Carbonate precipitation

Mix calcium chloride and baking soda solutions to make a precipitate of calcium carbonate - the chemical that makes up shells and some rocks.
Science content
Biology: Features, Adaptations of Living Things (K, 1, 3, 7)
Earth/Space: Rock cycle, Earth Materials, Natural resources (5)
  • baking soda solution: 1 teaspoon baking soda (sodium bicarbonate) in 40ml water
  • calcium chloride solution: 1 teaspoon calcium chloride in 40 ml water (pure calcium chloride needed - purchase food grade; I found sidewalk de-icer was not pure enough to make a clear solution)
  • squeeze bottles for the above two solutions, one per table grou.. Label as 'carbonate' and 'calcium')
  • clear dishes e.g. small petri dishes
  • black paper to place under the dishes (to highlight the white precipitate forming
  • white shells (e.g. from the beach)
  • white chalk or other white calcium carbonate rock e.g. limestone
  • optional: molecule models to show the chemical reaction: one CO2 and one H2O molecule per student pair, or group

The carbon dioxide in the air dissolves in the ocean, and becomes part of the shells of ocean animals and is also made into rock.
This activity shows the chemistry of how that happens.

First model the chemical reaction with molecule models, if available:
Ask students to combine their carbon dioxide molecule and their water molecule to make one molecule. Tell them that there are several possibilities, but the molecule that forms in the ocean has a double bond and is symmetrical. They should eventually arrive at H2CO3 (carbonic acid).
In the ocean, this molecule loses its H atoms to make carbonate, which animals use to make their shells.
(Some rocks are also made by this same chemical reaction.)

Show students an oyster shell, or other white shell. This shell is made from calcium carbonate.

Tell students they will do some chemistry to make their own shell material.
Ask students to place their clear dish on top of the black paper, then squeeze a little 'carbonate' and a little 'calcium' into the dish - a white precipitate forms.
They made a new chemical, calcium carbonate. A chemical reaction happened.
Animals in the ocean do the same chemical reaction to form their shells from ocean molecules.


Molecules are the funnest part of this. Mixing calcium and carbonate looks cool for a quick wow, but then you are done (and a lot of prep ;)

Grades taught
Gr 2
Gr 3
Gr 5
Gr 6