*However an excessive quantity was added and there was a solid remnant
Summary of results: D and E were soluble in a non-polar solvent.
NOTE: Neither H nor B was soluble in either polar or non-polar solvents
SECTION B: Conductivity Testing
Aim of section
“To test the conductivity of each of the eight unknown substances both in solution and in a solid state to give an indication of aqueous/solid conductivity, and hence to assist in determining the arrangement of electrons within the solid.”
METHOD:
- Attach power-pack to ammeter
- Connect a crocodile clips to each graphite rod connecting to ammeter
- Set ammeter to 5 Amps
- Either place graphite rods at opposite sides of beakers containing solutions obtained in Section A*, or at either ends of a small quantity of the solid in lid of bottle
*For solutions where cyclohexane was the solvent, either decant the solution into larger (100ml) beaker, or process a larger quantity of the solution in a 100ml beaker to begin with. If solution has been discarded during Section A, then reprocess in much the same way
- Turn power pack on and observe initial reading on ammeter, ensuring that graphite rods remain as distantly apart as the beaker will allow, or that they remain a similar distance apart while making contact with the solid
- Adjust sensitivity of ammeter between the 5 Amp, 50mA and 500mA scale until the most accurate reading can be taken
SAFETY:
- Avoid emptying contents of beakers containing cyclohexane or solutions of substance E into sink
- Avoid contact of graphite rods when power pack is on (i.e. leave power pack off when not in direct use)
- Do not touch un-insulated portion of graphite rods when power is running
- Start ammeter set to 5 Amps
- Avoid unprotected contact with substance E
RESULTS:
Testing
-
Polar Solutions: A, C, F, G
-
Non-Polar Solutions: D, E
-
Solids: B, H
A Table to Show the Conductivity of Each Substance in the Solid State:
A Table to Show the Conductivity of Each Substance in Aqueous Solution:
*Were not soluble in either polar or non-polar solvents
NOTE: Only parallax error is accounted for by uncertainty.
Summary of results: B and H were conductive in the solid state; A, C and F were conductive in aqueous solution only.
SECTION C: Testing Brittleness
Aim of section
“To test the brittleness of each of the eight unknown solids to give an indication namely of the rigidity of the substances structure.”
METHOD:
-
Place a small quantity (≈ 1 spatula) of the solid on the wooden impact board
- Strike the substance lightly (or as required to test the ease with which it may or may not deform in any way) with hammer
- Record any deformation, shattering or effects of blows
- Repeat as necessary to fully observe effects
SAFETY:
- Take special care that protective glasses are worn
- Do not test substance E
RESULTS:
A Table to Show the Effects of Hammering on Each Solid
*E was not meant to be tested
Summary of results: Every tested solid with the exception of B – which flattened – shattered, although D did so with less distinct fracturing.
SECTION D: Finding the Melting Point
Aim of section
“To find the melting point of each of the eight unknown solids to assist in the assessment of the strength of bonds within each solid, and hence to aid in the determination of types of bonds present.”
METHOD:
- Set up all apparatus on heatproof mat
- Set up the retort stand and the boss and clamp
- Set tripod and gauze
- Set Bunsen below tripod
-
Fill 200ml beaker with 100ml paraffin oil1 and place it on gauze
- Take a mortar and pestle to solids unable to fit in boiling tubes (namely C and D)
-
Prepare four boiling tubes2 containing small quantities of the unknowns (spanning 10-20mm up the walls of the tubes
-
Begin to warm paraffin oil (not above ≈ 50°C) if desired
- Attach first two boiling tubes to thermometer with rubber bands (1 each) so that they find their bases at the same level as that of the thermometer
- Clamp thermometer to retort stand, its base approximately central in the volume of paraffin oil
- Note the melting point of each solid, replacing each boiling tube after their contents have melted until melting points for all four test-tubes have been obtained
- Make notes on any additional observations
1Paraffin oil heats more slowly and consistently allowing for a more gradual and measurable increase in temperature
2Only four unknowns are suitable for testing in this manner – C, D, F and G
SAFETY:
- Perform this experiment in an active fume cupboard (particularly with respect to observing the demonstration of E
- Avoid inhalation of any fumes
- Ensure that apparatus is cooled before it is packed away
- Ensure that a match is ready to ignite the Bunsen immediately upon the mains being switched on, and that the Bunsen is on safety when first lit
- Do not discard spent boiling tubes haphazardly, rather dispose of them either in a glass bin or through a lab technician
- Do NOT empty paraffin oil down the sink
RESULTS:
A Table to Show the Melting Points of Each Unknown Substance
*Demonstrated by teacher, Ms Tachas
Summary of results: A, B and H displayed substantially higher melting points that the other substances, with D showing the lowest and H the highest.
Part Three: Interpreting and Analysing Collated Results – the Conjecture
Intention
“To summarise the determined properties of each unknown solid; to analyse these properties and conjecture the structure and bonding present in each substance.”
UNKNOWN SOLID A:
-
Soluble in polar solvent (H2O)
- Substance is polar or ionic itself due to the fact that polar solutes are generally only soluble in polar solvents (as a result of ion-dipole interactions)
- Conductive when in solution
- When in solid state either the electrons or charged ions are not free moving
- Solid shatters when impacted
- Rigid internal structure often associated with a lattice
-
High (801°C) melting point
“A” is soluble in polar solvents and is only conductive in the aqueous state. Since neither metals nor covalent network structures are soluble in water (or, indeed, generally), this – broadly speaking – leaves only two feasible bonding possibilities: covalent molecular, or ionic. Two properties of A, however, distinguish it as an ionic solid – it shatters distinctly (ionic substances fracture under a shearing force due to rigid lattice structure), and its melting point is high at 801°C. Covalent molecular substances exhibit neither conductivity in solution nor such high melting points, because the forces of attraction between the molecules are weak (e.g. van der Waals’/dispersion forces) and easily broken by small quantities of thermal energy. Conversely, ionic substances exhibit very high melting points due to their strongly bound lattice structure. In the case of A, given the fairly high melting point and conductivity in aqueous solution (ionic substances also only conduct electricity in the molten [because their ions become free-moving], and aqueous solution [due to ion-dipole bonding allowing the movement of charged particles] state), it is likely to have a network lattice. So A is:
“an ionic substance with a rigid network lattice structure of an infinite array and strong bonds in three dimensions.”
UNKNOWN SOLID B:
- Insoluble in both polar and non-polar solvents
- Conductive when in solid state
- Electrons are free moving in solid state
- Solid deforms when impacted
- Strong but flexible internal structure
-
High (419°C) melting point
Substance B is insoluble in both polar and non-polar solvents, which implies that it is either a metal or a covalent network structure (which both find themselves largely insoluble). Since it is conductive when in a solid state, and covalent network structures are not conductive (with the exception of graphite, which is a covalent layer lattice structure), this practically leaves metallic bonding as the only logical form of bonding within B. Metallic bonds can conduct electricity when in the solid state due to the free moving ‘sea’ of delocalised electrons that shift between the lattice of metal cations – this not only allows metals to conduct electricity in the solid state, but also results in the malleable and ductile qualities of metal, hence the fact that B deforms upon impact with a hammer. This is further confirmation of the metallic structure of B, which is again corroborated by its high melting point (another result of the strong electrical forces existing between the cation lattice and the delocalised electrons). So it is quite clear that B is:
“a metallic substance consisting of a regular array of metal ions surrounded by a ‘sea’ of delocalised valence electrons, with strong electrostatic forces of attraction between the positive cations and negative electrons. Additionally, due to its relatively high melting point, “A” probably has an h.c.p. or c.c.p. ion arrangement (coordination number of 12) – although this is less certain.”
UNKNOWN SOLID C:
- Conductive in aqueous solution
- When in solid state either the electrons or charged ions are not free moving
-
Fairly low (130°C) melting point
Substance C is formed through polar bonds, is soluble in water, and is only conductive only in aqueous solution. These properties distinguish it as ionic, since covalent molecular substances do not conduct electricity and neither covalent network structures nor metals are soluble in water. Another indicative property is that C shatters on impact with a hammer, again symptomatic of an ionic substance – the rigid lattice structure of an ionic bond and the alignment of charges into a regular array effectively result in a large-scale fracture of an ionic solid as like charges align and repel. However, the m.p. of C is much lower than would be expected of an ionic solid, and this is due to a less efficient coordination within the lattice. So C is:
“an ionic substance with a rigid lattice structure of an infinite array and strong bonds in three dimensions and a poor efficiency coordination.”
UNKNOWN SOLID D:
- Soluble in non-polar solvent
- In neither solid nor aqueous solution state are charged particles free to move
- Shatters/crumbles but with less defined fractures – it breaks up without fracturing
-
Not entirely rigid internal structure
-
Lowest (66°C) melting point
- Weak intermolecular bonds
“D” is soluble only in non-polar solvents and is not conductive. This indicates, along with its exceptionally low melting point (due to the presence of only weak intermolecular bonds requiring only a small quantity of thermal energy to break) that the substance is covalent molecular. Since it is a non-polar covalent molecular, no other intermolecular forces but those of van der Waals’ dispersion forces can exist. This is the weakest type of bonding, and exists between all molecules, and the melting point of this solid is so low because van der Waals’ forces are all that exist. So this substance is:
“a non-polar covalent molecular substance with weak intermolecular dispersion forces serving as the only intermolecular bonding force, accounting for the low boiling point of substance D.”
UNKNOWN SOLID E:
- Soluble in non-polar solvent
- In neither solid nor aqueous solution state are charged particles free to move
- Weak intermolecular bonds
“D” is soluble only in non-polar solvents and is not conductive even when in solution. This indicates that the bonding of substance D is covalent molecular, and that since the solid sublimes at 113.5°C the intermolecular bonding is fairly weak – again the only intermolecular forces of attraction are van der Waals’/dispersion forces, which are the weakest kind of weak force! Hence, this substance is:
“a non-polar covalent molecular substance with weak dispersion forces acting as the only intermolecular bonds.”
UNKNOWN SOLID F:
- Substance is polar/ionic itself
- Conductive in aqueous solution
- In aqueous solution charged particles free to move
-
Relatively high (175°C) melting point
F is soluble in a polar solvent and conductive in aqueous solution, properties typically associated with ionic substances (soluble due to ion-dipole bonding, conductive due to free movement of ions after dissolved), it also shatters in reaction to a shearing force, and has a fairly high melting point. All these properties indicate:
“an ionic substance with a rigid network lattice structure of an infinite array and strong bonds in three dimensions.”
UNKNOWN SOLID G:
- Substance is polar/ionic itself
- In neither solid nor aqueous solution state are charged particles free to move
- Less distinctly fractured
-
Relatively high (182°C) melting point
- Strong intermolecular bonding
G is both polar and non-conductive, thus indicating a strongly bonded polar covalent molecular substance. The high melting point of this unknown can be attributed to the relatively strong forms of weak intermolecular bonding; either dipole-dipole or even hydrogen bonding is likely influencing intermolecular bonding in this substance, and of course there are van der Waals’ forces acting simultaneously. The result of the polarity of this covalent molecular substance is substantially stronger intermolecular forces, and hence an increased melting temperature. Substance G is:
“a polar covalent molecular substance with intermolecular dipole-dipole/hydrogen bonds acting simultaneously with van der Waals’ dispersion force to induce a relatively high melting point”
UNKNOWN SOLID H:
- Insoluble in both polar and non-polar solvents
- Conductive when in solid state
- Electrons are free moving in solid state
- Solid crumbles/shatters when impacted
- Non-ionic yet strong and inflexible internal structure
-
High (3367°C) melting point
Substance H is insoluble in both polar and non-polar solvents, yet conductive in a solid state. This indicates that H is either a metallic or covalent network/lattice structure. Since this solid crumbles and shatters on impact, rather than deforming as a metal would, H is obviously a covalent lattice structure – as supported also by its high melting point (a result of strong bonds within any lattice structure). To determine the type of covalent lattice structure, the fact that H is conductive in a solid state must be taken into account. In a covalent network lattice, all the electrons are occupied and held in place by the strong three dimensional covalent bonds between the atoms, and as such are unable to conduct electricity. Covalent layer lattices, however, have strong bonds only in two dimensions, with intermediate gaps between each two-dimensional layer in which delocalised electrons are free to move and conduct electricity. Each layer is held together only by dispersion forces between each layer. This layer lattice structure would account for the conductivity of substance H, while also allowing it to have such a high melting temperature (due to remaining strong bonds in three dimensions) and to be practically insoluble. So H is:
“a covalent molecular layer structure of an infinite array with strong covalent bonds in two dimensions and weak van der Waals’ forces in one, with clouds of free moving delocalised electrons moving between each layer of covalent bonds.”
Conclusion:
It was a fairly simple matter, once certain properties of each substance had been discovered by the experimentation process’ described above, to determine the structure and bonding of each solid through critical analysis based on an understanding of the nature of various forms of bonding that can occur.
Christopher Bolton
A ionic 3d network
UNKNOWN SOLID B:
B metallic bonding
UNKNOWN SOLID C:
C ionic → network lattice?
UNKNOWN SOLID D:
D VDW covalent molecular one → non-polar
UNKNOWN SOLID E:
E VDW covalent molecular one → non-polar
UNKNOWN SOLID F:
F ionic → network
UNKNOWN SOLID G:
G covalent with hydrogen bonding and/or dipole-dipole
UNKNOWN SOLID H:
H covalent layer structure strong electrostatic bonds in 2D weak (VDW) in 1D
A ionic 3d network
B metallic bonding
C ionic → network lattice?
D VDW covalent molecular one → non-polar
E VDW covalent molecular one → non-polar ZINC
F ionic → network
G covalent with hydrogen bonding and/or dipole-dipole PROBABLY SUGAR
H covalent layer structure strong electrostatic bonds in 2D weak (VDW) in 1D GRAPHITE