Quick Summary
- Graphite has distinct physical properties different from other carbon allotropes
- Each property connects directly to its layered crystal structure
- Properties include black color, soft texture, electrical conductivity, and high melting point
- Understanding these properties explains graphite’s many practical uses
- WAEC questions often ask you to link properties to structure and uses
What Are Physical Properties?
Physical properties are characteristics you can observe or measure without changing the substance into something else. For graphite, you can see its black color, feel its slippery texture, measure its density, and test its electrical conductivity without turning it into a different chemical.
These properties make graphite very different from diamond even though both are pure carbon. The difference comes from how the carbon atoms arrange themselves in each allotrope.
Detailed Physical Properties of Graphite
1. Color and Appearance
Property: Graphite is black and opaque. You cannot see through it at all.
Explanation: The delocalized electrons in graphite absorb all wavelengths of visible light. No light passes through, making it completely opaque. The absorbed light energy spreads throughout the structure.
Comparison: Diamond is colorless and transparent when pure because all its electrons are locked in covalent bonds and cannot absorb visible light.
2. Lustre
Property: Graphite has a metallic lustre. It shines like metal when light reflects off its surface.
Explanation: The flat layers of graphite create smooth crystal surfaces that reflect light well. The free electrons also contribute to this metallic appearance, similar to how metals shine.
Practical observation: When you write with a pencil, the graphite marks on paper have a slight shine, especially if you rub them smooth.
3. Texture and Hardness
Property: Graphite is very soft with a slippery, greasy feel. It ranks only 1-2 on the Mohs hardness scale (talc is 1, the softest mineral).
Explanation: The layers in graphite are held together by weak van der Waals forces. These forces are much weaker than the strong covalent bonds within each layer. When you touch graphite or press on it, the layers easily slide over each other. This sliding creates the slippery feeling.
Why it matters: This softness lets graphite leave marks on paper (pencils), reduce friction between moving parts (lubricants), and break away easily when you rub it.
Key point for exams: The layers themselves are NOT weak. Each layer has very strong covalent bonds. Only the forces BETWEEN layers are weak.
4. Density
Property: Graphite has a density of 2.3 g/cm³.
Explanation: The layered structure of graphite has significant empty space between the layers. This spacing is about 3.35 Ångströms (0.335 nanometers), which is much larger than the distance between atoms within a layer. These gaps reduce the overall density.
Comparison: Diamond has a density of 3.5 g/cm³ because its atoms pack more tightly in a three-dimensional tetrahedral network with less empty space.
5. Electrical Conductivity
Property: Graphite is a good conductor of electricity. This is unusual for a non-metal.
Explanation: Each carbon atom in graphite forms only three covalent bonds using three of its four valence electrons. The fourth electron from each atom is delocalized, meaning it is not tied to any particular atom. These free electrons can move along the layers when you apply a voltage, creating an electric current.
Important detail: Graphite conducts electricity well along the layers (parallel to the sheets) but conducts poorly perpendicular to the layers. This directional conductivity is called anisotropic conductivity.
Why diamond does not conduct: In diamond, all four valence electrons from each carbon atom form strong covalent bonds. No electrons are free to move, so diamond cannot conduct electricity.
6. Thermal Conductivity
Property: Graphite conducts heat moderately well, better than most non-metals but not as well as metals.
Explanation: The delocalized electrons and vibrations of the carbon atoms transfer heat energy through the structure. Like electrical conductivity, thermal conductivity is better along the layers than across them.
7. Melting and Boiling Points
Property: Graphite has no true melting point at normal pressure. It sublimes (changes directly from solid to gas) at about 3,600°C. At very high pressure, it can melt at around 4,000°C.
Explanation: The strong covalent bonds within each layer require enormous energy to break. Breaking these bonds is necessary for graphite to melt or boil. The weak forces between layers allow them to separate at much lower temperatures, but this is not melting.
Practical importance: This very high sublimation point makes graphite useful for high-temperature applications like crucibles for melting metals and electrodes in electric arc furnaces.
8. Density Compared to Water
Property: Graphite sinks in water because its density (2.3 g/cm³) is greater than water’s density (1.0 g/cm³).
Practical note: If graphite powder falls into water during laboratory work, it will sink to the bottom, though it may take time if the particles are very fine and trap air bubbles.
9. Chemical Stability
Property: Graphite is relatively unreactive at room temperature but more reactive than diamond.
Explanation: The layered structure exposes more carbon atoms at the edges of layers, making them available for chemical reactions. In diamond, the three-dimensional network protects most atoms from attack.
Summary Table of Physical Properties
| Property | Value/Description | Cause (linked to structure) |
|---|---|---|
| Color | Black, opaque | Delocalized electrons absorb all visible light wavelengths |
| Lustre | Metallic, shiny | Flat crystal surfaces reflect light; free electrons present |
| Hardness | Very soft (1-2 Mohs scale) | Weak van der Waals forces between layers allow easy sliding |
| Texture | Slippery, greasy feel | Layers slide over each other easily |
| Density | 2.3 g/cm³ | Large spacing between layers creates empty space in structure |
| Electrical conductivity | Good conductor | One delocalized electron per carbon atom moves freely along layers |
| Thermal conductivity | Moderate conductor | Free electrons and atomic vibrations transfer heat energy |
| Sublimation point | ~3,600°C at normal pressure | Very strong covalent bonds within layers need high energy to break |
| Solubility in water | Insoluble | Strong covalent bonds not broken by water molecules |
| Reactivity | More reactive than diamond | Exposed atoms at layer edges available for reactions |
How Properties Relate to Uses
| Property | Use | How the Property Helps |
|---|---|---|
| Soft and slippery | Pencil “lead” | Layers rub off easily onto paper to leave black marks |
| Soft and slippery | Lubricant in machines | Reduces friction between moving metal parts |
| Conducts electricity | Electrodes in batteries | Allows electric current to flow in and out of battery |
| Conducts electricity | Electrolysis electrodes | Carries current without reacting with chemicals being electrolyzed |
| High sublimation point | Crucibles for melting metals | Does not melt even at temperatures that melt iron or steel |
| High sublimation point | Electric arc furnace electrodes | Withstands extreme temperatures in steel making |
| Black color | Pigment in paints and inks | Provides deep black color that does not fade |
| Chemically stable | Nuclear reactor moderator | Does not react with radioactive materials at high temperatures |
| Slippery layers | Brake linings | Helps brake pads slide smoothly and resist heat |
Common Exam Mistakes
Based on WAEC Chief Examiner reports, students commonly make these errors:
- Stating properties without explaining the cause: Writing “graphite is soft” gets only 1 mark. Writing “graphite is soft because weak van der Waals forces between layers allow them to slide easily” gets full marks. Always link properties to structure.
- Confusing “weak layers” with “weak bonds in layers”: The layers are NOT weak. Each layer has very strong covalent bonds. Only the forces BETWEEN layers are weak. Many students lose marks by saying “graphite has weak bonds.”
- Wrong electron count: Saying “graphite has four free electrons per atom” or “some electrons are free” is too vague. Be specific: “Each carbon atom has ONE delocalized electron” (three are used in bonding, one is free).
- Claiming graphite melts easily: Some students say graphite melts at low temperature because it is soft. This is wrong. Softness relates to forces between layers. Melting requires breaking strong covalent bonds within layers, needing very high temperature.
- Not comparing with diamond: When questions ask you to compare graphite and diamond, some students describe only one. You must discuss BOTH and highlight the differences clearly.
- Mixing properties with uses: When asked to state properties, students sometimes write uses instead. “Used in pencils” is a use, not a property. The property is “soft and leaves black marks.”
- Incomplete density answers: Writing just “2.3 g/cm³” without units loses marks. Always include units in all measurements.
- Wrong directional conductivity: Advanced students should know graphite conducts better along layers than across them. Saying “conducts equally in all directions” is incorrect.
Practice Questions
Multiple Choice Questions
- The density of graphite is lower than diamond because:
- a) Graphite has weaker bonds than diamond
- b) Graphite has larger spaces between its layers ✓
- c) Diamond has more carbon atoms per molecule
- d) Graphite contains impurities
- Graphite conducts electricity because it has:
- a) Mobile ions between layers
- b) Weak forces that allow electron movement
- c) One delocalized electron per carbon atom ✓
- d) Metallic bonding like copper
- Which property of graphite makes it suitable for use in pencils?
- a) High melting point
- b) Electrical conductivity
- c) Soft layers that slide easily ✓
- d) Low density
- The forces between layers in graphite are:
- a) Strong covalent bonds
- b) Weak van der Waals forces ✓
- c) Ionic attractions
- d) Metallic bonds
- At normal pressure, graphite:
- a) Melts at 3,600°C
- b) Sublimes at 3,600°C ✓
- c) Boils at 3,600°C
- d) Decomposes at 3,600°C
Essay Questions
- (10 marks)
a) State FIVE physical properties of graphite. (5 marks)
b) For each property stated, explain how it relates to the structure of graphite. (5 marks)
Examiner’s tip: Use a table format for clarity. Column 1: Property (e.g., “Soft texture”). Column 2: Structural explanation (e.g., “Weak van der Waals forces between layers allow layers to slide over each other”). This format shows clear thinking and is easy to mark.
- (8 marks)
a) Explain why graphite conducts electricity while diamond does not, even though both are forms of carbon. (4 marks)
b) State TWO uses of graphite that depend on its electrical conductivity. (2 marks)
c) Explain why graphite conducts electricity better along its layers than perpendicular to them. (2 marks)
Examiner’s tip: Draw simple diagrams showing electron arrangement in both graphite and diamond. Visual answers often earn bonus marks for clarity. For part (c), you need to mention that delocalized electrons move easily along layers but not between them.
- (6 marks)
Compare the physical properties of graphite and diamond under the following headings:
a) Hardness (2 marks)
b) Electrical conductivity (2 marks)
c) Density (2 marks)
Examiner’s tip: For each heading, state the property for BOTH substances and briefly explain the structural reason for the difference. Example for hardness: “Graphite is very soft (1-2 Mohs scale) because layers are held by weak van der Waals forces that allow sliding. Diamond is very hard (10 Mohs scale) because each carbon atom forms four strong covalent bonds in a rigid 3D network.”
- (4 marks)
A student claims that graphite is weak because it is soft. Explain why this statement is incorrect.
Examiner’s tip: Distinguish between the strong covalent bonds WITHIN each layer and the weak forces BETWEEN layers. Explain that softness comes from layers sliding, not from weak bonding within the layers. This tests your understanding of structure at a deeper level.
Practical Question
- (5 marks)
Describe how you would demonstrate THREE different physical properties of graphite in the laboratory. State what you would observe in each case.
Examiner’s tip: Choose properties you can actually test: (1) Electrical conductivity – connect graphite rod to battery and bulb, observe bulb lights up. (2) Softness – rub graphite on paper, observe black mark left behind. (3) Density – place graphite in water, observe it sinks. Be specific about observations.
Memory Aids
Remember key properties with “BSLED”:
- Black and opaque
- Soft and slippery (low hardness)
- Lustre (metallic shine)
- Electricity conductor (unlike diamond)
- Density is 2.3 g/cm³ (lower than diamond’s 3.5)
Remember why graphite is soft: “Weak between, strong within” – weak forces BETWEEN layers (allow sliding), strong bonds WITHIN layers.
Remember conductivity: “3 + 1 rule” – each carbon forms 3 bonds, leaving 1 electron free to conduct electricity.
Graphite vs Diamond quick comparison:
- Graphite: 3 bonds, 1 free electron, soft, conducts, black
- Diamond: 4 bonds, 0 free electrons, hard, insulator, clear
Related Topics
Study these related topics to build comprehensive understanding:
- Graphite – Full overview of graphite including occurrence and chemical properties
- Allotropes of Carbon – Learn how different carbon structures create different properties
- Physical Properties of Diamond – Compare and contrast with graphite
- Carbon – Understand the element and its bonding capabilities
- Uses of Diamond – See how opposite properties lead to different applications