Does Current Run Forever in Water?

A voltage supply connected to electrodes in water can drive a current, but voltage alone does not guarantee that current will continue indefinitely. The circuit must be complete, the water must conduct enough charge, and reactions at the electrodes must be able to proceed. In an ordinary setup, those conditions change over time.

The key distinction is that current is not a substance that gets used up. Rather, a sustained current transfers charge and usually causes chemical changes in the water or electrodes.

What carries current through water?

In a metal wire, mobile electrons carry most of the electric current. In water, the charge carriers are mainly dissolved ions: positively charged ions move toward the negative electrode, and negatively charged ions move toward the positive electrode. Even apparently clean water contains some ions, but pure water has very few, so its electrical conductivity is low. Tap water, salt water, and water containing other dissolved substances usually conduct much better.

A complete circuit is essential. With two electrodes connected to opposite supply terminals and immersed in the same conducting liquid, charge can pass through the liquid and return through the supply. If there is no return path, there may be a brief redistribution of charge, but there is no sustained current.

What happens at the electrodes?

Ions carry current through the liquid, but electrons cannot simply pass from a metal electrode into the water as though the water were another wire. Charge transfer between the metal and the liquid takes place through chemical reactions at the electrode surfaces. This is why sustained direct current through an electrolyte is normally accompanied by electrochemistry.

If the electrodes are sufficiently inert and water electrolysis is the main reaction, a simplified representation of the half-reactions in neutral solution is:

$$\begin{aligned} \text{Cathode: } & 2\mathrm{H_2O}+2e^-\\rightarrow \mathrm{H_2}+2\mathrm{OH^-} \\\\ \text{Anode: } & 2\mathrm{H_2O}\\rightarrow \mathrm{O_2}+4\mathrm{H^+}+4e^- \end{aligned}$$

The overall reaction is water splitting into hydrogen and oxygen. In a real cell, the electrode material and the substances dissolved in the water can change which reactions occur. For example, an electrode may corrode, or dissolved ions may react instead of water.

The reversible voltage for splitting water is about 1.23 V under standard conditions. A practical electrolytic cell generally needs a higher applied voltage because of electrode overpotentials and resistance in the liquid and circuit. The exact operating voltage depends on the cell and its conditions; 1.23 V is not a universal sharp threshold for every setup.

Why current does not stay constant forever

An idealized supply can keep providing energy, but that does not keep the cell itself unchanged. In a real water cell, the current may decrease, increase, or stop as conditions evolve. Common causes include:

• Changing liquid composition: Electrolysis changes local acidity and ion concentrations near the electrodes. The conductivity and electrode reaction rates can shift.
• Gas bubbles: Hydrogen or oxygen bubbles can cover part of an electrode, reducing the area available for reaction and raising resistance.
• Electrode changes: Reactive electrodes may dissolve. Other materials can become coated or passivated, making charge transfer harder.
• Changing gas pressure: In a sealed container, accumulating gas raises pressure and affects the reaction. A finite sealed cell cannot keep electrolyzing unchanged forever.
• Finite materials and energy: Water, dissolved reactants, electrode material, and the supply itself are not unlimited in an ordinary setup.

In a well-designed electrolyzer with adequate feed water, suitable electrodes, and a path for gases to leave, a steady current can be maintained for a long time. It is not the water alone that makes this possible: the apparatus must keep the chemical and electrical conditions suitable.

A worked example: what does 1 ampere do?

Suppose an electrolyzer carries a steady 1 A for one hour, and assume all the charge goes toward splitting water. The total charge is

$$Q=It=(1\\,\mathrm{A})(3600\\,\mathrm{s})=3600\\,\mathrm{C}.$$

Faraday’s constant is about $96{,}485\\,\mathrm{C/mol}$ of electrons. Producing one mole of hydrogen requires two moles of electrons, so the amount of hydrogen is approximately

$$n(\mathrm{H_2})=\frac{3600}{2(96{,}485)}\\approx 0.0187\\,\mathrm{mol}.$$

That is about 0.42 L of hydrogen at 0 °C and 1 atm, using the ideal-gas molar volume of about 22.4 L/mol. The corresponding oxygen volume is about half as large. The reaction also consumes about 0.34 g of water. These figures assume ideal reaction efficiency; side reactions and gas losses can make the actual amounts different.

This example shows what a sustained current does: it continuously drives chemical conversion. It does not show that an arbitrary supply connected to arbitrary water will produce 1 A. Pure water may carry only a small current, while a more conductive solution or different electrodes can allow much more.

Common mistakes

• Assuming water is a perfect insulator: Pure water conducts poorly, but real water usually contains ions and can conduct. Poor conduction is not the same as no conduction.
• Treating water like a metal wire: The mobile charge carriers in the liquid are primarily ions, and electrode reactions connect ionic current to electron current in the wires.
• Thinking the voltage source guarantees unlimited current: Current also depends on resistance, electrode reactions, and the changing chemical state of the cell.
• Assuming all current makes hydrogen and oxygen: The electrodes or dissolved substances may undergo other reactions, especially when the electrodes are not inert.
• Confusing a long-lasting current with a perpetual process: Maintaining current requires ongoing energy transfer and, in practice, suitable reactants and operating conditions.

So, current can be sustained through water while the circuit and electrochemical reactions remain viable. In a real finite cell, it will not run forever without changes to the water, electrodes, gases, or supply.