Reactivity of Alkaline Earth Metals with Water

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Group 2 • Alkaline Earth Metals

Reactivity with Water

Beryllium's inertness, the steam trap, and floating Calcium.

By chemca Team • Updated Aug 2026

Compared to the Alkali Metals (Group 1), the Alkaline Earth Metals (Group 2) react much less vigorously with water. Because their ionization enthalpies are higher and they must lose two electrons to react, the kinetics of their reaction with water are significantly slower, leading to fascinating exceptions at the top of the group.

1. The General Reaction and Trend

When alkaline earth metals ($M$) react with water, they displace hydrogen to form a metal hydroxide and release Hydrogen gas ($H_2$).

$$M_{(s)} + 2H_2O_{(l)} \rightarrow M(OH)_{2(aq/s)} + H_{2(g)} \uparrow$$

The Trend in Vigor:

• Reactivity increases rapidly down the group ($Be \lt Mg \lt Ca \lt Sr \lt Ba$).
• As atomic size increases, ionization enthalpy decreases, making it easier for the metals to release their two valence electrons to reduce the $H^+$ in water.
• Unlike Group 1, these reactions are generally not exothermic enough to ignite the escaping hydrogen gas.

2. The Beryllium Exception

Beryllium ($Be$) is completely inert to water. It does not react with cold water, hot water, or even red-hot steam.

Why does Be not react?

1. High Ionization Enthalpy: Beryllium is extremely small, making it very difficult to remove its electrons thermodynamically.
2. Protective Oxide Layer: The surface of Beryllium is covered by a very thin, tough, and impermeable layer of Beryllium Oxide ($BeO$) that physically shields the bulk metal from the water molecules.

3. Magnesium: The Hot Water & Steam Trap

Magnesium ($Mg$) reacts very sluggishly with cold water (the reaction is almost negligible because of its protective oxide layer and the insolubility of $Mg(OH)_2$ which coats the metal).

However, it reacts vigorously under heated conditions. The products differ based on the state of the water!

A. Reaction with Hot Water

When placed in boiling water, Magnesium forms Magnesium Hydroxide and Hydrogen gas.

$$Mg_{(s)} + 2H_2O_{(l, \text{ hot})} \rightarrow Mg(OH)_{2(s)} + H_{2(g)} \uparrow$$

B. Reaction with Steam (Highly Tested)

When Magnesium burns in steam (gaseous water), it forms Magnesium Oxide (NOT hydroxide!) because hydroxides decompose at high temperatures.

$$Mg_{(s)} + H_2O_{(g, \text{ steam})} \xrightarrow{\Delta} MgO_{(s)} + H_{2(g)} \uparrow$$

4. Ca, Sr, and Ba: The "Floating" Phenomenon

Calcium ($Ca$), Strontium ($Sr$), and Barium ($Ba$) react readily with cold water. The reaction becomes increasingly vigorous from Ca to Ba.

$$Ca_{(s)} + 2H_2O_{(l)} \rightarrow Ca(OH)_{2(aq)} + H_{2(g)} \uparrow$$

The Floating Calcium Phenomenon:

Unlike Lithium, Sodium, and Potassium (which are lighter than water), Calcium ($1.55 \text{ g/cm}^3$) is denser than water.

When dropped in water, a piece of Calcium initially sinks. However, as the reaction proceeds, the bubbles of Hydrogen gas ($H_2$) formed stick to the surface of the metal. This acts like a life jacket, making the overall density of the (metal + bubbles) less than water, causing the Calcium to rise and float to the surface. (Magnesium does this too in hot water).

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