GHS / HazCom term · Glossary
Water-Reactive Substance
Verified vs OSHA sources · 2026-10-05
Under GHS/HazCom, a solid or liquid chemical that, on contact with water, becomes spontaneously flammable or gives off flammable gases in dangerous quantities. The GHS class is specifically about emitting flammable gas — sodium hitting water releasing hydrogen is the classic case. Three categories rank the hazard by how fast the flammable gas evolves and whether it ignites on its own.
Also known as: water-reactive, emits flammable gases in contact with water
Water-Reactive Substance at a glance
- The GHS hazard is about emitting flammable gas on water contact (not merely reacting or heating).
- Category 1: reacts vigorously at ambient temperature and the gas tends to ignite spontaneously, or evolves ≥ 10 L/kg of gas over any one minute.
- Category 2: evolves flammable gas at ≥ 20 L/kg per hour (and not Category 1).
- Category 3: evolves flammable gas at > 1 L/kg per hour (and not Category 1 or 2).
In plain English
A material that reacts with water to make a flammable gas — sometimes catching fire on its own. Drop sodium or calcium carbide in water and it fizzes off hydrogen or acetylene that can ignite. So you keep these chemicals bone dry.
What the source says, in summary
Substances and mixtures which, in contact with water, emit flammable gases are solid or liquid chemicals which, by interaction with water, are liable to become spontaneously flammable or to give off flammable gases in dangerous quantities.
In context
A classification or labeling concept from the UN Globally Harmonized System, adopted into US law through OSHA's Hazard Communication Standard (29 CFR 1910.1200) and its appendices.
Where this is written in OSHA's rules
- 29 CFR 1910.1200 App B.12Read on eCFR / OSHA.gov
Example
Metallic sodium is water-reactive Category 1: dropped in water it reacts violently, releasing hydrogen that ignites from the reaction's own heat. Calcium carbide is another — it reacts with water to give off flammable acetylene gas, which is why carbide is stored sealed and dry.
Why it matters
Water-reactive chemicals turn ordinary firefighting or a damp storeroom into an ignition event — spraying water can feed the very reaction you're trying to stop. Correct classification drives dry storage, the “no water” firefighting placard, and the GHS02 flame pictogram, all of which prevent a bad situation from being made worse.
“Water-reactive” (GHS) means flammable gas specifically
Many chemicals react with water to release heat or a toxic gas — but the GHS “emits flammable gases in contact with water” class is narrower: it's about producing a flammable gas (like hydrogen or acetylene). A substance that reacts with water to give off a toxic, non-flammable gas is classified under different hazard classes.
Water-reactive categories (flammable gas evolution)
| Category | Criterion |
|---|---|
| 1 | Vigorous at ambient; gas ignites spontaneously, or ≥ 10 L/kg over any 1 minute |
| 2 | ≥ 20 L/kg per hour (and not Category 1) |
| 3 | > 1 L/kg per hour (and not Category 1 or 2) |
Water-Reactive Substance: frequently asked questions
- What is a water-reactive substance?
- A solid or liquid that, in contact with water, becomes spontaneously flammable or gives off flammable gases in dangerous quantities — such as sodium releasing hydrogen or calcium carbide releasing acetylene.
- How are water-reactive substances categorized?
- By how fast they evolve flammable gas: Category 1 (vigorous, spontaneous ignition or ≥ 10 L/kg per minute), Category 2 (≥ 20 L/kg per hour), and Category 3 (> 1 L/kg per hour).
- Why can't you use water on these chemicals?
- Because water is what triggers the hazard — it drives the reaction that releases flammable gas and heat. Firefighting and storage must keep these substances dry and use non-water methods.
Related terms
Free tools for this
Governing OSHA standards
More GHS terms
A GHS health-hazard class for the harmful effects that follow a single dose, several doses within 24 hours, or a 4-hour inhalation exposure. Chemicals are sorted into categories by route (oral, dermal, inhalation) using LD50/LC50 cut-offs — Category 1 is the most toxic. OSHA's HazCom standard uses Categories 1–4.
Aspiration HazardA GHS health-hazard class for chemicals that can be fatal if swallowed and then enter the airways — directly or by vomiting — causing chemical pneumonia or lung injury. Thin, low-viscosity hydrocarbons (gasoline, kerosene, lamp oil) are the classic example. It is Category 1, with the health-hazard pictogram (GHS08) and signal word “Danger.”
CarcinogenicityA GHS health-hazard class for chemicals that cause cancer or increase its incidence. GHS (and OSHA's HazCom standard) sorts carcinogens into Category 1A (known human carcinogen), 1B (presumed, from animal evidence), and Category 2 (suspected). Carcinogens carry the health-hazard pictogram (GHS08) and a hazard statement such as H350 (“May cause cancer”).
Chemicals Under PressureA physical-hazard class added by OSHA's 2024 HCS update (aligning with GHS Revision 7) for liquids or solids pressurized with a gas, held in pressure receptacles other than aerosol dispensers and not classified as gases under pressure. It is categorized by the flammability of its contents and always carries the gas-cylinder pictogram (GHS04) for the pressure hazard.
ClassificationThe process of evaluating the available scientific evidence for a chemical against the GHS criteria to decide whether it is hazardous and, if so, the hazard class and category of each effect. Classification is the first step in HazCom — its result determines every label element (pictogram, signal word, hazard and precautionary statements) and SDS content.
Corrosive to MetalsA GHS physical-hazard class for chemicals that, by chemical action, will materially damage or even destroy metals. It is a single category (Category 1), determined by corrosion-rate testing on steel and aluminum, and uses the corrosion pictogram (GHS05) with the hazard statement H290. It is a separate hazard from skin corrosion, even though both use GHS05.
Desensitized ExplosivesA physical-hazard class added by OSHA's 2024 HCS update (GHS Revision 7) for solid or liquid explosives that have been phlegmatized — wetted or diluted — to suppress their explosive properties for safe transport and storage. It has four categories based on the corrected burning rate, and uses the flame pictogram (GHS02), not the exploding-bomb.
Globally Harmonized System (GHS)An internationally agreed framework created by the United Nations (first adopted in 2003) that standardizes how chemical hazards are classified and communicated — using common hazard classes, pictograms, signal words (Danger / Warning), and coded hazard (H) and precautionary (P) statements on labels and safety data sheets. The US adopted GHS into law through OSHA's 2012 Hazard Communication Standard update.
Sources & verification
- 29 CFR 1910.1200 App B.12 (eCFR / OSHA.gov)
- OSHA — 1910.1200 Appendix B.12 (Water-Reactive Substances)
- OSHA — Hazard Communication (Physical Hazards)
Reviewed by HazComFast against eCFR, OSHA.gov, NIOSH, and the Federal Register. Last reviewed 2026-10-05. This glossary is general information, not legal advice; OSHA State-Plan states (e.g. California, Michigan) may adopt stricter requirements.
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