Age is Just a Number: Cross Linking Polymers

Kate McDaniel
Apr 25
4 min read

One of the core tenets of the OmniLearn method is that nearly all lab activities can be upleveled or down leveled to meet just about any age. When you are searching for activities to bring to your classroom don’t be afraid to widen the age bracket! With a little creativity and age appropriate language and safety practices, just about any experiment can be carried out from PreK to 12th grade!

In this series, we give examples on how we have taken high level science (often high school level labs) and brought them down for middle schoolers, elementary schoolers and even PreKers. Or the reverse! We take a simple activity and do a deep dive into the science. FREE resources included!

Cross Linking Polymers

The basic lab premise: Sodium alginate is a compound made from seaweed (algae!). When dissolved in water it makes a very thick jelly-like substance. While very viscous it is still technically a liquid. When you mix sodium alginate solution with a calcium chloride solution, the calcium will form links between the long chains of sodium alginate and you get a solid polymer. You can get round spheres by dropping drops of sodium alginate or you could get long worm-like shapes by pipetting a stream of alginate into the calcium chloride solution.

Safety

You can buy food grade calcium chloride and sodium alginate online here! And they are relatively inexpensive! Chefs often use these chemicals in molecular gastronomy to create spheres with liquid centers through a process called spherification. Since these chemicals are safe for ingestion, there are no safety concerns for little scientists accidentally tasting them. Of course, students should be held to regular lab safety rules and should not be allowed to consume their experiment!

High School

Students can explore the in-depth chemistry behind the spherification process, gaining a deeper understanding of the molecular interactions at play. They can accurately measure reagents using a graduated cylinder and a digital scale to prepare precise calcium chloride solutions, reinforcing key laboratory skills. You can cover advanced vocabulary, discussing concepts such as cross-linking polymers, metal ions, and the specific roles of sodium alginate and calcium chloride in gel formation.

High schoolers can take this lab step further by experimenting with other metal salts, like copper chloride. This lets them see how different metal ions (Copper vs. Calcium) affect the gel-forming process and gives them a better understanding of polymer chemistry. Since copper chloride is a bit more hazardous, it’s also a great opportunity for students to practice proper lab safety—wearing gloves, handling chemicals, and disposing of materials the right way.

Middle School

You can turn this reaction into a full scientific inquiry lab, giving students the opportunity to think critically about variables and controls. In this version, students are presented with a sodium alginate solution and three different clear liquids—calcium chloride, sodium chloride, and water. Their goal is to determine which one will cause the alginate to form a solid, leading them to think through the chemical interactions at play.

As part of the experimental design process, students can identify the independent variable (the type of liquid used), the dependent variable (whether or not gelation occurs), and the control variables (such as the volume of each solution, concentration, and temperature). Since accuracy is a key component of good experimental design, students should measure their solutions using a graduated cylinder with ±1-3 mL precision. While the exact volume of calcium chloride isn’t critical for the reaction to occur, it’s a great opportunity to reinforce the importance of consistency—ensuring that all three test solutions have the same volume. This helps eliminate confounding factors and teaches students why controlled, repeatable methods are essential in scientific research. Plus, it encourages them to think like real scientists while making predictions, recording observations, and analyzing their results.

Upper Elementary

At an upper elementary level (Grades 3-5), you can actually use this reaction to model frog eggs by adding poppy seeds to the sodium alginate. The poppy seeds represent the “yolk” of the frog egg and the alginate ball around it is like the egg white. It is important for students to let the alginate/poppy seed mixture drip slowly to get the nice spheres of frog eggs. In this case, we are using a different set of vocabulary like life cycle, frog eggs, tadpoles, froglet, and frog. Students can measure the premade calcium chloride solution using a graduated cylinder. Because the volume of calcium chloride does not matter, accuracy is not critical. Students should be able to measure to the specified volume within 5mL.

Lower Elementary

At a lower elementary school level (Grades K-2), you can cover states of matter. Students will still measure the calcium chloride using a graduated cylinder. But again, accuracy is not important. It is a great opportunity to give students exposure to the scientific equipment. This time they will have free reign to use a pipette to add the (colored) sodium alginate to their cups. They can explore what happens when they squeeze quickly vs slowly vs letting it drip out in drops. We would use vocabulary like solid, liquid, gas, volume, shape, size comparisons, alginate, pipette, and algae.

PreK

At a PreK level, it is largely the same as lower elementary. These steps will take longer and we will focus largely on the fine motor skills involved in carrying out the steps. While we would make a big deal about not putting anything in their mouths, the reagents are all food grade and non toxic so in the event of any “accidents”, students are still safe.