Question
Classify the following materials as conductors or insulators: copper, wood, iron, plastic, silver, rubber, graphite, distilled water. Explain what makes a material conduct or not conduct electricity.
Solution — Step by Step
Conductors are materials that allow electric current to flow through them easily. They have free electrons (or free ions in solution) that can move through the material.
Insulators are materials that do not allow electric current to flow through them easily. Their electrons are tightly bound and cannot move freely.
A third category — semiconductors — conduct partially (e.g., silicon, germanium). For this question, we focus on conductors and insulators.
| Material | Classification | Reason |
|---|---|---|
| Copper | Conductor | Metallic bonding — free valence electrons move easily |
| Iron | Conductor | Metal — free electrons; also magnetic properties |
| Silver | Conductor | Best electrical conductor at room temperature |
| Graphite | Conductor | Carbon allotrope with delocalized electrons in layer structure |
| Wood (dry) | Insulator | Covalent bonding, no free electrons; however, wet wood conducts |
| Plastic | Insulator | Polymer — all electrons tightly bonded to polymer chains |
| Rubber | Insulator | Used for insulating electrical wires — no free charges |
| Distilled water | Insulator (poor conductor) | No ions present; pure H₂O barely dissociates |
In metals, atoms give up their valence electrons to form a “sea” or “cloud” of free electrons shared by all atoms. These electrons are mobile — when a voltage (potential difference) is applied, they drift toward the positive terminal, constituting an electric current.
Better conductors have more mobile electrons and fewer collisions. Silver has the most mobile electrons with fewest collisions → best conductor. Nichrome has many collisions → high resistance → used in heater elements.
Graphite is unusual: it is the only non-metal that is a good conductor of electricity.
In graphite, each carbon atom forms three covalent bonds in a planar layer structure. The fourth electron of each carbon is delocalized — free to move across the entire layer. These delocalized electrons allow graphite to conduct electricity along the layers.
This is why graphite is used in dry cells (electrodes) and as a lubricant at high temperatures.
Diamond (also pure carbon) is an insulator — all four valence electrons form covalent bonds with neighbouring carbons; no free electrons.
Distilled water is a very poor conductor because it contains almost no ions. Water molecules (H₂O) barely ionise () — at room temperature, only about 1 in water molecules is ionised.
Tap water conducts because it contains dissolved salts (Na⁺, Ca²⁺, Cl⁻, etc.) that carry charge.
Salt solution is a good conductor — the ions carry the current. This is why a simple circuit tester (bulb + battery + electrodes) works with salt water but not distilled water.
Why This Works
The key to conductivity is the availability of charge carriers — mobile charges that can flow in response to an electric field:
- Metals: Mobile electrons (electron conduction)
- Electrolyte solutions: Mobile ions (ionic conduction)
- Graphite: Delocalized electrons in layers
Insulators have all their electrons tightly bound in bonds or localised atomic orbitals — no free charges to carry current.
Conductivity is not a binary property — it’s a spectrum. Resistivity () quantifies how strongly a material resists current flow: for silver it’s Ω·m; for rubber it’s around Ω·m — a difference of !
Alternative Method — Band Theory (for higher classes)
In band theory (Class 12 / JEE):
- Conductors: Conduction band and valence band overlap, or conduction band is partially filled → electrons easily move to higher energy levels
- Insulators: Large band gap (>3 eV) → electrons can’t jump from valence to conduction band → no current
- Semiconductors: Small band gap (0.5–2 eV) → electrons can jump with some energy input
CBSE Class 6 and Class 8 science questions on conductors/insulators focus on practical identification. For exams: memorise that metals are conductors (except liquid metals = electrolytes in some contexts), and most non-metals are insulators except graphite. The exceptions (graphite, electrolyte solutions) are almost always part of the question.
Common Mistake
Students often say “all non-metals are insulators.” Graphite is a clear counterexample — it is a non-metal that conducts electricity. Similarly, solutions of ionic compounds (like NaCl dissolved in water) conduct electricity even though the pure ionic compound as solid has limited conductivity (ions are fixed in the crystal lattice). The rule is about charge carriers, not about being a metal or non-metal.