What Happens to Aspirin in Water?: The Chemical Process of Hydrolysis

October 14, 2025 •

4 min read

Many people have noticed that old aspirin tablets can develop a distinct vinegar-like odor, a tell-tale sign of chemical decomposition. So, what happens to aspirin in water or even just moist air is a gradual process called hydrolysis, which breaks the drug down into less effective compounds.

Quick Summary

Aspirin undergoes hydrolysis when exposed to water, breaking down into salicylic acid and acetic acid. This process, accelerated by heat and moisture, reduces the drug's therapeutic potency over time.

Key Points

Hydrolysis Reaction: When aspirin (acetylsalicylic acid) is exposed to water, it breaks down through hydrolysis into salicylic acid and acetic acid.
Loss of Potency: The decomposition of aspirin into salicylic and acetic acid reduces its effectiveness as an anti-inflammatory, analgesic, and anti-platelet agent.
Vinegar Smell: The formation of acetic acid during hydrolysis is responsible for the distinct vinegar-like odor often associated with degraded aspirin.
Factors Influencing Rate: The rate of hydrolysis is significantly accelerated by increased temperature and alkaline (high pH) environments.
Proper Storage is Key: To prevent moisture-induced degradation, aspirin should be stored in a cool, dry location, away from heat and humidity like a bathroom.
Gastrointestinal Risk: The products of degradation, particularly salicylic acid, can increase the risk of stomach irritation and gastrointestinal side effects.
Slight Solubility: Regular aspirin is only slightly soluble in water, but soluble tablets use buffers to speed up dissolution by changing the pH.

The Chemical Process of Hydrolysis

Aspirin, known chemically as acetylsalicylic acid ($C_9H_8O_4$), is an organic compound with an ester functional group. When it comes into contact with water, it can undergo a chemical reaction called hydrolysis. In this process, a water molecule ($H_2O$) attacks the ester bond, splitting the aspirin molecule into two new, simpler compounds: salicylic acid and acetic acid.

This reaction can be represented by the following simplified equation: $C_9H_8O_4 + H_2O \rightarrow C_7H_6O_3 + C_2H_4O_2$ (Acetylsalicylic acid + Water $\rightarrow$ Salicylic acid + Acetic acid)

The Products of Decomposition

Salicylic Acid: This compound is the primary degradation product of aspirin hydrolysis and is what gives the drug its original source material from willow bark. It contains a phenol group, which can be used in lab tests to detect the presence of degraded aspirin. While salicylic acid also possesses therapeutic effects, it is more irritating to the stomach and not as effective as intact aspirin.
Acetic Acid: The other product is acetic acid, the main component of vinegar. It is the formation of this compound that causes the characteristic vinegar smell from older aspirin tablets that have been exposed to moisture.

Factors Influencing Aspirin's Breakdown in Water

The rate of hydrolysis is not constant; it is influenced by several external factors. Understanding these helps explain why proper storage is so critical for maintaining drug efficacy.

Temperature

Higher temperatures significantly increase the rate of hydrolysis. For example, studies have shown that while hydrolysis at room temperature is slow, it accelerates dramatically when heated. This is why storing medicine in hot locations, like a car or a humid bathroom, is discouraged.

pH Level

Aspirin's stability is heavily dependent on the pH of its environment.

It is most stable in a mildly acidic solution, with an optimal pH range of 2 to 3. This is similar to the environment in the stomach.
In alkaline (basic) solutions, the hydrolysis reaction proceeds much faster. This is relevant to the intestine, where conditions are more basic, and to effervescent tablets designed to dissolve quickly in water.

Humidity and Moisture

Even without being fully submerged in water, aspirin can undergo hydrolysis. Tablets exposed to the ambient moisture in the air will slowly break down over time. This is why expired tablets, or those stored improperly, begin to degrade and develop that vinegar smell.

Solubility versus Degradation

It is important to distinguish between aspirin's solubility and its degradation via hydrolysis. The two are related but are not the same process.

Insoluble vs. Soluble Aspirin

Regular Aspirin: Aspirin is only slightly soluble in water, dissolving at a rate of approximately 3 mg/mL at 25 °C. This low solubility is part of why it's more stable in dry conditions, but it also means it doesn't immediately form a clear solution when dropped in a glass of water, instead often clumping together.
Effervescent Aspirin: Effervescent formulations are designed to dissolve rapidly. They contain aspirin along with a buffering agent, typically sodium bicarbonate. When these tablets are added to water, the buffer reacts, increasing the pH and converting the aspirin into a more water-soluble salt. The resulting carbon dioxide fizz helps break the tablet apart, speeding up the dissolution process.

Comparison: Undegraded vs. Hydrolyzed Aspirin


Characteristic	Undegraded Aspirin (Acetylsalicylic Acid)	Hydrolyzed Aspirin (Salicylic Acid + Acetic Acid)
Appearance/Smell	White, crystalline, typically odorless in dry conditions.	Can appear crumbly or discolored, with a distinct vinegar odor.
Potency	Full therapeutic effect as an anti-inflammatory, analgesic, and anti-platelet agent.	Reduced or negligible therapeutic effect, as it's no longer intact acetylsalicylic acid.
Chemical Composition	Acetylsalicylic Acid ($C_9H_8O_4$)	A mixture of Salicylic Acid ($C_7H_6O_3$) and Acetic Acid ($C_2H_4O_2$).
Solubility in Water	Slightly soluble, but readily absorbed in the stomach.	Salicylic acid is slightly soluble, but overall, the chemical properties have changed.
Stomach Impact	Can cause irritation by inhibiting prostaglandins that protect the stomach lining.	Contains additional free acids (salicylic and acetic), which may exacerbate gastrointestinal irritation.

Implications for Users and Storage

For anyone using aspirin, especially for long-term conditions like heart health, the stability of the medication is paramount. Hydrolysis compromises the drug's intended action and can introduce unwanted side effects.

Reduced Therapeutic Effect: As aspirin degrades, it loses its full therapeutic properties. An older tablet may not provide the same anti-inflammatory or anti-platelet benefits as a fresh one. For critical applications like preventing heart attacks or strokes, this loss of potency could be dangerous.

Increased Gastrointestinal Irritation: The salicylic acid produced during hydrolysis can cause more severe stomach irritation than intact aspirin. This is a major concern for individuals with pre-existing gastrointestinal issues.

Best Storage Practices: To maximize the shelf life and efficacy of aspirin, it should be stored in a cool, dry place. The bathroom medicine cabinet, with its high humidity and temperature fluctuations, is one of the worst places to store it. A sealed container in a kitchen pantry or bedroom drawer is a much better option. Tablets that are chipped, discolored, or smell like vinegar should be discarded safely.

Conclusion

While aspirin is a common and highly effective medication, its interaction with water is a clear example of how environmental factors can affect drug stability. The process of hydrolysis, leading to the formation of salicylic acid and acetic acid, is a chemical reality that reduces the drug's potency and can increase side effects. Consumers must be aware of this process and practice proper storage to ensure the medication they take is both safe and effective. The simple observation of a vinegar smell is an important warning sign that the medication should be replaced.

For more information on proper medication storage and disposal, the InfantRisk Center offers helpful guidance.

Frequently Asked Questions

The chemical name for aspirin is acetylsalicylic acid.

Old aspirin tablets can smell like vinegar because the active ingredient, acetylsalicylic acid, has undergone hydrolysis due to moisture exposure, producing acetic acid, which is the main component of vinegar.

Yes, temperature has a significant effect. The rate of hydrolysis increases with higher temperatures, which is why it is recommended to store aspirin in a cool, dry place.

Yes, effervescent tablets contain buffering agents like sodium bicarbonate. These react in water to form a more soluble salt of aspirin, allowing it to dissolve and disperse more quickly, which is different from the slow degradation of a standard tablet.

No, you should not take aspirin that smells like vinegar. The odor indicates that the drug has degraded into less effective compounds, and its therapeutic potency may be significantly reduced or altered.

You should store aspirin in its original container in a cool, dry, and dark place, away from heat and humidity. The bathroom medicine cabinet is not an ideal storage location.

Hydrolysis is a chemical reaction in which aspirin reacts with water, breaking down the molecule into its degradation products, salicylic acid and acetic acid.