Water-reactive substances

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Water-reactive substances[1] are those that spontaneously undergo a chemical reaction with water. Some of the metals that are reactive with water are the alkali metals (lithium, sodium, potassium, rubidium, and caesium), alkaline earth metals (barium, radium, sodium, calcium, and magnesium), and aluminum alkyls.[2] These metals are usually stored in containers with oil so that, if any water is brought into the container, it and the oil will separate, protecting the metal.

Water-reactive substances are classified as R2 under the UN classification system and as Hazard 4.3 by the United States Department of Transportation.

Some water-reactive substances are also pyrophoric, like aluminum alkyls.

Group 1: Alkali metals[edit]

Group 1: Alkali metals
Reaction of sodium (Na) and water

The alkali metals (Li, Na, K, Rb, Cs, and Fr) are the most reactive metals in the periodic table - they all react vigorously or even explosively with cold water, resulting in the displacement of hydrogen.

The Group 1 metal (M) is oxidised to its metal ion, and water is reduced to hydrogen gas (H2) and hydroxide ion (OH-), giving a general equation of:

2M(s) + 2H2O(l) ⟶ 2M+(aq) + 2OH(aq) + H2(g) [3]

The Group 1 metals or alkali metals become more reactive with water as you go down the Group.

Group 2: Alkaline earth metals[edit]

Group 2: Alkaline earth metals

The alkaline earth metals (Be, Mg, Ca, Sr, Ba, and Ra) are the second most reactive metals in the periodic table, showing an increase in reactivity with water (or steam) as you go down the group. Beryllium (Be) is the only alkaline earth metal that does not react with water or steam, even if metal is heated to red hot[4], due to its relatively small size and high ionization energy. Additionally, beryllium has a resistant outer oxide layer that lowers its reactivity at lower temperatures, unless the metal is heated at 750°C by which the layer would have been broken, exposing beryllium[5].

Magnesium shows insignificant reaction with water, but burns vigorously with steam or water vapor to produce white magnesium oxide and hydrogen gas:

Mg(s) + 2H2O(l) ⟶ Mg(OH)2(s) + H2(g)

Note: A metal reacting with cold water will produce metal hydroxide. However, if a metal reacts with steam, like magnesium, metal oxide is produced as a result of metal hydroxides splitting upon heating[6].

The hydroxides of calcium, strontium and barium are only slightly water-soluble but produce sufficient hydroxide ions to make the environment basic, giving a general equation of:

M(s) + 2H2O(l) ⟶ M(OH)2(aq) + H2(g) [7]

Aluminium alkyls[edit]

Aluminium alkyls refer to a family of organo-aluminiums and are strong reducing agents. Not only do they react violently with water, they are also corrosive and spontaneously combust in air, classifying them as pyrophoric. Some examples are trimethyl and triethyl aluminiums[8] .


  1. ^ "The MSDS HyperGlossary: Water Reactive". Interactive Learning Paradigms Incorporated. Retrieved 2007-05-10. 
  2. ^ "Water-Reactive Chemicals". University of Pennsylvania, Environmental Health & Radiation Safety. Retrieved 4 February 2018. 
  3. ^ Landas, Trevor. "Reactions of Main Group Elements with Water". Chemistry LibreTexts. Retrieved 9 February 2017. 
  4. ^ Pilgaard, Michael. "Beryllium: Chemical Reactions". Michael Pilgaard's Table of the Elements. Retrieved 16 February 2018. 
  5. ^ Clark, Jim. "Reactions of the Group 2 Elements with Water". ChemGuide. Retrieved 16 February 2018. 
  6. ^ Clark, Jim. "Reactions of the Group 2 Elements with Water". ChemGuide. Retrieved 16 February 2018. 
  7. ^ Landas, Trevor. "Reactions of Main Group Elements with Water". Chemistry LibreTexts. Retrieved 16 February 2018. 
  8. ^ National Oceanic and Atmospheric Administration. "Aluminium alkyls". CAMEO Chemicals. Retrieved 16 February 2018.