Raven lays a printed list on the Bench. "Formulas," she says. "Each one is the recipe for one molecule or one unit of a substance."
Rocket reads the first one. "H2O. Two hydrogen, one oxygen. We built that with beans."
"Now read the baking soda one," Raven says. "What do you notice?"
Rocket squints. "Carbon, hydrogen, sodium, three oxygen. That is six atoms in one unit."
Nova hovers over the list. "Add up the atoms in the reactants," she says. "Then look for the same atoms in the products. Where did each one go?"
"Some went into the gas," Rocket says. "One carbon and two oxygens. The rest stayed in the bag."
Raven writes "same atoms, new homes" at the top of the page.
The table below lists molecular formulas from PubChem, a chemistry database run by the National Institutes of Health.
A formula is the recipe for one molecule or one unit. A small number after a symbol tells how many of that atom are in it.
When no number follows a symbol, there is just one of that atom.
Table salt is an extended structure rather than a molecule, so its formula gives the recipe for one repeating unit.
| Substance | Formula | Carbon | Hydrogen | Oxygen | Sodium | Chlorine | Atoms in one unit |
|---|---|---|---|---|---|---|---|
| Water | H2O | 0 | 2 | 1 | 0 | 0 | 3 |
| Carbon dioxide | CO2 | 1 | 0 | 2 | 0 | 0 | 3 |
| Acetic acid (in vinegar) | C2H4O2 | 2 | 4 | 2 | 0 | 0 | 8 |
| Sodium hydrogen carbonate (baking soda) | CHNaO3 | 1 | 1 | 3 | 1 | 0 | 6 |
| Sodium acetate | C2H3NaO2 | 2 | 3 | 2 | 1 | 0 | 8 |
| Table salt (sodium chloride) | NaCl | 0 | 0 | 0 | 1 | 1 | 2 |
| Table sugar (sucrose) | C12H22O11 | 12 | 22 | 11 | 0 | 0 | 45 |
Formulas can be written in more than one order. PubChem writes baking soda as CHNaO3 and some books write NaHCO3. Both describe the same six atoms.
Here is a counting model for the bag. One acetic acid unit and one baking soda unit hold 3 carbon, 5 hydrogen, 5 oxygen and 1 sodium atom.
Suppose the products are one carbon dioxide, one water and one sodium acetate. Count them: 3 carbon, 5 hydrogen, 5 oxygen, 1 sodium. The counts match.
The gas is carbon dioxide, and the other products stay in the liquid. A model that keeps every atom is a model that keeps the mass.
| Statement | True or false? |
|---|---|
| A formula is the recipe for one molecule or one unit. | ? |
| CHNaO3 and NaHCO3 describe different substances. | ? |
| Sucrose has 11 oxygen atoms in one molecule. | ? |
| Table salt is a single molecule rather than an extended structure. | ? |
Strong counting. Tomorrow you will find conservation of mass at work in a burning log and a rusting nail.