Seaweed Jelly

balls-1651081_1920Seaweed Jelly is fun for all age groups! It can work as a demonstration but is best as an interactive activity. It’s difficult not to be captivated by the amazing way the seaweed extract forms a jelly!

For very young children, seaweed jelly is a nice way to engage them with how fun science can be. For older children, you can start to introduce various scientific concepts such as polymers or electrostatic interactions. This version of the experiment is aimed at younger children so if you want a bit more depth, click here (link will be added soon)!


Have you ever picked up a piece of seaweed at the seaside? Or touched some seaweed in the sea? If you have, you might have noticed it feels really rubbery. The reason for this is some really special chemistry that is going on in the cells and structures of the seaweed.

Inside many types of seaweed is a really long molecule called alginate. Alginate is a type of molecule called a polysaccharide. It’s similar to things like starch, which is found in potatoes and pasta. To make our seaweed jelly, we’ll use a solution of alginate.


Add 1 g of alginate powder to 100 ml of cold water. You can buy food-grade alginate from a few websites, such as sous chef. Sometimes the alginate can get very clumpy (if you live in a hard-water area). You can solve this problem by gradually sieving in the powder while vigorously stirring the water. Or you can mix the alginate with some sugar before adding it to the water. Once you’ve mixed it thoroughly, you can add some food colouring to make your seaweed jelly look more exciting.

To make up a calcium lactate solution (which can also be purchased from a food supplier such as sous chef), dissolve 2g of the powder in 100 mL of water, and stir until it all dissolves.

Making jelly seaweed

This works best if you have a clear glass or plastic container and a plastic pipette. Put some of the calcium lactate in the container. Then suck up some of the alginate solution into the pipette. If you want to make little spheres, add the alginate solution into the calcium lactate drop by drop. To make jelly worms, squirt out the alginate solution very quickly.

When you add the alginate to the calcium lactate, the alginate takes calcium ions from the solution. Alginate has a negative charge and calcium ions have a positive charge and so they bind together very strongly. It’s this alginate-calcium link that makes the alginate go rubbery. With the droplets and worms, you can squeeze them and they will pop! The unreacted alginate inside is released.


Children of all ages will love this experiment, but beware it can get very messy! It’s valuable for very young children to show them that science is fun. It also introduces them to scientific equipment. You can even buy mini lab coats! For older children (Key Stage 2), you can introduce the concept of solutions and even let them help to make the solutions for the jelly. Concepts such as molecules and ions aren’t introduced in the curriculum until Key Stage 3.


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