Does Sodium Produce Water When It Reacts With Water?

Sodium does not produce water in its direct reaction with water. Instead, water is a reactant: sodium reacts with it to form sodium hydroxide and hydrogen gas. The reaction gives off heat, and that heat can sometimes ignite the hydrogen, creating a second reaction that does produce water.

The main reaction

The balanced chemical equation is:

$$2\mathrm{Na} + 2\mathrm{H_2O} \rightarrow 2\mathrm{NaOH} + \mathrm{H_2}$$

The products are sodium hydroxide and hydrogen gas. Sodium hydroxide dissolves in the water that remains, so the liquid becomes a strongly alkaline solution. The equation does not show water as a product because water is used up in the reaction.

In ionic form, the reaction is:

$$2\mathrm{Na} + 2\mathrm{H_2O} \rightarrow 2\mathrm{Na^+} + 2\mathrm{OH^-} + \mathrm{H_2}$$

The sodium ions and hydroxide ions together make sodium hydroxide in solution. The equation also shows where the hydrogen gas comes from: hydrogen atoms from water combine to form H₂.

Why the reaction makes hydrogen

This is a redox reaction, meaning electrons are transferred. Each sodium atom loses an electron and becomes a sodium ion:

$$\mathrm{Na} \rightarrow \mathrm{Na^+} + e^-$$

Water molecules accept those electrons. In the process, some of their hydrogen atoms form hydrogen gas, while the remaining part forms hydroxide ions. The half-reactions balance when two sodium atoms transfer two electrons to the water:

$$\begin{aligned} 2\mathrm{Na} &\rightarrow 2\mathrm{Na^+} + 2e^- \\ 2\mathrm{H_2O} + 2e^- &\rightarrow \mathrm{H_2} + 2\mathrm{OH^-} \end{aligned}$$

Adding the half-reactions gives the ionic equation above. There is no step in this net reaction that creates water molecules.

When water can appear afterward

The reaction releases heat. If enough heat is generated, hydrogen can ignite by reacting with oxygen in the surrounding air:

$$2\mathrm{H_2} + \mathrm{O_2} \rightarrow 2\mathrm{H_2O}$$

This combustion of hydrogen does produce water. It is a secondary reaction, separate from sodium reacting with water. So an observer might see water formed during the overall event if the hydrogen burns, but the direct sodium–water reaction consumes water.

The two reactions can happen close together, and the second may be difficult to distinguish from the first by sight. That does not mean the first reaction produces water. It means hydrogen made in the first reaction has reacted with oxygen in the air.

Worked example: tracking the products

Suppose 2.30 g of sodium reacts completely with excess water. Using sodium’s molar mass, 22.99 g/mol:

$$2.30\ \mathrm{g\ Na} \times \frac{1\ \mathrm{mol\ Na}}{22.99\ \mathrm{g\ Na}} \approx 0.100\ \mathrm{mol\ Na}$$

The balanced equation shows that two moles of sodium consume two moles of water and produce two moles of sodium hydroxide, along with one mole of hydrogen. Therefore, 0.100 mol of sodium:

• consumes 0.100 mol of water, about 1.80 g;
• produces 0.100 mol of sodium hydroxide, about 4.00 g;
• produces 0.0500 mol of hydrogen gas, about 0.101 g.

The accounting makes the direction clear: some water is consumed, while sodium hydroxide and hydrogen are formed. If the hydrogen then burns, that later reaction can make some water, but its amount depends on how much hydrogen actually burns and how much oxygen is available.

Common mistakes

• Calling the liquid product “water.” The remaining liquid is mostly water if water was in excess, but it contains dissolved sodium hydroxide. The solution is not pure water.
• Confusing hydroxide with water. Hydroxide ions, OH⁻, are products; water molecules, H₂O, are reactants in the main equation. They are different chemical species.
• Combining two reactions without labeling them. Sodium reacting with water makes hydrogen. Hydrogen burning in oxygen can make water. Keep these equations separate to answer which reaction produces what.
• Assuming the reaction is safe because sodium is a solid. Sodium reacts rapidly and exothermically with water. The hot, caustic solution and possible ignition of hydrogen make this a hazardous reaction, not a suitable home experiment.