Lost Labels Lab: An Inquiry-Based Solubility and Double Displacement Chemistry Lab

Quick Summary

The Lost Labels Lab is an inquiry-based unknown identification chemistry lab where students use solubility rules and double displacement reactions to match three unlabeled chemicals to their identities. This lab works well after teaching solubility and precipitation reactions and requires minimal instructions, encouraging authentic scientific reasoning.

What Is the Lost Labels Unknown Identification Lab?

Many chemistry students complete labs by following step-by-step instructions without ever making real decisions. This unknown identification lab flips that model. In the Lost Labels Lab, students must use solubility rules and double displacement reactions to identify three unlabeled chemicals—without instructions. The result? Students repeatedly say, “We actually did real chemistry.”

How the Inquiry-Based Lab Structure Works

The lab is very simple and has a simple goal.  Three labels have fallen off beakers of chemicals.  The students have to match them up.  That’s it.  Three labels, three chemicals.  You can download the free, brief handout here. 

I use potassium chloride, magnesium carbonate, and lead (II) nitrate.  I use this lab just after we have learned about solubility and how double displacement reactions occur.  In fact, I give a quiz over solubility during the first five minutes of the lab.  This gets their minds focused on solubility. (If you need a tool to help students predict solubility, check out our solubility rules download for free.)

The challenge of the lab is that there are only materials, and no instructions.  I have the lab groups get together and write a lab plan before they even put on lab coats or touch a chemical.  They have to check off their methods with me before they go.  I won’t, however, tell them if their methods will work, I only check for a complete thought.  I had students write this plan yesterday, “We will dissolve the unknowns in water and then mix them with the other chemicals to figure it out.”  While not necessarily incorrect, that obviously is a plan to fail and get confused.  They had to choose what chemicals they would react it with and what they expected to see.  Once they had their methods, they went to lab.

How Students Identify Magnesium Carbonate Using Solubility

Everyone thinks they know what solubility is until they have to determine if an unknown is soluble or not.  When magnesium carbonate is put into water it makes a cloud.  The students have a hard time discerning whether or not to call it aqueous.  I let them make the decision.  This is also clouded (pun intended) by the fact that they know one of the unknowns is insoluble in water and lead (II) nitrate takes quite a while to dissolve.  I don’t help them make the call on what they think is soluble.  They usually get it straightened out.  This allows them to identify magnesium carbonate.

Pro-tip: Come up with a quick way to remember unknowns.  I do a lot of unknown labs, and I want to know what they are working on.  My secret algorithm is alphabetical order by the cation name.  Please don’t tell the kids.

Using Double Displacement Reactions to Identify Unknowns

Once magnesium carbonate is known, they have to choose a double displacement reaction to separate the lead (II) nitrate from potassium chloride.  This is where I helped them out.  The three known chemicals that I gave them to work with were sodium chloride, sodium nitrate, and silver nitrate.  Two out of the three would actually work.  But sodium nitrate doesn’t help.  I tell them before the lab starts that there are tools and chemicals out that they will not use.  This raises their level of concern, because they don’t want to feel foolish in front of their classmates.  That pressure then leads to better planning.

The kids write out the reactions that they have chosen to complete and then do them.  The freedom that comes from giving them the choice makes them feel inspired.  Even if it is just a choice of three chemicals.  They like the fact that they solved it differently than the group next to them.

Confirmatory Tests and Student Reasoning

Some groups will just barely finish in time (90 minute block).  Others, who more quickly arrived at their plan, can finish much faster.  Kids will finish and bring their papers to me. 

“Mr. Koch, is this right?”

And I never tell them the answer.  I only ask them why they believe their answer.  Sometimes in the defense of their answer, they find issues with their logic.  I send them back to the lab to figure it out.  Other times, they are supremely confident.  This is when I allow them a confirmatory test.  I tell them that if they can dream up a chemical that would help them prove what is in beaker A, then they can do it.  Generally speaking, they choose simple and safe reactions that can be done.  If they choose something further out (Yesterday’s furthest out there choice was uranium nitrate) then I redirect them.  They are amazed that I have these chemicals that they think they just dreamt up.

“Mr. Koch, is copper (II) chloride a real thing?”

“Yes.”

“Do you have it?”

“Of course.

Their eyes got big, and they giggled with excitement.  I was like Willy Wonka.  They ran their test with copper (II) chloride, and it confirmed their experiment.  They were so excited! 

“We actually did real chemistry!”

Yes.  Yes, they did.

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