Rubidium and water
Rubidium is a highly reactive alkali metal. When solid rubidium contacts water, it reacts violently to form rubidium hydroxide and hydrogen gas, releasing heat. The hydrogen may ignite, and the reaction can cause fire or an explosion. This describes metallic rubidium; dissolved rubidium in natural water is present as the ion Rb+ and does not react with water in this way.
Reaction of rubidium with water
The reaction is:
2 Rb(s) + 2 H2O(l) → 2 RbOH(aq) + H2(g) + heat
Rubidium hydroxide dissolves in water and makes the solution strongly alkaline. The heat generated can ignite the hydrogen. Contact between alkali metals and water must therefore be treated as a serious fire and explosion hazard; do not attempt demonstrations or handling outside a properly equipped laboratory.
Why the reaction is so vigorous
Rubidium is below potassium in Group 1 of the periodic table. Its outer electron is held less strongly, so it reacts more readily with water than lithium, sodium or potassium. Caesium, which is below rubidium, is still more reactive. Research on alkali-metal droplets has also described rapid charge-driven breakup sometimes called a Coulomb explosion. This helps explain how fresh metal surface can be exposed during the reaction, but it should not be presented as the sole cause of the reaction or as a rubidium-specific observation.
Rubidium ions in water
In water, rubidium from dissolved salts occurs mainly as the hydrated monovalent ion Rb+. It is chemically distinct from metallic rubidium and is not spontaneously explosive. Its concentration and mobility depend on the source water and associated anions. Rubidium is generally a trace constituent rather than a major component of ordinary fresh water.
Rubidium removal from water
Where rubidium removal is required, the suitable process depends on its concentration, the competing ions and the required water quality. Cation-exchange media can capture Rb+, although sodium, potassium, calcium and magnesium may compete for exchange sites. Reverse osmosis can reduce dissolved salts, but actual rejection and recovery should be confirmed for the specific water composition. Testing and process design should use a complete water analysis.
For comparison, see Potassium and water and the chemical properties of rubidium.
Last reviewed: 8 November 2026 by Lenntech
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