Note from ScienceLine moderator:

Before reading the following answers, please consider that answers #1 to #5 respond to readers level K-12, however, part of answer #2 and answers #6 to #10 have a high level of technical language which should be for more advanced students.

Baking soda and vinegar react with each other because of an acid-base reaction. Baking soda is a bicarbonate (NaHCO₃) and vinegar is an acetic acid (HCH₃COO). One of the products this reaction creates is carbon dioxide.

You can make your own vinegar and baking soda bottle rocket! Take a piece of paper and put some baking soda on it. Wrap it up like a burrito and roll the ends tight. Pour some vinegar into an empty bottle. Put your baking soda paper roll in the bottle, cork the bottle, shake it up and quickly place it on the ground. See how high your rocket will go!

Baking soda and vinegar react with each other because they exchange atoms. In this case, they are exchanging a proton, which is a subatomic particle that makes atoms. You can also think of a proton as a Hydrogen atom that is missing an electron. In this reaction, baking soda acts as a base, and takes a proton from vinegar, which is an acid. The reaction releases gas because when the baking soda receives the proton, it transforms into water and carbon dioxide.

Note from a different scientist (AS):

This answer above and answer 5 below give generally correct descriptions of the reaction, but arguably suffer from being insufficiently explicit. The reaction of baking soda (sodium bicarbonate) and vinegar (acetic acid) is as follows. Note that all of this occurs in an aqueous (i.e., water-based) solution, with the water typically provided by the vinegar as it is ~5% acetic acid and 95% water.

1a. The sodium bicarbonate dissociates into a sodium cation (Na+) and a bicarbonate anion (HCO_3-). The charges arise because single valence electron of Na stays with the bicarbonate.

1b. The acetic acid dissociates into a hydrogen cation (H+) and an acetate anion (CH₃COO-). Again, the charges result from the single valence electron of H staying with the acetate.

2a. The bicarbonate anion (HCO_3-) bonds with a hydrogen cation (H+), forming carbonic acid (H₂CO₃). Note that the bond is formed by 2 electrons of the HCO_3- filling the (empty) valence shell of H+; the H+ ion does not contribute any electrons to this bond.

2b. The acetate anion (CH₃COO-) and sodium cation (Na+) remain as charged species surrounded by water molecules. (Sodium acetate is soluble in water, so no solid compound is formed, and neither substance will decompose in plain water.)

3. The carbonic acid (H₂CO₃) decomposes into water (H₂O) and carbon dioxide (CO₂).

Before the reaction, the H atoms and the associated electrons are part of acetic acid molecules; after the reaction the H protons are part of water molecules, while the electrons originally with the H atom are still with the acetate ion. Although not a direct exchange between acetic acid and (sodium) bicarbonate, the hydrogen nucleus has been transferred to a new compound (and the electrons have not). This is the Bronsted definition of acid-base reactions.

As described above and in the existing responses, the hydrogen nucleus is associated with different anions before and after the reaction, but it remains a hydrogen.

The main problem with the current responses (here above, and #5) is that they answer the question "What is the reaction between baking soda and vinegar?" rather than "Why is there a reaction between baking soda and vinegar?" Admittedly the former could be the intended question (and the answers therefore appropriate), but the answer to the latter is addressed in only the briefest manner in Answer 5. The basic idea is that the rearrangement of electrons/bonds produces a more stable configuration; the reaction occurs because it leads to a reduction in energy of the system. This is the realm of Chemical Thermodynamics. [These (long) videos - might be a good introduction.]

Baking soda is sodium bicarbonate: each molecule of baking soda contains a sodium atom, a hydrogen atom, an oxygen atom, and a carbon dioxide molecule.

Vinegar contains acetic acid, each molecule of which contains a hydrogen atom, and an acetate ion.

When combined, the hydrogen atom in the acetic acid meets up with the hydrogen and oxygen atoms in the baking soda to form a molecule of water, while the acetate ion grabs onto the sodium atom and forms a salt, sodium acetate. The carbon dioxide molecule, free of its other chemical bonds, can now escape, and bubbles forth as a gas.

They react because baking soda is a base and vinegar is an acid dissolved in water. When you mix an acid and a base in a solvent, which in this case is the water, usually the acid transfers hydrogen to the base to form a new compound. In this case, the new compound is called carbonic acid, which then decomposes to water and carbon dioxide gas. (A more precise definition of acids and bases states that a base is a compound that wants to donate electrons, and an acid is a compound that wants to accept electrons.)

Baking soda and vinegar react with one another because they both have a lot of energy that they don't want and they can help each other get rid of it! You might think this explanation is too simple, but it's true to what's happening.

Before we go into more detail, let's be clear about our materials. The chemical name for baking soda is sodium bicarbonate. Its chemical formula is NaHCO₃, meaning it's made of one sodium atom, one hydrogen atom, one carbon atom, and three oxygen atoms. Vinegar is a mixture of acetic acid and water. Dilute acetic acid is the chemical name for vinegar, and its chemical formula is CH₃COOH. From here on out I will write the chemical formulas in parentheses.

Baking soda is a base, and vinegar is an acid. An acid is a chemical that wants to get rid of a proton, or a positively charged hydrogen atom. A base is a chemical that wants a proton. When you mix an acid with a base exciting things can happen because the acid is ready to give away its proton and the base is right there to receive it!

Water is often added to acids and bases to tone down the intensity of this exchange. Water also acts as host in which the acid and base can break apart and react. In water, baking soda breaks apart into a positively-charged sodium ion (Na+) and a negatively charged bicarbonate ion (HCO_3-). An ion is a charged atom or molecule. Acetic acid doesn't break apart on its own in water as much as sodium bicarbonate; it's mostly diluted so it's not as strong.

When we mix baking soda and acetic acid in water together, acetic acid gives its proton to the broken-apart baking soda and together they form sodium acetate (CH₃COONa), water (H₂O), and carbon dioxide (CO₂). These products are created quickly, and the carbon dioxide comes out as a gas, so the whole event is spectacular as you've seen!

By reacting with each other, the acidic acetic acid and the basic sodium bicarbonate give up a lot of their energy and create things that have a lower energy relative to each other. The universe favors things at their lowest energy, and so we see a lot of exciting reactions involving acids and bases.

Concerning the answers #2 and #5 above, the important thing to note is that a hydrogen atom is just a proton and an electron with no neutrons. (Ok, actually in about 0.01% of hydrogen atoms there is a neutron in the nucleus in addition to the proton. We call those atoms deuterium, or heavy hydrogen). Since hydrogen is just a proton and an electron, a H+ ion is the same things as a proton, and chemists use the terms H+ and proton interchangeably. This is completely correct, if a little jargony. For examples of this language, you can search "proton transfer" online.

However, for the purposes of describing baking the soda and vinegar to a broad audience, there is no need to talk about protons as subatomic particles in the way answer 2 did. What they probably should have done is replace the word proton with "hydrogen ion," and not mentioned the part about a proton being a subatomic particle at all. However, rest assured that all of the Scienceline answers to this question are talking about hydrogen ions moving from one molecule to another, and none of them are talking about the proton from the hydrogen combining with another nucleus, which would be a nuclear reaction.

Concerning the answers #2 and #5 above, chemists and even chemical engineers have a habit of referring to a hydrogen atom with a +1 charge as a "proton" in these types of reactions. For better or worse the terminology most people use is "proton." I think to avoid confusion it would be easier just to call it a "positively charged hydrogen" or "a hydrogen with one less electron."

Concerning the answers #2 and #5 above, the hydrogen atom is indeed just a proton and an electron. The atom with a proton and a neutron in the nucleus is still hydrogen, but as a heavy hydrogen isotope has a special name - deuterium. So a proton is indeed (or can act as) a chemical entity, the cationic form of a hydrogen atom.

For answers #2 and #5 above, a proton (H+) is easily transferred between many molecules. Protons come off acidic molecules and join with a water molecule to form H₃O+.

Protons are also the positively charged particles in the nucleus, but the protons in hydrogen atoms (H) on molecules will easily leave if the atom next to the hydrogen atom attracts electrons. Oxygen atoms (O) in molecules often attract electrons, becoming O-. Carbon atoms (C) don't usually attract electrons, so C-H (carbon-hydrogen) bonds don't lose their protons.

In reference to answers #2 and #5 above, the bench top chemical reactions all involved transfer of an electron from one atom to another or the sharing of an electron between two atoms when in the initial state the shared electron was NOT being shared. Nuclear reactions are a different kettle of fish and such nuclear reactions are not operating at the low energies of bench top chemistry. One needs to go to millions or tens of millions of degrees in order to initiate reactions, generally speaking. Nuclear reactions can be triggered by firing neutrons and other particles at the nucleus of atoms… but this requires very high speeds kinetic energies and such kinetic energies are far, far above the typical kinetic energy of colliding atoms under normal low temperature/pressure conditions.