Is Baking Soda and Vinegar an Exothermic Reaction?

The reaction between baking soda and vinegar is endothermic, meaning it absorbs heat from its surroundings rather than releasing it. If you touch the container after mixing the two, you’ll feel it get noticeably cooler. This surprises many people because the reaction looks so energetic, with all that fizzing and foaming, that the assumption is it must be throwing off heat. The reality is more interesting: this is a reaction that gets cold and still happens on its own, driven by something other than energy release.

Why the Mixture Gets Cold

When you pour vinegar onto baking soda, the acetic acid in the vinegar reacts with sodium bicarbonate. The products are sodium acetate, water, and carbon dioxide gas. The enthalpy change for this reaction is positive, which in chemistry terms means the reaction needs to pull in energy from its environment to proceed. That energy comes in the form of heat drawn from the liquid and the container, which is why the temperature drops.

In classroom settings, students consistently observe this cooling effect when the two substances are mixed together. The temperature drop is real, measurable, and consistent with the reaction’s positive enthalpy change.¹ If this were an exothermic reaction, you would feel warmth instead. That difference is one of the easiest ways to tell the two apart without any lab equipment beyond your hand.

Why It Still Happens Spontaneously

Here is where the reaction gets genuinely interesting: it absorbs heat, yet it runs on its own the instant the two ingredients touch. No flame, no spark, no external energy input needed. For many people, this seems contradictory. We tend to think of spontaneous reactions as things that release energy, like a fire burning or a match striking. But energy release is only one factor that determines whether a reaction will happen on its own.

The other factor is disorder. Chemists call it entropy, but the idea is intuitive: nature favors processes that spread things out and increase randomness. When baking soda reacts with vinegar, you start with solids and a liquid, and you end up with dissolved ions, water, and a gas flying off in every direction. That is a massive increase in disorder. The reaction produces enough of this entropy gain that it more than compensates for the energy penalty of absorbing heat.¹ The reaction goes to completion not because it is energetically favorable, but because the universe’s preference for disorder overwhelms its preference for lower energy.

This is actually an important concept that baking soda and vinegar illustrates better than almost any other kitchen experiment. Most spontaneous reactions people encounter in daily life are exothermic: burning wood, rusting iron, an ice pack activating. Baking soda and vinegar is a vivid counterexample showing that absorbing heat does not prevent a reaction from happening on its own.

Why the Fizzing Fools People

The main reason people assume this reaction is exothermic is the drama of it. The vigorous bubbling, the foam spilling over the sides of the container, and the hissing noise all look and sound like something releasing a lot of energy. In our everyday experience, big visible reactions tend to be the hot ones: think of volcanoes, fireworks, or baking soda combined with something that actually is exothermic. The visual intensity of the carbon dioxide escaping creates an impression of heat that simply is not there.

Another factor is that many school volcano demonstrations pair baking soda and vinegar with red food coloring and soap to create dramatic “lava” flows. Kids come away associating the reaction with eruptions and heat, even though the mixture they just created was getting colder the whole time. The emotional memory of the demonstration overrides the thermodynamic reality.

It also does not help that the word “reaction” itself carries connotations of explosive energy in everyday language. A chemical reaction that absorbs heat while looking vigorous challenges the casual mental model most people carry around. The truth is that the bubbling is just gas escaping from liquid, a purely physical process driven by the carbon dioxide becoming less soluble once it forms. It does not require or indicate heat release.

How Much Cooling Actually Happens

The temperature drop you can expect depends on the amounts used and the starting temperature, but in a typical kitchen experiment mixing a couple of tablespoons of baking soda with a cup of vinegar, you’ll see something in the range of a few degrees Celsius drop. It is not dramatic enough to make ice or frost, but it is clearly noticeable if you touch the container or dip a thermometer in.

For context, this puts it in the same general range as the cooling you feel when you dissolve certain salts in water, like ammonium nitrate in a cold pack. The mechanism is different, but the experience for your hand is similar: the container gets cool to the touch over a short period. The cooling effect is temporary because the liquid eventually returns to room temperature as it absorbs heat from the surrounding air.

In more controlled settings, the endothermic character of sodium bicarbonate reactions has practical value. Researchers studying acid spill remediation found that sodium bicarbonate actually lowers solution temperatures during neutralization, in sharp contrast to other common alkaline agents like calcium hydroxide, which release substantial heat and can cause dangerous secondary effects like vaporization and fume spread.² This cooling property makes sodium bicarbonate a safer choice for neutralizing concentrated acid spills in industrial settings, precisely because it does not dump heat into an already dangerous situation.

Breaking Down What Happens Step by Step

The overall reaction looks simple on paper: sodium bicarbonate plus acetic acid gives sodium acetate, water, and carbon dioxide. But the process actually involves a couple of steps happening in rapid succession. First, the acid donates a proton to the bicarbonate ion, forming carbonic acid. Carbonic acid is extremely unstable and almost immediately breaks apart into water and carbon dioxide gas. That decomposition is where most of the disorder increase comes from, because one dissolved molecule turns into a gas molecule that escapes into the atmosphere and a water molecule that joins the surrounding liquid.

Each step has its own energy profile. The acid-base neutralization part, where the proton transfers, is only mildly energetic on its own. The decomposition of carbonic acid into water and carbon dioxide is the step that really drives the entropy change. The combined effect of all these steps, taken together, is a net absorption of heat. No individual step releases enough energy to make the overall process exothermic.

The Carbon Dioxide and Where It Goes

The carbon dioxide produced is the whole reason this reaction is useful in baking, cleaning, and science demonstrations. In baking, the same basic chemistry is at work when baking powder reacts with moisture and heat in a batter. Baking powder typically contains sodium bicarbonate along with one or more dry acids. When mixed into a wet batter, the acid and bicarbonate react to produce carbon dioxide, which aerates the batter by expanding existing air bubbles and creating new ones.³ Some of the gas is released during mixing at room temperature, while the rest is released during baking as heat accelerates the reaction.

In a vinegar-and-baking-soda volcano, the carbon dioxide has nowhere useful to go, so it just foams out of the container. In a sealed bottle, that gas builds pressure, which is why some people use the reaction to launch bottle rockets. The gas itself is at room temperature or slightly below it when it forms, reinforcing that no heat is being generated. Compare this to, say, a combustion reaction, where the gases produced are extremely hot.

Common Mixups With Other Reactions

Part of the confusion comes from the fact that there are closely related reactions that are exothermic. For example, mixing a strong acid like hydrochloric acid with a strong base like sodium hydroxide is genuinely exothermic, and the solution heats up. People sometimes mentally group all acid-base reactions together and assume they all release heat. But the baking soda and vinegar reaction involves a weak acid and a weak base, and the carbonate decomposition step absorbs enough energy to flip the sign.

Another common confusion is with hand warmers or heating packs, which are sometimes described in similar “mixing two chemicals” terms. Those typically use iron oxidation or crystallization of supersaturated salt solutions, both of which release heat. The takeaway is that “two chemicals reacting” tells you nothing about whether heat is released or absorbed. You have to look at the specific chemicals and the specific bond changes involved.

Calcium hydroxide, mentioned earlier in the context of acid spill cleanup, is a useful comparison. When calcium hydroxide neutralizes an acid, the reaction is strongly exothermic and can generate enough heat to boil liquid, spread toxic fumes, and cause burns.² Sodium bicarbonate performing the same neutralization job while cooling the mixture down is a striking illustration of how much the choice of base matters, even when the end goal is the same.

Testing It Yourself

If you want to confirm the endothermic nature of this reaction at home, the setup is simple. Put a few tablespoons of white vinegar in a glass or plastic cup and note the temperature with a kitchen thermometer. Add a tablespoon of baking soda, stir gently, and watch the thermometer. You should see the reading drop within seconds. The effect is clearest if you use room-temperature vinegar and do the experiment quickly enough that the surrounding air does not warm the mixture back up before you take your reading.

For a slightly more dramatic version, use a thin-walled metal cup. Metal conducts heat well, so you’ll feel the cooling through the walls of the cup almost immediately after mixing. This is the same principle used in instant cold packs, though cold packs use ammonium nitrate dissolving in water rather than an acid-base reaction.

You can also do the inverse comparison to really drive the point home. Dissolve a small amount of sodium hydroxide (sold as lye for drain cleaning) in water in a separate container, and you will feel that container get warm. That is an exothermic dissolving process. The contrast between the two containers makes the difference between exothermic and endothermic vivid in a way that no textbook diagram can.

Why This Matters Beyond the Kitchen

The endothermic nature of the baking soda and vinegar reaction has real practical implications outside of science fair demonstrations. In fire extinguishers, some older designs used sodium bicarbonate solutions precisely because the compound absorbs heat while neutralizing acids and smothering flames with carbon dioxide. The cooling effect is a bonus on top of the gas production.

In medicine, sodium bicarbonate solutions are used intravenously to treat certain types of acidosis. The endothermic character is not the primary reason for its medical use, but the fact that it does not generate heat when neutralizing acid in the body is a meaningful safety advantage over stronger bases that could cause thermal injury to tissues.

Industrial cleaning applications also take advantage of baking soda’s gentle chemistry. When sodium bicarbonate is used as an abrasive cleaner or deodorizer, its reactions with mild acids in food residues or organic matter are endothermic and produce only carbon dioxide, water, and harmless salts. There is no risk of heat damage to surfaces, no toxic fumes, and no dangerous byproducts. This is why baking soda shows up in so many household cleaning tips: it is chemically active enough to be useful but thermodynamically gentle enough to be safe on almost any surface.

Endothermic Does Not Mean Weak

One last misconception worth addressing is the idea that because the reaction absorbs heat, it must be somehow less “real” or less powerful than an exothermic one. The baking soda and vinegar reaction goes to completion. If you mix stoichiometric amounts, meaning the right proportions so neither is left over, every bit of baking soda will react and every relevant molecule of acetic acid will be consumed.¹ The reaction is thorough and irreversible under normal conditions. The carbon dioxide leaves the system entirely, which pulls the equilibrium forward so there is nothing left to reverse.

The reaction’s vigor, its speed, its completeness, and the volume of gas it produces are all independent of whether it releases or absorbs heat. Plenty of exothermic reactions are sluggish and incomplete. Plenty of endothermic ones are fast and thorough. The direction of heat flow tells you about energy balance, not about how impressive or useful the reaction is. Baking soda and vinegar is proof of that: cold to the touch, yet powerful enough to inflate a balloon, launch a toy rocket, or leaven a loaf of bread.