Conduction in liquids and gases

An ion is a charged atom (or group of atoms).

• Positive ion: atom lost one or more electrons → net positive charge.
• Negative ion: atom gained one or more electrons → net negative charge.

A classic formation example: when magnesium combines with chlorine, magnesium gives up two valence electrons. Each chlorine atom gains one electron. Result: a positive magnesium ion and two negative chloride ions bound in magnesium chloride — a metallic salt similar in concept to table salt (sodium chloride).

In liquids and gases, ion movement — not individual electron drift through a metal — carries the current.

Pure distilled water is an excellent insulator. Add an acid, alkali, or metallic salt, and the solution becomes a conductor. The dissolved compound splits into ions that migrate when a voltage is applied.

Solutions of acids, alkalies, and metallic salts are called electrolytes.

Demonstration logic: two electrodes in a glass, lamp in series as a current limiter. Pure water → lamp off. Salt added → lamp glows; brightness rises until the solution saturates and lamp resistance limits current.

Not every ion is a single atom. Sulfuric acid in water separates into hydrogen ions and a sulfate ion (a group of atoms carrying extra electrons). Copper sulfate in water yields cupric ions and sulfate ions — the copper plating bath used in industry.

These compound ions matter because battery and plating chemistry depends on them, not only on H⁺ and metal ions.

Electroplating deposits atoms of one metal onto another surface using an electrolyte and direct current.

Rules that always apply:

1. Electrolyte contains positive ions of the metal being plated.
2. Object to be plated → cathode (negative terminal — electrons enter the workpiece).
3. Anode (positive terminal) is made of the plating metal.
4. DC only — polarity controls which ions move where.

Positive metal ions move to the cathode, pick up electrons, and become neutral metal atoms on the surface. Coating thickness depends on time and current. Electroplating also refines copper for wire: impurities stay at the anode; pure copper deposits on the cathode.

Electrolysis is the reverse idea: use electric current to split a compound into its elements. Historically this made aluminum practical — aluminum oxide is abundant, but separating aluminum from oxygen by chemistry alone was costly until electrolysis scaled up.

In the cell, electrons are removed from oxygen ions and returned to metal ions; neutral metal atoms form at one electrode and oxygen at the other.

At normal pressure, air is an insulator. Raise voltage across a gap until ionization potential is reached → an arc forms; current jumps the gap. Once established, the arc has very low resistance; voltage can often be reduced while current continues because ionized gas molecules maintain conduction.

Factors affecting breakdown voltage:

Welding: touch electrode to work, withdraw slightly — arc starts, then holds at lower voltage.

In gas conduction the dominant mechanism is electron impact. An electron freed from the cathode accelerates toward the anode. If it gains enough energy before hitting a gas molecule, it knocks out more electrons → avalanche current. Molecules left short of electrons become positive ions drawn to the cathode.

Low pressure lets electrons travel farther and reach ionization energy; high pressure means frequent collisions without enough energy — no sustained glow.

Applications: sodium/mercury/neon lighting (color from gas type), historical cathode-ray displays, X-ray production when energetic electrons strike metal or glass targets.

Situation. A helper tests “continuity” of battery acid by dipping two bare meter leads into a cell while the charger is connected. The meter reads current; they declare the electrolyte “good.”

What went wrong. They measured ion current through a live electrolyte, not safe cell health. They bypassed normal limits, risked sparking at the leads, and ignored specific gravity and loaded voltage — the proper battery checks (covered in later lessons).

Applied lesson. Liquid conduction needs ions and a complete external path. Treat live electrolytes like energized conductors: correct PPE, no improvised probes across charged cells, use a hydrometer or approved tester.