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Growing salt crystals

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Objective

Grow crystals from a salt solution

Materials

clean clear glass jar (jam jar or similar)

pot

200 ml water

salt (approx 1/8 to 1/4 cup)

food colouring (optional)

string or wool (the thicker and rougher the better)

pencil or ice cream stick

These crystals were made from iodised salt and grew overnight

Instructions

Difficulty: 

Time: approx 10 minutes plus growing time

Adult supervision: yes

Dangers: hot water

What type of salt is best?

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Iodised will work but the iodine and any anti-caking agents can affect crystal growth. Plain salt is better​ .

Epsom salts and alum are preferred.

  1. Put the water in the pot and heat until nearly boiling.

  2. Remove the pot from the heat.

  3. Add 1/8 cup of salt and stir. The salt should all dissolve.

  4. Keep stirring in more salt until no more will dissolve.

  5. If using food colouring, add a few drops.

  6. Carefully pour the water into the jar.  

  7. ​Tie the string or wool around the pencil or ice cream stick. Trim the string or wool so it is just above the bottom of the jar

  8. Leave the jar somewhere where it will not be bumped (so the crystals aren't disturbed).

  9. Depending on the type of salt used, you should see crystals forming overnight.

The Science

The solution you have made is super saturated. When water is heated, it is able to hold more salt than it would normally hold (compared to if the water is cold). As the water cools, the extra salt comes out of the solution and attaches itself to the string as a crystal.

Further investigation

  • Try different types of salt - are the crystals different? Did they take longer to grow?

  • Try different types of string or wool. Which one grew the most or the best crystals?

  • Can you identify the type (shape) of your crystals?

Further information

The following website has detailed information about growing salt crystals 

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What type of string should I use?

As mentioned above, the thicker and rougher the better as the fibres give the crystals more surface area to adhere to.

Thinner or smooth threads are not as successful to grow crystals on.

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Left: jute rope (4 mm thick); 8 ply knitting wool; string (2 mm thick) and sewing thread.

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Salt forms cubic crystals. You can see some are long.

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