Science Experiments

Crystals on a piece of wool

Sun, 27th Apr 2008

Listen Now    Download as mp3 Part 1,2 from the show The Sparkling Science of Gemstones

What you Need

Bicarbonate of soda

Some washing soda or bicarbonate of soda


some wool or string, natural fibres are probably best.

2 Mugs

A couple of mugs

A small saucepan

A small  saucepan

2 forks

A couple of forks

What to do

If you have some washing soda

Dissolve about 1/3 mug of  washing soda in 1 2/3 mugs of water, you may have to heat it in the saucepan to get it all to dissolve.

if you are using bicarbonate of soda add about 1/3 of a mug full to 1 2/3 mugs of water and then heat it until it starts to boil. At 80-90C it will start to fizz as carbon dioxide is driven off converting sodium bicarbonate (sodium hydrogen carbonate) into sodium carbonate - washing soda. Keep heating until it starts to boil properly - you will be able to tell because the bubbles will get a lot bigger.

While you are doing this set up two mugs on the plate or tray, with a piece of wool or string hanging loosely between them. You can secure the string  by tieing a loop in each end and catching the loop on a fork.

Now fill the two mugs so they are nearly full of washing soda mixture, and make sure the string is hanging a bit below the level of the water.

Put the whole thing somewhere warm such as the airing cupboard and wait for 2-3 days.


Be very careful when pouring the boiling liquids as they are very hot!

Don't use an aluminium pan as it will dissolve in the washing soda.

You can try the same experiment with different concentrations of washing soda or using other things that will dissolve well in your kitchen.

What may happen

You should find that small crystals grow on the string. If you wait long enough you may find that you start to form a stalagtite on the dip of the string. If you


Crystals growing on the wool

Crystals will form on the bits of wool.

If you make a very strong solution you may also find that the crystals form inside the liquid on all available surfaces.



Why does it happen?

The sodium carbonate you buy is in fact millions of tiny crystals. A crystal is a repeating array of billions of atoms or molecules in a regular pattern. There can be more than one type of molecule in a single crystal. In sodium carbonate crystals there are Sodium ions, Carbonate ions and water molecules.


A crystal

A crystal dissolving

A crystal is a repeating array of atoms or molecules. There can be more than one type of molecule but it is easier to think of only one.

When a soluble crystal is put in water the water molecules tend to knock molecules off the crystal, dissolving it.

When you put the crystals in water the water molecules will bash into the sides of the crystal and tend to knock molecules off, these will wander off amongst the water and then eventually rejoin the crystal. The warmer the water the more violent the battering by the water molecules so it is harder for the crystals to reform and the water can hold more molecules in solution.

What happened in the experiment?

The wool attracts water well and because it has many closely spaced fibres surface tension draws the water up the wool, it can then run down the other side. In fact it acts as a syphon pulling water and the dissolved sodium carbonate up the short side and along the wool.

This means you have a string permanently soaked in sodium carbonate solution. The string has a large surface area so lots of water evaporates leaving behind the sodium carbonate, which will then form crystals.



Water syphoning

The water is strongly attracted to the wool fibres, so they act a bit like a pipe, so the water can syphon over the lip of the mug and into the centre. 

The water evaporates

Because the wool has a high surface area lots of water evaporates from it. This means that the  liquid can't hold all the stuff that is dissolved in it so some comes out of solution forming new crystals.



Is this used in the real world?

Evaporating solutions is how most of the crystals you find in everyday life are made, such as salt or sugar. Sea salt is manufactured in huge shallow ponds in warm countries which sea water is allowed into and is then allowed to evaporate by the sun leaving the salt behind to form crystals that you put on your chips.


A salt evaporation pond for the manufacture of sea salt. Phgcom @ wikipedia


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