How to Write Equations for Metals Reacting With Water

When a metal reacts with water, the metal is oxidized and water is reduced, usually producing hydrogen gas. The exact products depend on the metal and whether it meets cold liquid water, hot water, or steam. To write the equation, identify those conditions, select the products, and then balance the atoms.

The usual reaction patterns

Many reactive metals form a hydroxide and hydrogen when they react with liquid water. For an alkali metal, written as $M$, the balanced pattern is:

$$2M + 2H_2O(l) \rightarrow 2MOH(aq) + H_2(g)$$

For a metal that forms a $2+$ ion, such as calcium, the pattern is:

$$M + 2H_2O(l) \rightarrow M(OH)_2 + H_2(g)$$

These are useful patterns, not universal rules. Some metals do not react appreciably with water under ordinary conditions, and some react only with steam. The conditions can also affect whether the metal product is an oxide or hydroxide.

A step-by-step method

1. Specify the water and conditions. Is it cold liquid water, hot water, or steam? Include the state symbol when useful: $(l)$ for liquid and $(g)$ for gas.
2. Decide whether the metal reacts. Very reactive metals can react with cold water. Other metals react slowly, need heating, or do not react with water in ordinary conditions.
3. Write the products. With liquid water, reactive metals often produce a hydroxide and hydrogen. With steam, some metals produce an oxide and hydrogen.
4. Balance the equation. Change coefficients in front of formulas, not the subscripts inside formulas.
5. Check atoms and charge. Count each element on both sides. For ionic formulas, make sure the product formula reflects the metal’s usual ion charge.

Worked examples

Sodium with liquid water

Sodium forms $Na^+$, so its hydroxide is $NaOH$. The unbalanced equation is:

$$Na + H_2O(l) \rightarrow NaOH(aq) + H_2(g)$$

Hydrogen appears as $H_2$, so use two water molecules to provide two hydrogen atoms for it. Then balance sodium and sodium hydroxide:

$$2Na(s) + 2H_2O(l) \rightarrow 2NaOH(aq) + H_2(g)$$

Check: there are two sodium atoms, four hydrogen atoms, and two oxygen atoms on each side. The reaction is vigorous; the equation describes the chemistry, not a safe experiment to try.

Calcium with liquid water

Calcium forms $Ca^{2+}$, and hydroxide is $OH^-$. Two hydroxide ions are needed for a neutral compound, giving $Ca(OH)_2$. The balanced equation is:

$$Ca(s) + 2H_2O(l) \rightarrow Ca(OH)_2 + H_2(g)$$

The parentheses mean that the formula contains two hydroxide groups. The equation has one calcium atom, two oxygen atoms, and four hydrogen atoms on each side. Calcium hydroxide is only slightly soluble, so a more detailed equation may show it as a solid or as an aqueous product depending on how the reaction mixture is represented.

Iron with steam

Iron reacts with steam at high temperature to form magnetite, $Fe_3O_4$, and hydrogen. Start with the skeleton equation:

$$Fe + H_2O(g) \rightarrow Fe_3O_4 + H_2$$

The oxide contains three iron atoms and four oxygen atoms, so use $3Fe$ and $4H_2O$. That gives eight hydrogen atoms, requiring $4H_2$:

$$3Fe(s) + 4H_2O(g) \rightarrow Fe_3O_4(s) + 4H_2(g)$$

This differs from the hydroxide pattern because the high-temperature steam reaction produces an oxide.

Why these are redox reactions

In these reactions, metal atoms lose electrons, so the metal is oxidized. Hydrogen in water gains electrons and forms hydrogen gas, so it is reduced. For example, sodium changes from oxidation state $0$ in $Na$ to $+1$ in $NaOH$. Hydrogen changes from $+1$ in water to $0$ in $H_2$.

You do not usually need to write the electron-transfer steps to produce a balanced molecular equation. They are useful, however, for explaining why hydrogen is formed and checking that the proposed products make chemical sense.

Common mistakes

• Assuming every metal reacts with water. Copper, for example, does not displace hydrogen from ordinary liquid water. A protective surface layer can also prevent or slow a reaction.
• Using hydroxide for every condition. Steam reactions may form an oxide instead. Magnesium, for instance, forms magnesium hydroxide slowly with cold water but magnesium oxide with steam.
• Changing subscripts to balance. Do not turn $H_2O$ into $H_3O$ to fix an atom count. Adjust coefficients instead.
• Forgetting hydrogen is diatomic. Write hydrogen gas as $H_2$, not $H$.
• Ignoring the metal’s charge. Calcium hydroxide is $Ca(OH)_2$, not $CaOH$, because two hydroxide ions are needed to balance $Ca^{2+}$.

For any particular metal, check its behavior under the stated conditions before applying a general pattern. Beryllium, for example, is protected by a surface oxide and does not react readily with water; magnesium’s behavior changes substantially between liquid water and steam.