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Michael Yaworski
Partner: Robbie Embro
Mrs. Johnson
SCH 4UI
Date Experiment Performed: 3 November 2014
Date Report Written: 9 November 2014
Classifying Mystery Solids
Introduction:
All solids have similar properties in that they have a definite shape and volume, are
virtually incompressible and do not flow. However, solids also vary substantially in other
properties such as hardness, conductivity and melting point. Their properties are determined by
elements the solid is composed of, the type/strength of the bonding between atoms or ions, the
strength of the forces within the substance, the shape of the compound and the arrangement of
compounds within the substance. The four categories of solids are the following: ionic crystal,
molecular crystal, metallic crystal and covalent network crystal.
An ionic crystal consists of a 3-D arrangement of ions (positively charged and negatively
charged particles). The attraction between the ions is called ionic bonding. Ionic crystals are
relatively hard and brittle, have high melting points, are soluble in water, and conduct electricity
in liquid state and as a solution, but not as a solid.
A molecular crystal consists of a 3-D arrangement of neutral molecules (nonmetal atoms
sharing electrons). Molecular crystals are relatively soft, have low melting points, and are
nonconducting of electricity as a liquid, solid, or in a solution.
A metallic crystal consists of a 3-D arrangement of metal cations. Metallic crystals range
from soft to very hard; are ductile and malleable; are shiny and silvery; conduct electricity in
solid and liquid states; are not soluble in water.
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A covalent network crystal consists of a 3-D arrangement of covalent bonds between
atoms. A covalent network crystal is very hard, brittle, has a very high melting point, and is
nonconducting of electricity in any state.
Purpose: The purpose of this lab was to classify each of the nine mystery solids into one of the
four categories of solids: ionic, molecular, metallic or covalent network crystal. The observations
were meant to help classify the solids into categories based on the specific properties of each
category.
Procedure:
1. The physical properties of the solid were noted by touching it and looking at it.
2. The electrical conductivity of the solid was tested using a conductivity apparatus.
3. The hardness of the solid was tested by applying force to it with a mortar and pestle.
4. The melting point of the solid was tested by applying heat to it while in a crucible using a
hot plate.
5. The electrical conductivity of the liquid (if melted in step 4) was tested using a
conductivity apparatus.
6. The solubility of the solid was tested by putting it in water on a spot plate and stirring it.
7. If soluble in step 6, the electrical conductivity of the aqueous solution was tested using a
conductivity apparatus.
Each step was repeating for every solid before moving on to the next step.
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Observations:
Table 1 – observations used to distinguish properties of each of the solids
Mystery
Solid
Physical
Description
Hardness
Conductive
as Solid
2a
- white flakes
soft
No
3a
- powder
- white
- soft
soft
No
4
- silver
- dull
- in chunks
very hard
Yes
very hard and
brittle, shattered
when crushed
No
5
6a
7
8
8a
9a
- crystal chunks
- jagged shape
- white
- translucent
- small white
crystals
- shiny
- silver
- thin strips
- very flexible
- brown and
black grains
- soft
- powder
- white
- soft
- tiny crystals
- white
hard and
crushable
No
soft, very
flexible
Yes
soft and
crushable
No
soft
No
soft and
crushable
No
Melting
Point
Relatively
low, melted
first
Did not
melt under
the heat
provided
Did not
melt under
the heat
provided
Did not
melt under
the heat
provided
Did not
melt under
the heat
provided
Did not
melt under
the heat
provided
Did not
melt under
the heat
provided
Did not
melt under
the heat
provided
Relatively
low, melted
second,
caramelized
Conductive
as Liquid
Soluble
Conductive
as solution
No
No
N/A
N/A
No,
maybe
partially
No
N/A
No
N/A
N/A
No
N/A
N/A
Yes
Yes
N/A
No
N/A
N/A
No
N/A
N/A
No,
maybe
partially
No
No
Yes
No
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Analysis:
The purpose of this lab was to classify each of the nine mystery solids into one of the four
categories of solids: ionic, molecular, metallic or covalent network crystal.
Ionic Crystals:
The properties of ionic compounds can be explained by their composition of cations and
anions in a 3-D arrangement. Ionic bonds are very strong compared to all intermolecular forces.
Ionic bonds are described as the simultaneous attraction between an ion and the oppositely
charged adjacent ions. Each ion is surrounded by six other oppositely charged ions due to the 3D shape (for example: front, back, left, right, up down). All of these electrostatic forces make for
a strong ionic bond, which explain the hardness and high melting point of an ionic crystal.
Because the bonds between ions are so strong, they are relatively hard to break and therefore are
relatively hard and have a relatively high melting point. Ionic bonds are also directional, which
explains why an ionic crystal is brittle. Since an ionic compound as a whole is neutral, it does not
conduct electricity as a solid. However, when an ionic compound dissolves in water, the ions
dissociate and therefore there are ions floating around in the water that are able to conduct
electricity. Therefore, an ionic compound in aqueous solution will conduct electricity. When the
ionic solid is melted into a liquid state, the ions are spread further apart and therefore may
conduct electricity.
An ionic crystal is typically soluble in water due to its high polarity. Water is also polar
and “like dissolves like” means that polar substances dissolve in other polar substances. Ionic
compounds are polar due to the large electronegativity difference between metals and nonmetals
(metals have low electronegativity and nonmetals have high electronegativity). However,
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polarity is typically only used to describe molecules because ionic compounds are known to be
very highly polar and are just considered “ionic”. However, some ionic compounds are not
soluble in water due to their shape, typically with the combination of polyatomic ions and
cations.
Metallic Crystals:
The properties of metallic crystals can be explained by their 3-D arrangement of metal
cations. The positive nuclei in metals and the loosely held, mobile valence electrons are bonded
together. This bonding is delocalized (electrons float between many atoms, not just two) and is
not directed between specific atoms. Therefore, it is nondirectional bonding (unlike ionic
bonding). All metals also have been observed to have a continuous and very compact crystalline
structure.
(Nelson – Chemistry 12)
Figure 1 – a model of the “sea” of valence electrons
There is the concept known as the “electron sea” model, which is illustrated in Figure 1.
Each of the positive nuclei is surrounded by a cloud, or “sea”, of valence electrons. That
negatively charged sea of electrons is attracted to each of the positively charged nuclei, which
bonds the metals together.
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The low ionization energy of metal atoms explains the loosely held valence electrons.
Metals atoms also have mostly empty valence orbitals (typically the p and d orbitals), which
explains the valence electron mobility.
The shiny and silvery properties of metals can be explained by the valence electrons
absorbing and emitting light from all wavelengths in the visible light spectrum. The flexible
property of metals can be explained by the nondirectional bonds which mean that the atoms can
move around and slide over each other while remaining bonded. The electrical conductivity of
the metals can be explained by the fact that valence electrons are mobile within the metal. A
power source can force electrons to move from one end of the metal to the other, thus conducting
the electricity. Most metals have high melting points and are relatively hard, but there are some
exceptions. The high melting points and hardness can be explained by the large electron sea
surrounding all positively charge nuclei which produces strong bonding. The bonds are hard to
break; therefore the metals are hard and have high melting points. However, there are some
exceptions, such mercury, which has a low melting point and is not hard. But generally, they are
hard or very hard and have high melting points.
Metals are also insoluble in water due to metals being nonpolar. Metals may consist of
one element or multiple elements, but all elements involved have similar (and low)
electronegativity. Therefore, metallic crystals are typically nonpolar due to a small
electronegativity difference.
Molecular Crystals:
The properties of molecular crystals can be explained by the 3-D arrangement of neutral
molecules. Other than large hydrocarbons and polymers molecular substances are crystals that
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have relatively low melting points, are soft and are nonconductors of electricity in any form.
Covalent bonds are stronger than ionic bonds, however, in a molecular crystal, neither bond is
what holds molecules together. Covalent bonds are the intramolecular forces which hold the
atoms within a molecule together. But the forces that hold molecules together to form a
molecular crystal are intermolecular forces. These intermolecular forces are much weaker than
covalent and ionic bonds, which explains why molecular crystals have a relatively low melting
point and are relatively soft. The intermolecular forces are London (dispersion), dipole-dipole,
and hydrogen bonding. As a comparison to ionic bonding, the dipole-dipole forces are the
electrostatic attraction between partially positive and partially negative ends of molecules.
However, this force is much weaker than the electrostatic attraction between fully charged ions
in an ionic compound. Individual molecules in a molecular crystal are neutral, so even when the
substance is separated into its individual molecules, it will not conduct electricity. Therefore, it is
a nonconductor of electricity in any state, or as a solution.
Molecules may or may not be soluble in water. It depends on the electronegativity
difference between the elements that are in the molecule, as well as the shape of the molecule.
The shape ultimately determines the solubility of the molecule. There is no general rule as to
whether or not they are soluble in water or not.
Covalent Network Crystals:
The properties of a covalent network crystal can be explained by its 3-D arrangement of
atoms that are held together by covalent bonds. As an example, diamond is one of the hardest
materials on Earth and is simply composed of carbons atoms. Each carbon atom is covalently
bonded to four other carbon atoms in a tetrahedral shape (from VSEPR theory). The individual
carbon-carbon single covalent bonds are not any stronger than other carbon-carbon single bonds,
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such as in a hydrocarbon. It is the interlocking structure that is responsible for the strength in the
overall material. It is the network of covalent bonds; the mass amount of the covalent bonds
within the crystal that is responsible for its properties. Covalent bonds are also the strongest
bonds that exist within substances. Covalent bonds are stronger than ionic bonds and much
stronger than intermolecular forces.
The structure of the covalent network also determines the degree of hardness it is. For
example, glass has the same chemical formula as quartz (SiO2), but lacks the crystalline structure
of quartz. Glass is not a crystalline because it does not have long-range order in its structure.
Quartz is perfectly ordered. Therefore, because of the structure, quartz is harder than glass.
Figure 2(a) compared to Figure 3(b) shows the comparison between quartz structure and glass
structure, respectively.
(Nelson – Chemistry 12)
Figure 2(a) – quartz structure
(Nelson – Chemistry 12)
Figure 2(b) – glass structure
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The hardness and very high melting point of covalent network crystals can be explained
by how strong the covalent bonding structure is within the crystal. It takes a significant amount
of energy to break so many covalent bonds, so the melting point is very high for this type of
crystal. It is much higher than ionic crystals and molecular crystals because the bonds are much
stronger. Because the atoms are not easily displaced from one another (due to the strong and
plentiful bonds), they are typically very hard compared to the other types of crystals. The
covalent bonds are also directional, because the bonds shared are specifically between two atoms
and do not move around the network. Therefore, because they are directional bonds, the covalent
network crystal is also brittle.
Electrons within a covalent network crystal are not free to move through the network
because they are held within atoms and the covalent bonds in a rigid structure. Therefore,
covalent network crystals are not conductors of electricity.
Covalent network crystals are not soluble in water due to them being polar substances.
The shape affects the polarity and because the net force of electrons being pulled from inner
atoms tends to be ⃗0, the substances are nonpolar.
Classifying the Mystery Solids:
Metallic Crystals:
The mystery solids 4 and 7 were both metals. When looking at Table 1 in the
observations section, solid 4 was silver, very hard and conductive as a solid. It also did not melt
(had high melting point) and was not soluble in water. All of those properties are consistent with
the properties of a metal, so it was categorized as a metal. The only property of a metal it did not
maintain was being shiny. This was because the element may have been tarnished or just an
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exception to the rule of being shiny. Not all metals are shiny, such as lead. Because the solid was
so hard, its flexibility was not tested.
Solid 7 had the exact same experimental results as solid 4, except that it was soft and
flexible instead of very hard, and was also shiny. Those properties are also consistent with the
properties of a metal, so it was categorized as a metal as well. Metals are flexible and shiny, and
can be soft.
The property that primarily showed that these solids were metals was the fact that it
conducted electricity as a solid. Metals are the only type of solid that do.
Covalent Network Crystal:
Mystery solid 5 was the only covalent network crystal. It was in the form of solid, jagged
crystal chunks, which is typically what covalent network crystals appear as (such as quartz).
They cannot be easily broken down into a fine powder or small pieces due to their hardness. It
was determined to be very hard and brittle (because it shattered), which is consistent with the
properties of a covalent network crystal. It is also consistent with the properties of an ionic
crystal, though. What distinguished it from being an ionic crystal was that it was not soluble in
water. Most ionic compounds are soluble in water, so it was most likely not an ionic compound.
It also did not conduct electricity in any form and had a relatively high melting point. All of
those properties are consistent with that of a covalent network crystal.
Ionic Crystals:
The only mystery solid that perfectly followed the properties of an expected ionic crystal
was solid 6a. It was hard, was not conductive as a solid, had a relatively high melting point, was
soluble in water, and was conductive of electricity in an aqueous solution. It was hard to tell
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whether it was brittle or not, though, because the presentation of the solid was in small crystals.
It was difficult to observe. The properties that primary showed that this solid was an ionic crystal
was that it was soluble in water and also conductive as a solution. Both of those properties
combined are unique to an ionic crystal compared to all of the other types of solids. It was not
conductive as a solid, meaning it was not a metal. It was soluble and conductive of electricity, so
it was not a covalent network crystal or a molecular crystal.
Molecular Crystals:
The remaining five mystery solids (2a, 3a, 8, 8a, and 9a) were each categorized as
molecular crystals, although their properties did not all match perfectly.
For solid 2a, it was soft, nonconductive of electricity in solid or liquid form, had a
relatively low melting point and was not soluble in water. This solid matched the properties of a
molecular crystal perfectly. The low melting point is the unique property of molecular crystals
compared to the others. The fact that it was not conductive as a liquid also ruled out it being a
metallic crystal or ionic crystal. It was also soft and flakey, which mostly ruled out being any of
the other crystals (they mostly hard).
Solid 9a had very similar experimental results to solid 2a except that it was soluble in
water. It melted almost at the same time as 2a and in the same heat conditions, so it also had a
relatively low boiling point. The only difference was that 9a was soluble in water, but that is still
consistent with the properties of a molecular crystal because as stated previously, molecular
crystals may or may not be soluble. It just depends on which molecular crystal it is. Because this
crystal was soluble in water, it was also tested for conductivity as an aqueous solution. It was not
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conductive as a solution, so that property is also consistent with that of a molecular crystal and at
the same time ruled out it being an ionic crystal.
Solid 3a and 8a had the exact same experimental results. However, they were clearly not
the same substance. They were each categorized as a molecular crystal, but with an exception in
one of the properties. They were soft, did not conduct electricity as a solid or in aqueous
solution, had relatively high melting points and had low solubility. They were observed to only
be partially soluble in the amount of water in the experiment, so they were considered to have
low solubility. Even though they were only partially soluble, they were observed to not be
conductive as a solution, which somewhat ruled out being ionic crystals. Ionic crystals would
have been conductive as a solution, but it is not very good evidence because they were not
complete solutions. That, combined with the fact that they were relatively soft ruled out them
being ionic. It was also ruled out to be covalent network crystals or metallic crystals because
they were soft, not conductive as solids (not metals) and partially soluble in water. They seemed
to more so match the properties of a molecular crystal, although a molecular crystal should have
a relatively low melting point, but these solids had high melting points. That was determined to
just be an exception to the rule, but the evidence mostly led to categorize the solids as molecular
crystals.
However, solid 8a was actually sodium bicarbonate (baking powder; NaHCO3), which is
an ionic compound. Even though it was known that the solid was not actually a molecular
crystal, it was determined to be one based on the evidence. Sodium bicarbonate was soft (as
opposed to hard, which was expected for an ionic compound) because it was a powder. But,
sodium bicarbonate should have been soluble in water and also conductive of electricity in that
solution. It was possible that not enough water was used to create a solution, and therefore it
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appeared to have low solubility, as well as be nonconductive as a solution because there really
was not much of a solution. More accurate trials of the experiments would need to have been
performed to get a correct categorization.
Solid 8 had very similar experimental results to solids 3a and 8a. The only difference was
that solid 8 was observed to be completely insoluble in the amount of water provided. That
difference would not have affected the analysis, though. For the same reasons as solids 3a and
8a, solid 8 was categorized as a molecular crystal.
Sources of Error:
-
There may have been chemical impurities. The impurities may have affected the results
of the experiments by reacting differently to the experiments than the actual solid meant
to be tested would have. To fix this, completely pure chemicals would have needed to be
used.
-
There may have been cross contamination. The equipment used (such as crucibles, mortar
and pestle, spot plate, etc.) may have had residue from past experiments that could not be
completely cleaned off. The stirring rod used to stir and touch each solid may have cross
contaminated them with each other. Although the stirring rod was rinsed after each use, it
may not have been able to be completely cleaned. This would have affected the
experimental results in the same way that having impure chemicals would have. The
contamination may have affected the way the solids reacted to the experiments in an
unknown way. To fix this, completely clean (possibly new) equipment would have
needed to be used for the experiments.
-
The water used to test solubility was not measured and the solids used for the
experiments were not massed. The proper amount of water may not have been used to
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dissolve the arbitrary amount of the solid being tested. When the solubility of the solid in
water was being tested, that property was recorded assuming that the correct ratio of
water to solid was being used. If the correct ratio was not used, the solubility test may
have been invalid. Also, each of the solids used for testing were not based on a specific
sample mass, so the results may have been relatively different for each solid. To fix this,
the water would need to be measured and the solid would need to be massed in order test
the solubility with the correct amounts.
-
The hot plate may have distributed the heat to each crucible in different amounts. The
crucibles on the edge of the hot plate may have gathered less heat; therefore the relative
melting points being observed may have been skewed. To fix this, equivalent heat would
need to be transferred to each crucible containing the solids by using more hot plates or a
larger one.
-
There was also human error when recording the observations. The people recording the
results may not have had a complete understanding of what was happening in the
experiments. Therefore, the solubility observation, the physical description, etc. may
have been recorded incorrectly. There was no way to fix this because it was human error,
except possibly a more qualified scientist to be more accurate in analyzing what was
happening.
-
The lab experiments were only completed once. To gather evidence of properties that is
consistent, the experiments would have needed to be completed more than one time. Due
to other sources of error, lab results will not always be the same and therefore need to be
done repeatedly to get consistent results.
Works Cited:
Kessel, Hans Van. Nelson Chemistry 12. Toronto: Thomson Nelson, 2003. Print.