Thies1

Sparta high school /
NONPOLAR, POLAR, IONIC AND COVALENT COMPOUND STRUCTURE AND PROPERTY /
LAB NUMBER 1 /
RACHEL THIES /
9/2/2015 /

INTRODUCTION

There are two major types of compounds found on Earth. These compounds are ionic and covalent. They differ in the way that they are bonded. Bonding happens when an atom needs to gain, lose or share electrons to be stable. All atoms must have a full outer shell of electrons to be stable. Ionic compounds transfer electrons from the outermost electron orbital from one atom to another. Generally, they consist of a positively charged metal and a negatively charged nonmetal. These opposite charges hold the two atoms together. Covalent compounds share electrons rather than transferring them from orbital to orbital. There is not a positively or negatively charged atom involved in a covalent compound. Typically, the atoms involved in a covalent compound are both nonmetals. Covalent compounds may be much bigger than an ionic compound because they can be made up of two to thousands of atoms at a time. Ionic compounds are known to have much stronger bonds than those that are covalently bonded. Within these two types of compounds, there is one more characteristic that they can be classified into: polarity. A polar substance is one that shares electrons unequally and has a partial positive and a partial negative end. A nonpolar substance shares equally throughout the structure and will have no charges. Polar substances generally can be dissolved in other polar substances, and nonpolar substances can be dissolved in other nonpolar substances. This pattern follows the phrase, “like dissolves like”. Certain polar substances like water can also exhibit a phenomenon called Hydrogen bonding, which occurs when the partially charged ends of a compound weakly attract nearby compounds of the same structure.

The purpose of this lab is to investigate the properties of polar, nonpolar, ionic and covalent substances due to their chemical structure. Examinations were done with each applicable compound such as crystal structure, hardness, solubility, melting point, and surface tension. Prior to entering lab, predictions were made. If crystal structures were observed under a microscope, then ionic substances would show irregularly patterned crystals and covalent substances would show regularly patterned crystals. If hardness was observed, then ionic substances would show a much higher level of hardness than a covalent substance because of relative bond strengths. If melting point was observed, then ionic substances would take a much longer time and higher temperature to melt than a covalent substance because of relative bond strengths. If surface tension was observed, than the polar substance would exhibit a force that would allow a small object to float and a nonpolar substance would not allow this phenomena. If solubility was observed, then a polar substance like salt and sugar would dissolve in a polar substance like water and a nonpolar substance would dissolve in a nonpolar substance. Unlike substances, like oil and water; oil and sugar; and oil and salt would not dissolve one another.

MATERIALS AND METHODS

  • Polar ionic substance (salt)
  • Polar covalent substance (sugar)
  • Nonpolar substance (vegetable oil)
  • Polar substance (water)
  • Two 250 mL beakers
  • Hot plate
  • Stirring rods
  • Compound light microscope
  • Glass slides
  • Petri dish
  • 2 paper clips

First the crystal structure was observed under a microscope. To do this, small crystals of each of the solids, salt and sugar were placed on separate glass slides and placed on the stage to be observed one at a time. Next, the solid substances were tested for hardness. For this test, a fingernail was used to scratch and break the solid. An actual numeric value was not recorded, but rather a relative comparison of each of the solids. Next, each solid substance was measured for melting point. 1 gram each of salt and sugar were placed in a beaker and placed in the center of a hot plate.A temperature value was not collected, rather a time value that it took one to melt in relation to the other. Next, surface tension was examined. To do this, equal amounts (5 mL each) of vegetable oil and water were placed in a petri dish. A paper clip was placed on the very outer surface of the liquid line in an attempt to have it float. Next, solubility was tested. To do this, 1 gram of each solid was dissolved in each liquid. Equal amounts (5 mL) of each liquid were mixed as well. This gave the following potential solubility combinations: salt and water, sugar and water, oil and water, sugar and oil, salt and oil. Observations were recorded.

RESULTS

In the first test, sugar was arranged into perfect cubes and salt appeared irregularly shaped under the microscope. In the hardness test, it was found that sugar crystals were much easier to chip with a fingernail than salt crystals. In the melting point test, the sugar melted in 3 minutes, whereas salt never changed in state or color. In the surface tension test, the petri dish containing the water was able to hold a paper clip, whereas the petri dish with the oil alone, the paper clip sunk to the bottom. In the solubility test, oil and water, oil and sugar, and oil and salt all did not dissolve. The only dissolved combinations were sugar and water, and salt and water. See Table 1 below for an organized representation of this data.

Table 1- Results From Tests 1-4

Test / Test Description / Substance / Result
1 / Crystal structure / Salt / Cube
Sugar / Hexagon
2 / Hardness / Salt / Very difficult to chip with fingernail
Sugar / Very easy to chip with fingernail
3 / Melting point / Salt / Neither melted nor changed color
Sugar / Completely melted and changed color within 3 minutes
4 / Surface tension / Oil / Paper clip did not float
Water / Paper clip did float
5 / Solubility / Salt and water / Dissolved
Sugar and water / Dissolved
Oil and water / Did not dissolve
Salt and oil / Did not dissolve
Sugar and oil / Did not dissolve

CONCLUSIONS

In this lab, the purpose was to investigate the properties of the compounds oil, water, sugar and salt and draw conclusions that relate the structure of that compound to its properties. Looking at the crystals under the microscope became difficult because of the quality of the microscopes given. However, there were regularities seen on both crystals. Sugar exhibited hexagonal crystallization, whereas salt crystals showed perfect cubes when subjected to light. This was different than was predicted. The hardness and melting points of sugar and salt relate to the bond strengths of those compounds. Because the bond strength of an ionic compound is generally stronger than one with covalent bonding, then the hardness and melting point of salt was also greater than sugar, which is just as was predicted. Just as predicted, surface tension action was only present in the petri dish, as this is the only compound in the test that was expected to exhibit hydrogen bonding. The weak attractions between the water molecules held the paper clip in suspension in the water dish, whereas there were no attractions in the dish with the oil. As stated in the introduction, salt and sugar are both polar molecules. Because sugar and salt were both observed to dissolve in water, which is also polar, but not oil, which is polar, the original prediction that “like would dissolve like” can be supported. All of the original predictions were supported with the exception of the crystalline structure.

DISCUSSION

Some sources of error in this lab led to the rejection of one of the original predictions. One issue was the inconsistency of the crystal sizes of the salt. This made viewing the crystal much more difficult. In the future, a mortar and pestle will be used to combat this issue. Another issue with the crystal structure examination was the resolve power of a few of the microscopes. The objective seems to show very little clarity, which made it difficult to conclude at the close of the lab.

It is important to consider the implications that this lab has on real life. Ionically bonded compounds will generally have a much higher boiling and melting point than covalent compounds because of their strong bonds. They will tend to be much harder as well. This makes them better at withstanding high heat than a covalent compound, which can have many industrial benefits. Surface tension is not the only phenomena that hydrogen bonding is responsible for. Hydrogen bonding will also allow for water to stick to itself in droplets (cohesion), for water to stick to other materials (adhesion) and for water to rise up a tube (capillary action). It also makes water stay liquid at a much higher temperature than most compounds and it also allows it to resist a temperature change (which means that it is slow to heat and slow to cool) that would cause them to evaporate or freeze. This contributes to the major amount of liquid water in the world. Water rises up tubes in plants to travel to leaves. With respect to the idea that “like dissolves like”, this idea that polar substances can only dissolve polar substances and nonpolar substances can only dissolve polar substances, there is great significance as well. Nonpolar vitamins in the human body need nonpolar fat or oil to dissolve and be used. Any polar substance can be well dissolved in water, making it the universal solvent. Oil and water were found to not mix well, which gives an explanation to why having a nonpolar component to the cell membrane would help it maintain itsmoisture level.

LITERATURE CITED

None.