Physics · Lesson 24

Conduction through liquids & gases

JAMBWAECNECO

What the syllabus asks

JAMB: Conduction of electricity through liquids and gases: electrolysis, Faraday’s laws, applications; discharge through gases, hot cathode, uses

WAEC: Conduction of electricity through liquids: electrolytes and non-electrolytes, electrolysis, ions, electrodes, Faraday’s laws of electrolysis, electrochemical equivalent, voltameters, applications (electroplating, refining, extraction); conduction through gases: discharge at low pressure, glow discharge, cathode rays, applications (fluorescent and neon lamps), lightning

NECO: Conduction of electricity through liquids: electrolytes and non-electrolytes, electrolysis, ions, electrodes, Faraday’s laws of electrolysis, electrochemical equivalent, voltameters, applications (electroplating, refining, extraction); conduction through gases: discharge at low pressure, glow discharge, cathode rays, applications (fluorescent and neon lamps), lightning

What you will learn

Distinguish electrolytes from non-electrolytes and explain conduction by ions.
Describe the movement of ions and products at electrodes in simple electrolytic cells.
State Faraday’s laws and use m = zIt and m = MIt/(nF).
Describe how to determine the electrochemical equivalent of copper experimentally.
Explain electroplating, refining and extraction of metals.
Describe conduction through gases at low pressure and its applications, and explain lightning.

Key points

Electrolytes conduct by ions and are decomposed; mercury and metals conduct by electrons.
Cations go to the cathode (−); anions go to the anode (+).
Faraday 1: m = zIt (t in seconds, z in kg/C).
Faraday 2: same Q → m ∝ M/n; F = 96 500 C per mole of electrons; m = MIt/(nF).
Copper electrodes in CuSO₄: anode loses mass, cathode gains; concentration constant.
Electroplating: object = cathode, plating metal = anode, its salt as electrolyte; d.c. only.
Gases conduct when ionised; low-pressure discharge gives coloured glows; cathode rays are electrons.
Applications: fluorescent and neon lamps, sodium lamps; lightning and lightning conductors.

Start here: Which liquids conduct?

Metals conduct because they contain free electrons. Most liquids have no free electrons, yet some still conduct. Test a few liquids with a battery, two carbon rods and a bulb:

Liquid	Bulb lights?	Type
Salt solution	Yes	Electrolyte
Dilute sulphuric acid	Yes	Electrolyte
Copper(II) sulphate solution	Yes	Electrolyte
Sugar solution	No	Non-electrolyte
Distilled water	Very faintly / no	Very weak
Paraffin (kerosene)	No	Non-electrolyte
Mercury	Yes	Liquid metal (electrons)

An electrolyte conducts by the movement of ions and is decomposed chemically as it conducts. This process is electrolysis.

Mercury conducts like any metal, through free electrons, and is not decomposed, so it is not an electrolyte.

Worked example 24.1

Why does solid sodium chloride not conduct, but molten or dissolved sodium chloride does?

In the solid, Na⁺ and Cl⁻ ions are fixed in the crystal lattice.
When molten or dissolved, the ions are free to move and carry charge.

Ions must be free to move

⚠️ Common mistake: saying electrons flow through the electrolyte. In the liquid, charge is carried by ions; electrons flow only in the external wires.

Try it yourself. Is ethanol solution likely to conduct?

Show solution

No; ethanol is a covalent molecule that does not form ions.

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Practice questions

1. Find the mass of copper deposited by 0.5 A in 2 hours (z = 3.3 × 10⁻⁷ kg/C).

Show worked answer

Q = 3600 C; m = 1.19 g.

2. How long does 4 A take to deposit 2.64 g of copper?

Show worked answer

Q = 2.64 × 10⁻³/3.3 × 10⁻⁷ = 8000 C; t = 2000 s.

3. A silver and a copper voltameter in series: 0.635 g copper is deposited. Find the silver mass (Ag 108, n = 1; Cu 63.5, n = 2).

Show worked answer

mol e⁻ = 2 × 0.635/63.5 = 0.02; Ag = 0.02 × 108 = 2.16 g.

4. State Faraday’s first law.

Show worked answer

The mass liberated is directly proportional to the quantity of charge passed.

5. Describe how to nickel-plate an iron key.

Show worked answer

Key as cathode, nickel anode, nickel salt solution, steady d.c.

Key words

Electrolyte: A compound that conducts electricity when molten or dissolved, and is decomposed by it.
Non-electrolyte: A liquid that does not conduct, e.g. sugar solution, paraffin.
Electrode: A conductor through which current enters or leaves an electrolyte.
Anode: The electrode connected to the positive terminal.
Cathode: The electrode connected to the negative terminal.
Ion: An atom or group of atoms carrying an electric charge.
Voltameter: A vessel containing electrodes and electrolyte in which electrolysis occurs.
Electrochemical equivalent (z): Mass deposited by 1 coulomb of charge; unit kg/C.
Faraday constant (F): Charge carried by one mole of electrons, about 96 500 C/mol.
Electroplating: Coating an object with a thin layer of metal by electrolysis.
Ionisation: The production of ions from neutral atoms or molecules.
Glow discharge: Light emitted when current passes through a gas at low pressure.

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