Yaworski 1 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. Yaworski 2 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. Yaworski 3 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 Yaworski 4 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, Yaworski 5 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. Yaworski 6 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 Yaworski 7 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, Yaworski 8 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 Yaworski 9 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 Yaworski 10 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 Yaworski 11 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 Yaworski 12 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 Yaworski 13 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 Yaworski 14 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.