This lesson is appropriate for middle school science students in class sizes up to 20 students (more if you have additional support in the classroom).
Kids love to experiment. Give them some baking soda, vinegar, lemon juice, and various other sundries and they will mix and match and explore every possible outcome. Isn't that what experimentation is? Exploring all of the various outcomes so that a hypothesis can be supported by conclusive evidence? Well, maybe giving kids a pile of supplies and letting them have at it isn't quite the true type of experimentation we want to harness in the middle school science classroom. So how do we get kids excited about experimentation while still giving them the ins and outs of the scientific method without making it seem like a humdrum step-by-step procedure when science is anything but a humdrum step-by-step, paint-by-number practice? I have found that this lab meets many of those requirements while really sparking the scientist that is innately in each and every child.
The problem given to the students is simple... "which household liquid; vinegar, milk, lemon juice, makes the best invisible ink with exposure to heat?" Students are then given the task to go through a somewhat rigid set of steps to help them determine which of the three liquids works the best based upon qualitative observation. The lab requires basic supplies and tools and can be accomplished easily in 1-1/2 class periods (about 90 minutes).
White Distilled Vinegar
Milk (whole, 1%, 2%, etc...)
Lemon Juice Concentrate
Electric hotplates / burners
Test tube clamps or tongs
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Step 1: Get the Materials Together and Ready for the Lab
Warning - This lab does involve the use of hot plates in the classroom and paper being put near the hot plates (on low). I have completed this lab with sixth through seventh grade students for the past three years and have had no problems. This is after a very lengthy discussion on proper lab safety, classroom etiquette and behavior, and the almighty riot act. I only run three burners at a time in one set location and have water near by just in case a kid needs to dip their test sample (if it's smoking). I also monitor the entire area like a hawk.
You will need only a small amount of liquids for the lab since the students will be using such a small amount for their testing. I usually have the lemon juice and vinegar in the supply closet and bring in a cup or two of milk from home on the lab day. You will need some type of holding mechanism for the paper slips. I have a large number of old test tube clamps that I use and they work perfectly. As for the burners, ceramic burners are the best choice as they typically will not ignite paper quickly unlike coil burners which ignite paper very fast. I use a bunch of recycled cookie containers for the water dunking areas and for organizing the supplies for each table.
Step 2: Set Up Supplies
Instead of writing the names of each liquid on the cups I create a simple key that is written on the board where a letter represents each liquid. You will need four cups for each group. One for each liquid and one for the discard container. Pour in a small amount of liquid into each dixie cup with the letters marked on them and place them, along with three q-tips, into the plastic containers to be easily distributed to the class.
Step 3: Day #1 - Set Up and Preparation
You will need to make sure to print all of the lab materials I provided in the supplies section at the start of the instructable. Print a classroom set of the lab instruction sheets, so that each student can use one throughout both days of the lab. Print off one copy of the student sheet for each student you plan on completing the lab with and print off one set of test strips for each group.
Distribute the instruction sheet first and take the time to review it with the students, making sure to stress the importance of them following the directions carefully and completely during the entire lab. After you feel confident that your students know the expectations and are prepared for the lab you can distribute the test strip sheets to each group. Hand out a set of scissors to each group so that they can cut them apart and make sure that they are properly labeling the sheets based upon the directions in the lab instruction sheet. You can then distribute the lab supplies to each group so that they can write their group number on the test strips they have set up. This entire process will take approximately 35 minutes and combined with a lab safety lesson and review of the directions before starting it will take an entire class period. This is perfect since it allows the strips to dry overnight. Let them dry as much as possible before having the students paper clip their group's strips together for the next day.
Step 4: Day #2 - Experimentation!
The next day is where the lesson really shines. Before you get too excited about exposing the hidden messages you should hand out the student lab sheet to each student and review the requirements for the lab sheet. I use a standards based grading system with a grading scale I created (the speedometer) but you are welcome to change things up if it works better for you a different way. I typically will take the time to review exactly how I want the information to be detailed in the data table and I will do a short explanation of what it means to use the data table as evidence for the three focus questions on back.
Once the kids are all set with the lab sheet you can hand back their now-dried test strips and have them fill out the first section in the data table comparing the three samples to each other to determine which one is the most invisible at the start. I will typically put two groups at each burner station so as not to crowd them but if you wanted you could have specific students from each group come up to expose the invisible ink and maybe have three groups at a time... you'll have to feel it out. Review the directions of how to expose the message on the invisible ink test strips and make sure that the students understand that the first exposure is for ten seconds, the second for ten additional seconds, and the third for twenty additional seconds. You will definitely see the kids getting into the lab and getting excited about the invisible ink showing up with the heat. I always have kids that ask me if other liquids would work and I have encouraged them, with parent help, to try it at home with a light bulb, hair dryer, toaster oven, or other heat source (once again, with parent support). The lesson leaves multiple avenues to head towards when it comes to reviewing the lesson and discussing the results. I typically will use this presentation to help stir up more discussion and help us segue into our next experiment-based project. I hope you enjoy the lab and please post any comments or teacher notes if you use the lesson.
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