When Do Substances Conduct Electricity? A Phase-by-Phase Guide

A dry crystal of sodium chloride does not conduct electricity in an ordinary school conductivity test. Dissolve that same salt in water, and the solution conducts. Both samples contain \\(\ce{Na+}\\) and \\(\ce{Cl-}\\) ions, so what changed?

Before reading on, make a prediction: did the water create the ions, or did it change whether they could move? The ions were already present in the crystal. Dissolving the salt separates them and allows them to move through the water.

That is the key to almost every HSC question about electrical conductivity: identify the charge carriers, then decide whether they can move in the substance’s current state.

01The quick decision rule

Use these three steps whenever a question asks whether a substance conducts electricity.

1. Identify the possible charge carriers. Look for mobile ions or delocalised electrons.
2. Check whether they can move in the phase being tested. Ions locked into a solid lattice cannot carry current across the sample. Mobile ions in a liquid can.
3. Explain the result using the particles and their movement. Don’t stop at saying that a substance conducts because it is ionic, or doesn’t conduct because it is covalent.

The same substance can therefore conduct in one phase and fail to conduct in another. The bonding, structure, and physical state all matter.

02Identify the charge carriers

Ionic substances: the ions need to move

Solid sodium chloride contains positively charged sodium ions and negatively charged chloride ions. These ions occupy fixed positions in a regular crystal lattice. They vibrate, but they cannot move freely from one position to another through the solid.

Apply a potential difference across the crystal, and the ions still cannot travel through it to carry a sustained current.

Melting the sodium chloride changes the situation. The ordered lattice breaks down, allowing the ions to move through the liquid. Sodium chloride now conducts electricity.

Dissolving it in water produces the same essential change in mobility:

\\[ \ce{NaCl(s) -> Na+(aq) + Cl-(aq)} \\]

The notation \\(\ce{(aq)}\\) means that the ions are dissolved in water. Water molecules surround the separated ions, allowing them to move through the solution.

A useful way to picture this is a school assembly where every student has a marked spot on the floor. Everyone is present, but nobody can cross the hall. Melting or dissolving the salt is like clearing the floor so people can move.

The analogy has a limit: ions do not march neatly in straight lines. Their motion is largely random, but an applied electric field produces a net drift in a particular direction.

A substance that conducts through mobile ions when molten or dissolved is called an electrolyte in this context.

Remember the three cases:

• Solid NaCl: no significant conductivity because the ions are fixed.
• Molten NaCl: conducts because the ions are mobile.
• NaCl dissolved in water: conducts because the dissolved ions are mobile.

Not every ionic compound dissolves in water. An insoluble ionic solid will not form a conducting aqueous solution simply because you add water, although it may still conduct when molten.

Metals: electrons carry the current

Copper conducts electricity as a solid. Unlike solid sodium chloride, it does not need to melt before its charge carriers can move.

In the metallic bonding model, positive metal ion cores occupy the structure while valence electrons are delocalised across it. These electrons can move through the metal when a potential difference is applied.

The positive ion cores remain in their lattice positions in solid copper. The electrons, not the positive ions, are the main charge carriers.

Molten copper also conducts because mobile electrons remain available in the liquid metal.

This gives you an important comparison:

• Copper conducts as a solid because it has mobile electrons.
• Solid sodium chloride does not conduct significantly because its ions are fixed.
• Molten sodium chloride conducts because its ions become mobile.

The fact that all three samples contain charged particles is not enough to explain their behaviour. You must identify which particles can actually move.

Simple covalent molecules: dissolving is not the same as ionising

Consider sugar, such as sucrose.

Sugar dissolves in water, but a sugar solution does not conduct electricity significantly under ordinary school laboratory conditions. The dissolved sucrose particles remain neutral molecules. They spread throughout the water, but they do not become mobile ions just because they dissolve.

This is a common trap. Solubility tells you whether a substance dissolves; conductivity depends on whether mobile charged particles are available.

Hydrogen chloride provides an important exception to the rule that molecular substances do not conduct.

Before dissolving, hydrogen chloride consists of covalent molecules. When it dissolves in water, it reacts with the water to form ions:

\\[ \ce{HCl + H2O -> H3O+ + Cl-} \\]

The \\(\ce{H3O+}\\) ions are hydronium ions. The \\(\ce{Cl-}\\) ions are chloride ions. Both can move through the solution, so hydrochloric acid conducts electricity strongly.

Notice what this tells you. The original bonding type alone does not determine conductivity in water. You must consider what particles are actually present after the substance dissolves.

Covalent networks: why graphite conducts

Most simple covalent molecular substances do not conduct significantly. But graphite is a covalent network solid, and it conducts electricity.

In graphite, each carbon atom forms three covalent bonds with neighbouring carbon atoms. One valence electron per carbon atom is delocalised, meaning it is not confined to a single covalent bond. These electrons can move through the layers and carry electrical current.

Diamond has a different structure. Each carbon atom forms four covalent bonds, using all four of its valence electrons in those bonds. Diamond therefore has no comparable supply of mobile electrons and does not conduct electricity under ordinary conditions.

This is why the statement “covalent substances do not conduct electricity” is incorrect. Graphite and diamond are both covalent networks, but their structures give them different electrical properties.

03Phase-by-phase reference table

Use this table when you need to make a quick prediction in an exam. The answers describe ordinary HSC-level conditions and significant conductivity, rather than claiming that every material has mathematically zero or infinite conductivity.

Two qualifications are worth remembering.

• An aqueous solution conducts through ions only if enough mobile ions are present. A soluble ionic compound generally supplies these ions, whereas sugar does not. A covalent substance such as hydrogen chloride can also form ions by reacting with water.
• Pure or distilled water is a very poor conductor, rather than a perfect insulator. It contains tiny amounts of ions, and ordinary tap water often conducts better because it contains dissolved substances.

04Reading a conductivity test

A lamp or ammeter tells you whether current can pass through a sample. It does not, by itself, identify the charge carriers.

For a quantitative question, you may need to use the electrical measurements as well as the chemistry.

The relationship between resistance, potential difference, and current is:

\\[ R = \frac{V}{I} \\]

where:

• \\(R\\) is electrical resistance, measured in ohms (\\(\Omega\\)).
• \\(V\\) is potential difference, measured in volts (V).
• \\(I\\) is electric current, measured in amperes (A).

Conductance is the reciprocal of resistance:

\\[ G = \frac{1}{R} \\]

Conductance is measured in siemens (S). For the same conductivity cell, a higher current at the same voltage indicates greater conductance.

However, the measured current also depends on the electrode spacing, immersed electrode area, temperature, and other experimental conditions. To compare two samples fairly, keep these conditions the same.

05Common mistakes

– “Solid sodium chloride contains ions, so it must conduct.” The ions exist, but they are locked into fixed lattice positions. Having charge is not the same as being able to carry current.

– “Anything that dissolves in water conducts.” Sugar dissolves, but its molecules remain neutral. Dissolving a substance does not necessarily produce ions.

– “Covalent substances never conduct.” Graphite conducts through delocalised electrons, and hydrogen chloride forms ions in water.

– “Metals conduct because their positive ions move.” In a solid metal, the positive ion cores are fixed in the lattice. Delocalised electrons carry the current.

– “A lit lamp proves that every component of a mixture conducts.” A single component may provide a continuous conducting path while the other components remain non-conducting.

– “Four times the current means exactly four times as many ions.” Current depends on the overall conductivity and experimental conditions. Ion concentration matters, but ion mobility and interactions also affect the result.

06Questions and solutions

Question 1

A student tests solid sodium chloride, molten sodium chloride, and sodium chloride dissolved in water.

Which samples conduct electricity? Identify the charge carriers in each case and explain the difference between the solid and the two liquid samples.

Solution 1

Molten sodium chloride and sodium chloride dissolved in water conduct electricity significantly; solid sodium chloride does not.

In the solid crystal, \\(\ce{Na+}\\) and \\(\ce{Cl-}\\) ions occupy fixed lattice positions. They cannot move freely through the sample to carry current.

When the salt melts, the lattice breaks down and the ions become mobile. When the salt dissolves in water, the ions separate and become surrounded by water molecules, allowing them to move through the solution.

In both liquid samples, the charge carriers are ions. The difference is their mobility, not whether the ions exist.

Question 2

A student takes 20.0 mL of \\(0.50\ \mathrm{mol\,L^{-1}}\\) sodium chloride solution and dilutes it with water to a final volume of 100.0 mL.

(a) Calculate the final concentration of sodium chloride.

(b) Predict how the current will change if the diluted solution is tested using the same conductivity cell, at the same voltage and temperature.

(c) The student argues that the current must remain unchanged because no sodium chloride has been removed. Explain the flaw in this reasoning.

Solution 2

The final concentration is \\(0.10\ \mathrm{mol\,L^{-1}}\\), and the current should decrease under the stated conditions, although not necessarily by exactly a factor of five.

Part (a): Calculate the concentration.

During dilution, the amount of solute stays constant. Use:

\\[ C_1V_1 = C_2V_2 \\]

Here, \\(C_1\\) is the initial concentration, \\(V_1\\) is the initial volume, \\(C_2\\) is the final concentration, and \\(V_2\\) is the final volume.

Rearrange to find \\(C_2\\):

\\[ \begin{aligned} C_2 &= \frac{C_1V_1}{V_2}\\ &= \frac{0.50 \times 20.0}{100.0}\\ &= 0.10\ \mathrm{mol\,L^{-1}} \end{aligned} \\]

The volume units cancel because both volumes are in millilitres.

Part (b): Predict the current.

The diluted solution still contains mobile \\(\ce{Na+}\\) and \\(\ce{Cl-}\\) ions, so it still conducts. However, the concentration has fallen to one-fifth of its original value. There are fewer ions per unit volume available to carry charge, so the current is expected to decrease in the same cell at the same voltage and temperature.

Part (c): Explain the flaw.

The student is confusing the total amount of solute with its concentration.

The total amount of sodium chloride has not changed, but the ions are spread throughout a larger volume of water. The concentration falls, and so does the solution’s conductivity under these conditions.

The current does not have to fall by exactly five times. The relationship between concentration and conductivity is not perfectly proportional under all conditions because ion mobility and interactions between ions also matter.

Question 3

Copper wire, molten sodium chloride, and solid graphite all conduct electricity.

A student concludes that all three substances must contain mobile ions.

Explain why this conclusion is incorrect. For each substance, identify its charge carrier and relate the carrier to the substance’s structure.

Solution 3

The conclusion is incorrect because copper and graphite conduct mainly through electrons, whereas molten sodium chloride conducts through ions.

In copper, delocalised electrons move through the metallic structure when a potential difference is applied. The positive metal ion cores remain in their lattice positions in the solid.

In molten sodium chloride, the \\(\ce{Na+}\\) and \\(\ce{Cl-}\\) ions can move through the liquid. These mobile ions carry charge.

In graphite, each carbon atom forms three covalent bonds, leaving one electron per carbon atom delocalised. These electrons can move through the graphite layers and carry current.

All three substances conduct, but the observation does not imply that they share the same bonding or charge carriers. The structure of each substance determines which charged particles can move.

Question 4

Two equal-volume, dilute aqueous solutions have the same molar concentration. One contains hydrogen chloride, \\(\ce{HCl}\\), and the other contains sucrose, a type of sugar.

A student predicts that neither solution will conduct because both solutes are covalent molecular substances. Another student predicts that both will conduct because both solutes dissolve in water.

Identify which prediction is correct, and explain the conductivity of each solution. Use a chemical equation in your answer.

Solution 4

Neither prediction is correct: the hydrogen chloride solution conducts strongly, while the sucrose solution conducts very poorly under ordinary conditions.

Hydrogen chloride consists of covalent molecules before dissolving. In water, it reacts to form hydronium and chloride ions:

\\[ \ce{HCl + H2O -> H3O+ + Cl-} \\]

These ions are mobile and carry current through the solution. Hydrogen chloride is therefore a strong electrolyte in water.

Sucrose behaves differently. It dissolves, but the sucrose molecules remain neutral. They do not produce a significant concentration of mobile ions, so the solution conducts very poorly.

The first student’s mistake is assuming that the original bonding type alone determines conductivity after dissolving. The second student’s mistake is treating solubility as evidence of ion formation.

The important distinction is what particles are present in the solution and whether they carry charge.

Question 5

A technician presses a dry mixture of sodium chloride crystals and graphite powder between two electrodes. The conductivity tester lights.

Assume the graphite particles touch to form a continuous path between the electrodes. The technician then stirs the mixture into water, dissolves the sodium chloride completely, and filters out all the undissolved graphite. The clear filtrate also conducts electricity.

A student draws two conclusions:

• The dry sodium chloride must conduct electricity.
• Graphite ions must be responsible for the conductivity of the filtrate.

Evaluate both conclusions. Propose a set of control tests that would distinguish the role of each substance in the two experiments.

Solution 5

Neither conclusion is correct. The dry mixture conducts through the continuous graphite path, while the filtrate conducts because dissolved sodium chloride supplies mobile ions.

The tempting interpretation is that the same mixture conducts before and after adding water, so the same substance must be responsible both times. The crucial constraint is that the phase and composition of the sample change between the two tests.

Before adding water, the graphite particles form a continuous conducting path. Delocalised electrons can move through the graphite network and carry current between the electrodes. The sodium chloride crystals remain solid, with their ions fixed in their lattices. The fact that the mixture conducts does not show that solid sodium chloride conducts.

After adding water and filtering, the situation is different. Sodium chloride dissolves, producing mobile \\(\ce{Na+}\\) and \\(\ce{Cl-}\\) ions. The graphite remains insoluble and is removed by filtration. The ions in the clear filtrate carry the current; graphite ions are not responsible.

A suitable set of controls would be:

• Test dry sodium chloride alone and dry graphite alone using the same electrode arrangement. This distinguishes the behaviour of the two solids.
• Prepare a sodium chloride solution without graphite and test it using the same conductivity cell.
• Test the water used in the experiment as a blank, to check whether background conductivity affects the result.

These controls separate the two possible pathways: electronic conduction through solid graphite and ionic conduction through dissolved sodium chloride.

The deeper lesson is that a conductivity result applies to the actual sample and conditions tested. A mixture can conduct even when one of its components cannot, and two tests that both produce a lit lamp can have completely different explanations.

07What to check next: conductivity or electrolysis?

Once you can identify the mobile charge carriers, the next useful question is whether the current simply passes through the material or causes a chemical change.

That is the starting point for electrolysis. In an electrolytic cell, mobile ions carry charge through the electrolyte, while electrons carry charge through the external circuit. Chemical reactions occur at the electrode surfaces, where oxidation and reduction transfer charge between the electrodes and the reacting particles.

The distinction to carry forward is simple: ions move through the electrolyte, electrons move through the external circuit, and the electrode reactions connect the two.