Water-Reactive & Pyrophoric Chemicals: List, Hazards & Storage

What are water-reactive chemicals?

OSHA’s classification criteria define this hazard class as solid or liquid chemicals which, by interaction with water, are liable to become spontaneously flammable or to give off flammable gases in dangerous quantities (29 CFR 1910.1200 Appendix B, B.12.1).

The wording repays attention. The hazard is the gas, not the contact. Water-reactives are not corrosive to water and they do not explode because water offends them — they tear water apart, take the oxygen, and liberate the hydrogen or another flammable gas, releasing heat while doing it. The heat then ignites the gas the reaction just produced. The substance supplies fuel, the reaction supplies the ignition, and the surrounding air supplies the oxygen. Nothing else needs to go wrong.

Which gas appears depends on the chemistry:

The phosphides deserve a note. Aluminium phosphide is used as a fumigant precisely because it meets atmospheric moisture and releases phosphine; zinc phosphide works as a rodenticide by the same mechanism in stomach acid. For those products the reaction is the intended function. In a store room it is an uncontrolled release of a toxic flammable gas, which is why phosphides are kept sealed and dry.

GHS sorts the class into three categories by how fast the gas comes off:

How we classify this page’s substances

We assign a substance here from its GHS hazard statements — H260 or H261 for water reactivity, plus H250 and H251 for the pyrophoric and self-heating substances covered in the next section. In our database:

Two things stand out. First, 22 of the 24 water-reactives are Category 1 — the tier where the released gas tends to ignite by itself, without any external ignition source. This is not a class with a gentle end. Second, all 30 substances carry the signal word Danger, with no exceptions — unusual among our storage classes, and a fair summary of the class.

You may also notice the list is almost entirely metals, hydrides, phosphides, carbides and organometallics. That is not a sampling artefact. OSHA’s criteria state that the classification procedure need not be applied when a chemical’s structure contains no metals or metalloids, when handling experience shows it does not react with water, or when it simply dissolves in water to form a stable mixture (B.12.3). The hazard is essentially a metals-and-metalloids phenomenon.

Pyrophoric chemicals: ignition in air, without any water

A pyrophoric liquid or solid is one that, even in small quantities, is liable to ignite within five minutes of coming into contact with air (App. B, B.9.1 and B.10.1). There is no water in that definition, no spark and no external heat — only air. GHS gives pyrophorics a single category: either a substance does this or it does not.

A self-heating chemical (H251/H252) is the slower relative. It also reacts with air without any energy input, but it differs from a pyrophoric in that it ignites only in large amounts — kilograms — and after long periods, hours or days (B.11.1). Heat accumulates faster than the mass can shed it, and after an induction period the pile lights itself. A drum can do what a spoonful cannot, which makes the hazard a function of quantity rather than of the substance alone.

The pyrophoric entries in this class fall into three groups: white phosphorus, which ignites in air at barely above room temperature; finely divided metal powders — aluminium, magnesium, zinc, zirconium, cadmium — which are pyrophoric in that form although the bulk metal sitting on a shelf is not; and organometallic reagents such as n-hexyllithium, isobutyllithium and the organoaluminiums, along with tert-butylarsine.

Now the part that makes this class awkward, and worth a page of its own:

Those 7 are the reason a single storage rule cannot be written for this class. An organolithium reagent is pyrophoric and water-reactive: exclude water and it still burns; exclude air and moisture still destroys it. Nothing short of an inert gas blanket — nitrogen or argon — and a sealed septum protects it.

How water-reactives and pyrophorics must be stored

Start with the split that is easiest to get backwards. Sodium is stored under mineral oil or kerosene. White phosphorus is stored under water. Both appear on this page. The media are opposites because the hazards are opposites: sodium reacts with water and is indifferent to dry air; white phosphorus ignites in air and is indifferent to water. Apply either rule to the other substance and you cause the accident you were trying to prevent. Before choosing a storage medium, establish which of the two hazards — or both — a given substance actually carries.

The red pills above are mostly aqueous. Acids and bases are marked never store with, and the reason is more literal than it first appears: laboratory and industrial acids and caustics are overwhelmingly water solutions. Hydrochloric acid is hydrogen chloride dissolved in water; caustic soda is sodium hydroxide dissolved in water. Storing a water-reactive next to a bottle of dilute acid is storing it next to a reservoir of water that also happens to be corrosive, and the reaction with acid is typically more violent than with water alone. Oxidizers, oxidizing acids and organic peroxides are prohibited for the more familiar reason: they supply oxygen to a substance whose signature failure mode is producing its own flammable gas.

Flammable liquids are marked amber — treat that as a floor, not a ceiling. Our matrix rates the pairing keep separate on the basis of general international practice. US workplaces are held to something stricter and explicitly written down. Paragraph (d)(7)(iv) of the flammable liquids standard states:

Materials which will react with water shall not be stored in the same room with flammable liquids.

That is a room-level prohibition, and it is one of the very few inter-class storage rules stated outright in 29 CFR 1910.106. The logic is about the fire you might have to fight rather than the chemicals sitting quietly on a shelf: water is the primary suppression medium for a flammable liquid fire, and a room containing water-reactives is a room where using it makes things worse.

Reactive and alkali metals are the one green pill — they share this class’s hazards and its handling regime, so they can share its space.

Water is the wrong extinguishing medium here, by definition. For a Category 1 water-reactive, applying water adds fuel: it is the exact reaction the classification describes, now at fire-hose scale. Combustible metal fires need a Class D extinguishing agent — a dry powder that smothers rather than reacts. OSHA requires Class D agents to be distributed so the travel distance from a combustible metal working area to the nearest one is 75 feet (22.9 m) or less, and requires them wherever combustible metal powders, flakes or shavings are generated at least once every two weeks (29 CFR 1910.157(d)(6)). An ABC dry chemical unit is not a substitute. Worth confirming in advance: your standard extinguisher will not help, and reaching for it costs time.

Beyond that, the handling principles follow from the classification itself — sealed containers, a dry location with no sprinkler discharge overhead, no floor drains that could carry a spill to water, minimal working quantities, and an inert atmosphere for anything in the “both” column. Class-level guidance is a starting point rather than a verdict on any individual substance: check SDS sections 7 and 10, or look the substance up in the storage compatibility matrix.