went over recently that I don’t love the separation of mixtures lesson because it’s just so vocabulary heavy. After finding a way to get my students more engaged in that lesson, it came time to get them engaged in the separation of mixtures labs. Now don’t get me wrong, your girl LOVES a good distillation. But it’s near impossible to be able to do that in a standard high school chemistry lab. I’ve used and loved both of these separation of mixtures labs in my high school chemistry classroom and honestly, they’re not even that hard to do! Plus there’s a bonus in here, you may not have even realized was a separation of mixtures lab!
To preface, I teach 6 methods for separation of mixtures: distillation, evaporation, chromatography, precipitation reaction, separatory funnel and filtration.
Chromatography Lab
For this separation of mixtures lab, you’ll only need a few materials. If you don’t have what you need, you may be able to borrow from the art teacher in your school. You’ll need water, beakers, pencils or stirring rods, tape, coffee filters (or nice funnel filters) and washable markers (they need to be water soluble).
Student Procedure
Fold the coffee filter into a pizza slice to find the center. Unfold it and pop a tiny hole in the center. Put a rolled up piece of filter paper through this hole to make an umbrella out of the two filter papers. On the round piece of filter paper dot the different colors of ink in a circle around your pinhole. The circle should have a half inch radius. Put about a half inch of water into the beaker. Drop the “umbrella handle” into the water and let the water climb up the paper to the ink dots. As the water reaches the ink, it will spread. The more polar the ink, the more it will spread on the paper. (It clings to the water molecules and hitches a ride. Nonpolar inks won’t move as much).
Sequence of the Chemistry Course
I’ve written about this a few times before, but this lab is the reason I teach the Matter Unit so late in the year. Students need to know the concept of molecule polarity before they can truly understand how chromatography works. In my first year of teaching, I hadn’t put this together yet, and taught Matter really early in the year. To my embarrassment, I had to teach chromatography as “separation of mixtures based on differences in “molecular magnetism.” And then had to go back and change my language. If you need help organizing your chemistry course, check out my free chemistry curriculum outline. It’ll give you the reasoning as to why I teach what I teach in order, plus give you a day-by-day look at my chemistry course.
Evaporation Lab: Separation of Sand and Salt
This one is a lot of fun, and was born out of a pretty common state final exam question. “How should a student separate a mixture of sand and salt?” Well instead of hypothetically doing it, I decided we should do it for real.
Teacher Set Up
All I need to do is measure out sand and salt samples. I like to do some variety in my samples so students can’t just copy from each other. Also, my tip is to make a few extra samples in case your student spills the sample before even making it to their lab bench… (been there, done that). Obviously, make a note of the amount of sand and salt in each sample. I wouldn’t go beyond 5 grams total, but 3 is a pretty good sweet spot.
Student Procedure
Students will measure the full mass of the sample, then drop it in water to dissolve the salt. It’s best to use as little water as possible, because they’ll need to evaporate the water in order to get the salt back. Then the wet sample goes into a funnel with filter paper. The sand will be caught but the salt water goes through. The sand should be washed with water to make sure all the salt becomes part of the filtrate. The sand can be put in an oven or set on a counter for a day or so to dry out. Just make sure it’s not near your air vents! This could also be warmed on a hot plate if you need it to be quicker!
The water will need to be evaporated from the salt. This requires a hot plate, or a Bunsen burner set up. I prefer the hot plate simply because the Bunsen burners can be a pain and need a LOT more monitoring than the hot plate. Use what you have, and what you feel comfortable with!
Students then measure how much sand, and how much salt they recovered. With that sum, they do a percent error calculation based on the original sample size. If you choose to give them the data, they can do a percent error for each component of the mixture as well.
Reasons I Love this Lab
I love this lab for a few reasons. One – It feels like a college level lab, with a lot of moving parts, data collection and calculations. But it’s certainly within the capacity of a high school student. Two – It showcases some great lab techniques: evaporation, funnel set up, washing a sample, drying a sample, and precise data collection. Three – the students feel like scientists when they do it. I’m a big fan of the fun labs, like making ice cream and slime, but those make kids feel like kids. Some labs are inherently “science-y” and this is one of them. Four – it’s a good real lab situation that I know my students will ace on that final exam because they’ve done the whole thing. Five – it’s great prep work for solubility rules, which based on my chemistry curriculum outline, is just a few weeks away.
BONUS: Dehydration of a Hydrated Salt
In this lab, my students are removing water from a hydrated salt. I prefer copper (II) sulfate pentahydrate because I love the color change! While I prefer to use this lab in my Stoichiometry Unit when learning about percent composition, it’s also a good separation of mixtures lab. If you weren’t able to get to it during your Stoichiometry Unit, now is a great time to use it! Check it out inside my full year lab manual.
You can find all of these separation of mixtures labs in my full year chemistry lab manual. You can also find them individually in my TPT store.
