Warning: This activity requires cutting with scissors. Younger readers should get an adult's assistance. It also uses the toxic chemicals ammonia and bluing agent. Be careful not to get these, either in liquid or crystal form, into your eyes or your mouth. Read the warnings on the containers too. Wear old clothes which are allowed to get stains on them.
Use an old bowl and old measuring cups which will not be used for cooking.
You will need
Cardboard tube
Shallow bowl
Large jar
Old measuring cups
Spoon
Food dye (three or four colours of your choice)
Salt
Bluing agent (available at most supermarkets)
Ammonia (with supermarket cleaning products)
Scissors
Water
What to do
Cut one end of the cardboard tube into a shape resembling branching coral. Don't be afraid to be creative, and bend the card however you like.
Make five or six cuts lengthwise up the opposite end of the tube, each around 5cm long and spaced evenly around the rim. Fold out the cardboard tabs to form a base, allowing the tube to stand.
Place the tube upright into the shallow bowl. Dribble different colours of food dye over the branches in patterns. Take care not to get dye onto anything you don't want stained.
Using a measuring cup, mix up your salt solution in a large jar using the following amounts:
90ml of salt
90ml of bluing agent
15ml of ammonia
60ml of hot tap water
Stir the solution until as much of the salt as possible has dissolved. Pour the solution into the bowl with your cardboard tube, covering its base entirely to a depth of at least 2cm.
Place the bowl somewhere warm with lots of ventilation. Out in the sun is perfect.
After a day or two, make up another batch of the salt solution and top up the liquid in the bowl.
Over the next week, watch your coral bloom! If it is growing is little too slowly for you, you can use a cup to pour some of the salt solution over your coral every now and then to speed things up.
What colours can you see? Is any of the salt white?
What's happening?
In this activity, the solution moved up through the cardboard tube due to capillary action – the fibres of the cardboard soaked up the solution and drew it out into its branches. The ammonia and water then evaporated from the surface, leaving behind the crystals coloured with the food dye.
Table salt, or sodium chloride (chemical structure NaCl), dissolves into its component ions of sodium (Na+) and chloride (Cl-) when mixed in water. There should be more than enough salt in the mixture for the solution to be 'saturated', which means no more salt will dissolve into the water. As the ammonia and water evaporate, the sodium and the chloride recombine into solid crystals.
Bluing agent is actually used to make white fabrics look brighter. It is made up of ferric (iron) hexacyanoferrate particles suspended in water. This chemical breaks apart in ammonia, only to recombine with the sodium chloride crystals when the water and ammonia evaporates. This adds a nice rusty colour to the crystals.
Applications
The Burketown Bore
Real coral is made from calcium carbonate, rather than the cardboard and salt you used in this activity. To create coral, tiny animals called polyps remove certain minerals from sea water and excrete them around themselves, creating small protective dwellings primarily made of calcium carbonate. Over millions of years, as the coral structures pile up, layers of sedimentary limestone rock form.
In Burketown, North Queensland , there is another perfect example of minerals dissolved in water reforming into solid structures again. The Burketown Artesian Bore was drilled in 1897, and has been flowing ever since. Today a large mound of minerals has formed as the heated water has evaporated over the past century, creating an impressive structure around the old bore hole.
The stalactites and stalagmites often found in caves also result from dissolved minerals forming solids as the water evaporates. They can take anywhere from months to centuries to form into beautiful, delicate structures.
