How can you tell if distilled water is bad?

October 17, 2025 •

3 min read

While commercially bottled distilled water can theoretically last indefinitely if sealed and stored properly, its purity is highly susceptible to contamination once the container is opened. Knowing how can you tell if distilled water is bad is essential for ensuring safety, whether for drinking or use in sensitive appliances.

Quick Summary

Detecting compromised distilled water involves checking for visual changes, off-putting smells, or altered taste. Contamination often occurs after a container is opened and exposed to environmental factors, affecting its purity. Proper storage is key.

Key Points

Visible Contamination: Cloudy water, floating particles, or algae growth are clear signs that distilled water is no longer pure.
Off-Putting Smells: A chemical, plastic, or musty odor indicates contamination from the container or environment.
Taste Test: Pure distilled water is flat; any other flavor suggests the absorption of impurities.
The Evaporation Test: Boil a small amount in a clean pan; if residue remains after evaporation, the water contains impurities.
Proper Storage: Keeping opened distilled water in a cool, dark, and tightly sealed container prevents most forms of contamination.
Appliance Safety: Never use compromised distilled water in humidifiers or CPAP machines, as contaminants can damage the equipment and pose a health risk.

Distilled water is created by boiling and condensing steam, a process that removes nearly all impurities, minerals, and contaminants. This pure state, however, makes it highly absorbent and vulnerable to re-contamination from its surroundings, which is how it "goes bad". Spotting the signs of contamination is crucial for health and appliance maintenance.

The Visual Check

Pure distilled water should be completely clear and colorless, like regular water. Any deviation is a major red flag.

Cloudiness or Murkiness: Unlike hard water with mineral deposits, cloudy distilled water indicates foreign substances have been introduced. This could be anything from chemical particles to microbial growth. Always discard cloudy water.
Visible Particles: If you see any floating debris, fibers, or sediment in the water, it is no longer pure. This often happens if the container is not tightly sealed or if airborne dust and other particles have entered.
Algae Growth: If the water has been exposed to sunlight, especially in a clear container, you might see green or slimy algae begin to grow. Algae thrive in water and indicate bacterial and organic contamination. Never use or drink water with algae growth.
Container Degradation: Inspect the bottle itself for signs of warping, discoloration, or softening. If stored in a warm environment, especially in plastic, the container can degrade and leach chemicals into the water, affecting its quality.

The Smell and Taste Test

Pure distilled water is odorless and tasteless, though some find its lack of minerals gives it a "flat" taste. If you notice any distinct odors or flavors, the water is compromised.

Foul or Chemical Odor: A chemical or plasticky smell is a strong indicator that the container has leached chemicals into the water, often due to heat exposure. An earthy or musty smell can signal microbial growth.
Unusual Taste: A metallic, sour, or otherwise off-putting taste is a clear sign of contamination. The water has likely absorbed impurities from its surroundings or the container itself. While tasting is a reliable test, if the visual or smell tests already raise a flag, do not taste it.

At-Home Purity Tests

For a more definitive test without a lab, a simple boiling test can reveal if the water is no longer pure.

Take a small amount of the suspect distilled water.
Pour it into a clean, new saucepan or a glass container.
Bring the water to a rolling boil until it has completely evaporated.
After the container cools, inspect the bottom. Pure distilled water will leave behind no residue. The presence of a white, chalky, or other colored residue indicates mineral or solid impurities have entered the water.

Comparison of Good vs. Bad Distilled Water


Feature	Good Distilled Water	Bad Distilled Water
Appearance	Crystal clear, no particles	Cloudy, murky, or discolored; may contain visible sediment or algae
Odor	Odorless	Chemical, metallic, musty, or foul smell
Taste	Flat or neutral	Sour, bitter, metallic, or otherwise unpleasant
Container	Intact, no warping or damage	Cracked, warped, or discolored, especially if plastic
TDS Reading	Near zero (0-3 ppm)	Higher than zero, indicating dissolved solids
Evaporation Test	Leaves no residue	Leaves behind mineral deposits or film

Preventing Distilled Water from Going Bad

Proper storage is the best way to maintain the purity of distilled water after opening.

Store opened bottles in a cool, dark place, away from direct sunlight.
Keep the container tightly sealed to prevent airborne contaminants and odors from being absorbed.
Transferring the water from plastic to a clean glass container is a good practice to prevent leaching of chemicals over time.
Use the water within a few weeks of opening, especially if using it for sensitive applications like CPAP machines or drinking.

Conclusion

In conclusion, determining the quality of distilled water relies on a few key sensory indicators and simple at-home tests. Pure distilled water is transparent, tasteless, and odorless. Any changes in its visual appearance, smell, or flavor are a strong sign of contamination, and the water should be discarded. For critical applications, relying on a simple evaporation test or a TDS meter offers greater certainty. By being vigilant about storage and checking for the signs of compromise, you can ensure your water remains pure and safe for its intended use. For more details on water quality standards, you can refer to regulations set by official bodies like the FDA, which regulates bottled distilled water in the US.

Frequently Asked Questions

Once opened, distilled water can become contaminated by its environment. For critical applications or drinking, it's best to use it within one to four weeks, especially if not refrigerated.

If distilled water has been contaminated, it can contain bacteria, chemicals, or other impurities. Drinking it could potentially lead to health issues, depending on the type and level of contamination.

Pure, good-quality distilled water should be completely clear, free of any particles or cloudiness, and have no odor whatsoever.

Distilled water tastes flat because it lacks minerals. If it tastes metallic, sour, or like plastic, it has likely absorbed contaminants from the container or environment.

Yes. Using contaminated or 'bad' distilled water in a CPAP machine can introduce minerals or microbial growth. This can lead to mineral buildup, which can damage the machine over time and potentially expose you to harmful contaminants.

If the water is in an unopened, sealed bottle and has been stored properly, it should still be pure. However, once opened, the risk of contamination increases significantly over time, so it's safer to discard very old, opened water.

The easiest test is the visual and smell test. Pour some into a clear glass and check for cloudiness or particles. Smell it for any chemical or off-putting odors. For a more conclusive test, perform the boiling evaporation test.