Jelly seaweed

balls-1651081_1920Jelly seaweed 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, jelly seaweed is a nice way to engage them with how fun science can be.

Introduction activity

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 seaweed.

You can buy dried seaweed from asian supermarkets. It is easily rehydrated in warm water and allows children to explore the texture of seaweed if they’ve never picked up seaweed at the beach.

Background for teachers: 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.

What you’ll need

  • Sodium alginate powder (available online, often as a specialist food ingredient for ‘spherication’)
  • Calcium lactate (also available online)
  • Measuring jugs and a spoon for stirring
  • Plastic 3 ml pipettes (available online)
  • Small plastic pots or cups
  • Food colouring
  • Paper towel for spilled solutions

If your school/nursery is unable to afford the ingredients and materials for this activity, please contact us at the University of Birmingham and we will post them (UK only).


Add 1 g of alginate powder to 100 ml of cold water.  Sometimes the alginate can get very clumpy (if you live in a hard-water area). You can solve this problem by gradually adding the powder while vigorously stirring the water. Or you can mix the alginate powder with some sugar before adding it to the water. It helps to warm the water and it’s best to prepare this solution in advance to give the alginate time to dissolve. It will be gloopy a bit like runny honey. 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, dissolve 2g of the solid in 100 ml of water, and stir until it all dissolves.

Making jelly seaweed

This works best if you have a clear plastic container such as a plastic cup. Put it on a sheet of white paper so the colour shows up really well. Put 1-2 cm deep of calcium lactate in a container for each child.

Then give then a small pot of alginate solution – 1 between 2 is fine for this. Show them how to use the pipette to suck up some of the alginate solution and add it to the calcium lactate solution drop by drop. This will make little jelly beads.

To make jelly worms, squirt out the alginate solution very quickly into the calcium lactate.

Background for teachers: 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. The calciums ‘tie’ together the long ‘spaghetti-like’ alginate molecules to make a dense net – that’s what makes the ‘skin’ around the droplets and worms.

Children of all ages will love this experiment, but beware it can get very messy!

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.

Page author: Zoe Schnepp

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