Sterling silver lab: Force students to think at the start of AP Chem

The first lab of AP Chemistry is special. New surroundings. New expectations. New challenges. Over the years, I have changed how I conduct the classic lab, “What percent silver is in the sterling alloy?” I love how it gets their minds working quickly. The logic behind the lab can be applied at any level. Here is how I do it.

(Here is an AI-generated picture on how to do it. Ha Ha Ha! Don't worry. We are safe from AI for a few more minutes.)

Preparing for the Lab

I spend some time the day before the block to prepare the students. Sterling is an alloy made of some percentage of copper and silver. Their job in the lab is to determine the percentage of silver in the necklace piece they receive.

Pro Tip: Search the floors of the gym and classroom. You can often find unclaimed sterling necklaces lying around. I tell the kids in my classes that if they lose their jewelry around me, they will get it back… unless it’s sterling. Then it belongs to science. I cut the necklace into pieces weighing approximately 0.35 g.

We discuss how to ionize the metals using nitric acid. Since it is early in the year, I spend a little time talking about aqueous ions and reminding them how to write net ionic equations.

Starting the Methods

I provide the students with the methods for the lab up to the point where they have dissolved the necklace in nitric acid. Then I stop. There is nothing after that. It is the students’ job to create the rest of the method. They meet in their groups and often stare at each other sheepishly, hoping their partner has the method figured out. They don’t. And so the discussions begin. This is where the real magic of this lab happens. They get to work through wrong processes. I allow them to ask me questions, but I am pretty coy.

They frequently investigate a single displacement reaction. This isn’t an effective strategy since copper and silver are stacked together on the activity series. But I don’t tell them that. I let them ask me for an activity series, use it, and then get disappointed when they realize it won’t work.

Then they turn to double displacement. To see clearly what to add, they need to use solubility rules. Luckily for them, I already knew they would eventually ask for them, so I have them printed out.

Pro Tip: Teach the use of solubility rules in grade-level chemistry. Yeah, yeah, I know. AP only requires minimal memorization of them. I still think they are valuable in helping students understand why double displacement reactions work. I have my handout linked here.

The students then start to propose chemicals to use in order to precipitate the silver and leave the copper in solution. This feels like the Big Leagues for them. They can use whatever chemical they want (as long as I approve). Here’s the sneaky teacher trick: through their conversations, you can almost always gently guide them to using sodium chloride to accomplish this. They honestly feel great when they discover its use. This year, every single lab group ultimately ended up using sodium chloride. Some will try to use sodium iodide (I tell them it’s too expensive). Others will want to use bromides (which I have to save for a later lab). They come back to sodium chloride or potassium chloride. And honestly, they don’t care. They are just excited.

Writing Their Own Methods

Now that the students have picked their idea, they have to write the methods. I make them think about what states belong on the left side of a double displacement reaction (aqueous). What does that mean for their methods? I make them consider how much sodium chloride to add. Do they want the silver or the salt to be the limiting reagent? They have to do some stoichiometry to ensure they are adding the correct amount. Their methods get written into their digital notebook. Once that is complete, it is time for lab day.

Pro Tip: Make students highlight anywhere in their methods that they are taking a measurement that must be recorded. This cuts down on “I didn’t weigh my filter paper; can I just weigh another one?” idiocy.

Lab Day

Here are the methods that I provide for the students. This is also as far as I help them in the lab since they are using concentrated nitric acid.

Methods

1. Clean a 150 mL beaker with deionized water.

2. Place 20.0 mL of 6 M nitric acid in the beaker. While handling concentrated nitric acid, you must wear gloves.

3. The initial reaction must take place in the fume hood, as both reactions emit a toxic gas (NO2).

4. Weigh and record the mass of your alloy.

5. Place the alloy in the acid until it is completely dissolved. Cover the beaker with a clean watch glass.

6. Once the alloy is dissolved, take the solution, place it in a volumetric flask, and dilute it to 100 mL with deionized water.

   
   

From here on out, the students will follow their own methods (which I already approved the day before). At the end of the day, the students will have their silver chloride precipitate drying on a filter paper in the fume hood.

The Final Results

The next day, the students will weigh their dried silver chloride. They will use stoichiometry to calculate the mass of silver in the necklace and, therefore, the percent silver. I make many of my AP labs competitive. Only the top teams can receive an A on the results score of the lab. The worst team will get a B. I have the students write their answers on the whiteboard. Once all the answers are up, I dramatically reveal the right answer: 92.5% silver. There is much celebrating and also gnashing of teeth.

Ultimately, the students get to compete in a lab that connects to their real world. The best part is that it was their decision. They figured it out. They chose the chemicals. They know how to use stoichiometry. They leave the lab feeling like they are back in control of their chemical futures. That is a good way to start the year.

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