Where on your tongue do you feel the fizz of carbonation?
― Your tongue senses carbonation as sourness, not bubbles
Drink a fizzy soda and your tongue tingles. Most people think it's the feel of bubbles popping. But studies show that even when the bubbles are suppressed, the sting barely changes. What your tongue actually senses is a very faint "acid" formed when dissolved carbon dioxide transforms on its surface.
On a hot day, you take a sip of chilled soda water. Your tongue prickles, and a crisp sensation runs down your throat.
Now try drinking the same soda water after leaving it open for half a day. Bubbles are still rising a little, but that sting is almost gone. The warmed-up soda somehow tastes "flat," too.
So what exactly on your tongue is picking up that prickle? The answer turns out to be the mechanism behind "sourness."
The real cause: two "acid sensors"
The surface of your tongue's sour-taste cells carries an enzyme that rapidly reacts carbon dioxide with water. The cell senses the acid produced and sends a "fizz" taste signal to the brain.
Your mouth is also laced with nerves that carry pain and heat signals. These nerves also respond to acid. The prickly "itchy-sting" is thought to come from here.
In other words, the sting of carbonation is a slightly unusual sensation sitting between taste and pain. And bubbles are not the main actor. Let's look at this step by step.
Even with the bubbles gone, the sting remained
The idea that "the sting is the feel of bubbles popping" is a very natural one. In 2013, American researchers set out to test this directly.
They used a chamber that could be pressurized. At high pressure, carbon dioxide stays dissolved in water and can't form bubbles. Participants sipped soda water inside this chamber, and the strength of the sting was compared against normal air pressure.
The result: even with almost no bubbles forming, the strength of the sting barely changed. Bubbles only slightly altered the sensation — most of the sting came from the dissolved carbon dioxide itself.
So what is dissolved carbon dioxide actually doing on your tongue? Take a look at Figure 1.
Sour-taste cells turned out to be in charge of carbonation
In 2009, an experiment using mice confirmed this mechanism. Researchers disabled each type of taste cell in turn and checked the response to soda water.
Disabling the sweet, umami, or bitter cells left the response to carbonation intact. But when the "sour-taste" cells were disabled, the response to carbonation nearly disappeared.
The key was an enzyme on the surface of those cells. Carbon dioxide reacts with water to form acid, but on its own this reaction is slow. The enzyme speeds it up dramatically. In the few seconds a drink sits in the mouth, acid is generated right at the tongue's surface.
Even so, soda water doesn't taste "sour" the way lemon juice does. The acid is very dilute across the whole mouth, and signals from pain nerves overlap with it. The brain is thought to combine these into something distinct from "sour" — the sensation of "fizz."
A drug used to prevent altitude sickness (acetazolamide) weakens this enzyme's action. Climbers who take this drug have long reported that "carbonated drinks taste flat." This is considered evidence, even in the human body, that the enzyme is the key to sensing carbonation.
Why does cold soda sting more?
The source of the sting is the amount of dissolved carbon dioxide. And the colder the water, the more gas it can hold dissolved. Try changing the temperature with the slider in Figure 2.
Cold soda can hold onto more gas while it's in your mouth. That means more acid forms on your tongue, and the sting is stronger. Warm soda loses its gas quickly, leaving too little raw material for acid.
The same reason explains why soda left open goes flat. The air around us holds almost no carbon dioxide, so the dissolved gas gradually escapes into it. Even if bubbles are still visible, once the dissolved amount itself drops, the acid sensors on your tongue stop responding.
Half of the sting from carbonation comes from pain nerves. Chili heat and the sharp bite of wasabi work the same way, using pain nerves too. Why people actively enjoy a bit of pain is still one of the questions we don't fully understand.
Summary
The fizzy sting of carbonation isn't the feel of bubbles popping. Dissolved carbon dioxide turns into acid right on your tongue, via an enzyme. That acid is sensed by both sour-taste cells and pain nerves. Cold soda stings more, and soda left open goes flat, because the amount of dissolved gas is itself the raw material for the sting.
Your tongue doesn't taste the sound of bubbles —
it tastes freshly made acid.
You can read about how most of taste is actually decided by your nose in "Why does food lose its taste when you have a cold?", and about molecules directly pressing on skin sensors in "Why does mint feel cool when it isn't cold?". For a story about dissolved gas suddenly turning to bubbles, see "Why do volcanoes erupt?" too.
- Split the same soda water into two cups. Put one in the fridge and leave the other at room temperature for 30 minutes. Taste both and compare how strong the sting is.
- Stir the cold cup well with a spoon until the visible bubbles are gone, then take another sip. Check whether it still stings even with fewer bubbles.
- Hold a sip of soda water on your tongue without moving it for about 5 seconds. Notice whether the prickling continues even after the feel of the bubbles fades.
Stirring doesn't release all the dissolved gas right away. If the sting remains after the bubbles disappear from view, that's a clue that the source of the sting is the dissolved gas, not the bubbles. Anyone who dislikes carbonation, or young children, needn't force themselves to try this.
Want to know more? ― Terms, formulas, and textbook connectionsWe mark clearly whether each point is middle-school-level or university-level, so you know how deep you're going.
- JHSCovered in middle-school science
- HSCovered in high-school chemistry/biology
- HS+Advanced high-school content, or a textbook sidebar topic
- Univ.Not covered in high school; university-level content (physical chemistry, sensory physiology)
- ResearchNot yet settled even at university level — something researchers are actively studying
JHSTerminology: this phenomenon has names
- Carbonic acid: a very weak acid formed when carbon dioxide dissolves in water. This is why soda water is slightly acidic.
- Carbonic anhydrase: an enzyme that speeds up the reaction (and its reverse) between carbon dioxide and water into acid by millions of times. It's also found on the surface of sour-taste cells on the tongue.
- Trigeminal nerve: a nerve that carries pain, heat, and cold signals from the face and mouth to the brain. It carries the prickle of carbonation and the sharp bite of wasabi.
JHSHSCheck with a formula: how much gas is dissolved in 500 mL of soda?
The source of the sting is the concentration of dissolved carbon dioxide. How much gas dissolves in water can be estimated with Henry's law. Here C is the dissolved concentration (in mol/L), k is the solubility coefficient (mol/(L·atm)), and P is the pressure of the carbon dioxide in contact with the water (atm).
| In symbols | C ＝ k × P |
| In words | Dissolved CO2 concentration ＝ solubility coefficient × CO2 pressure in contact with the water |
| Where it comes from | The relationship when the number of molecules dissolving from the surface balances the number of molecules leaving the water (Henry's law). The dissolving rate is proportional to pressure and the escaping rate is proportional to concentration, so at balance, concentration is proportional to pressure. |
| Symbol | Meaning and unit |
| C | Concentration of dissolved CO2 in water (mol/L) |
| k | Solubility coefficient, which varies with temperature (mol/(L·atm)) |
| P | Pressure of CO2 in contact with the water (atm) |
| Solubility coefficient at 25°C | Roughly 0.034 mol/(L·atm) |
| Solubility coefficient at 0°C (same approximation as Figure 2) | Roughly 0.071 mol/(L·atm) |
| CO2 pressure inside a sealed container (example) | Taken as 3 atm |
| CO2 pressure in open air | Roughly 0.0004 atm |
| Weight of 1 mol of CO2 | 44 g |
| Volume of 1 mol of gas (25°C, 1 atm) | Roughly 24.5 L |
| Concentration inside the sealed container (mol/L) | 0.034 × 3 ＝ 0.102 |
| Converted to weight (g/L) | 0.102 × 44 ≒ 4.5 |
| Amount dissolved in 500 mL (mol) | 0.102 × 0.5 ＝ 0.051 |
| Volume if turned back into gas (L) | 0.051 × 24.5 ≒ 1.25 |
| Equilibrium concentration once opened (mol/L) | 0.034 × 0.0004 ＝ 0.0000136 |
| Ratio vs. sealed container (×) | 0.102 ÷ 0.0000136 ＝ 7500 |
| Ratio of solubility at 0°C vs. 25°C (×) | 0.071 ÷ 0.034 ≒ 2.1 |
A 500 mL bottle holds enough dissolved carbon dioxide that, turned back into gas, it would fill about 1.25 liters — roughly two and a half bottles' worth. Left open, the amount that can stay dissolved drops to about 1/7500, so gas keeps escaping over time. Chilling it roughly doubles how much can stay dissolved, which also means more raw material for acid on your tongue.
HSHS+Chemical equilibrium and the enzyme's role
HSCarbon dioxide reacts with water to form carbonic acid, part of which splits into hydrogen ions and bicarbonate ions (CO2 ＋ H2O ⇄ H2CO3 ⇄ H+ ＋ HCO3−). These hydrogen ions are the "acid" that sour-taste cells detect. The drop in gas solubility with rising temperature, and Henry's law itself, are both covered in high-school chemistry.
HS+The step where carbon dioxide reacts with water is said to take tens of seconds to minutes without an enzyme. Carbonic anhydrase speeds this up by orders of magnitude. The enzyme doesn't change where the reaction ends up (its equilibrium) — it only changes how fast it gets there. This is a textbook example of exactly the idea taught in high-school biology and chemistry.
Univ.Taste receptors and the temperature dependence of Henry's constant
A 2009 study showed in mice that sour-sensing cells (marked by PKD2L1) carry a membrane-anchored carbonic anhydrase (Car4) on their surface, and that this enzyme is necessary for tasting carbonation. On the pain side, reports suggest that the ion channel TRPA1, found in the free nerve endings of the trigeminal nerve, is activated when the inside of the cell becomes acidic. The temperature dependence of the solubility coefficient is expressed by the van 't Hoff equation, and the calculation in Figure 2 uses this same form.
📖 For the derivation of the formula and further reading: Henry's law (Wikipedia, Japanese) ／ Carbonic anhydrase (Wikipedia, Japanese)
ResearchWhat's still not fully understood
- Are bubbles really just a side actor? While suppressing bubbles doesn't much change the sting, some researchers think the tactile feel of bubbles contributes to a sense of "freshness" — how large a role they play remains unclear.
- Does the human tongue work the same way as a mouse's? The experiments disabling specific cell types were done in mice. In humans, reports that the enzyme-blocking drug changes taste suggest the same mechanism is likely at work.
- Why doesn't it taste "sour"? Even though sour-taste cells are involved, why soda water isn't perceived as sour like lemon is still being studied — including exactly how the brain combines multiple signals.
In other words, even the explanation in this article is "what we currently understand." Even a sensation as everyday as this has only had its mechanism understood in the last decade or so.
Textbook connections (by level)
| Level | Subject/unit | Where in this article |
|---|---|---|
| JHS | Science, properties of gases (CO2 dissolves in water and makes it acidic) | The section on CO2 turning into acid |
| HS | Chemistry, gas solubility, Henry's law | "Check with a formula," Figure 2 |
| HS+ | Biology, enzyme function / chemical equilibrium | How the enzyme speeds up the reaction |
| Univ. | Sensory physiology, taste receptors / physical chemistry | Sour-taste cells, carbonic anhydrase, temperature dependence |
| Research | Chemosensory research | The role of bubbles, the mechanism in humans |
| ― | Everyday connections | Why we chill soda, why we reseal it right after opening |
- Chandrashekar J. et al., "The Taste of Carbonation," Science 326 (2009)
- Wise P. M. et al., "The Influence of Bubbles on the Perception Carbonation Bite," PLoS ONE 8 (2013)
- Wang Y. Y. et al., "TRPA1 is a component of the nociceptive response to CO2," Journal of Neuroscience 30 (2010)
- Graber M., Kelleher S., "Side effects of acetazolamide: the champagne blues," New England Journal of Medicine 319 (1988)
- Sander R., "Compilation of Henry's law constants for water as solvent," Atmospheric Chemistry and Physics 15 (2015)
※This article is a general-audience science explainer. The figures given are approximate, meant to aid understanding of the mechanism. The pressure inside a container varies by product and temperature. If you are taking medication, please check with your doctor or pharmacist about its effects.