When sodium carbonate decomposes, it also leaves behind carbon dioxide, or CO2. You can find this out without knowing about carbonate reactions though. Chemical equations must be balanced, so whatever is on the left must also be on the right. There are 2 atoms of sodium, one atom of carbon, and three atoms of oxygon on the left. On the right, there is two atoms of sodium and one atom of oxygen. We are still missing one atom of carbon and one two atoms of oxygen, which, when combined, make a molecule of carbon dioxide.
