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Chemistry Lab: Classifying Unknown Substances

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Introduction

Classifying Unknown Substances Lab Hypothesis: If solution of compound and water has high conductivity level, compound is ionic. If compound dissolves in water quickly, it is ionic. If compound takes a long time to melt (high bp), it is ionic. If compound is crystalline, it is ionic. If solution of compound and water has low conductivity level, compound is molecular. If compound does not dissolve in water quickly, it is molecular. If compound takes a short time to melt (low bp), it is molecular. If compound is not crystalline, it is molecular. Materials: 1. compounds A, B, C 2. conductivity tester 3. graduated cylinder 4. 3 beakers 5. scoopula 6. stir stick 7. distilled water 8. deflagrating spoon 9. bunsen burner 10. magnifying glass 11. spot plate 12. stop watch Set up: DANIEL DRAW THE SET UP Procedure: Experiment 1 ? conductivity test (H20) 1. Pour 20 mL of distilled water into beaker 2. Mix compound A in water using stir stick until fully dissolved 3. Test solution's conductivity using conductivity tester 4. Record conductivity 5. Repeat 1-4 five times 6. Repeat 1-5 using compound B and C Experiment 2 ? solubility test 1. Pour 20 mL of distilled water into beaker 2. Mix compound A in water using stir stick 3. Record time for compound to dissolve in water with stop watch 4. ...read more.

Middle

Substance B began to boil at 4 seconds. Substance C began to boil at 10 seconds. Both substance B and C's liquid forms were a little brown in colour. Test four: Magnifying Glass Substance Does it form a crystal lattice? A Yes B Yes C No For this test, all 3 substances were observed under a magnifying glass. We observed the shape of each grain of the substances to see whether or not they were crystalline. Conclusions : Molecular substances have a low melting point due to the weak intermolecular forces that bond them (electron is shared between 2 atoms). As a result, molecular substances require little energy to break the bonds. Since temperature is related to energy, molecular substances have a low melting point. Ionic compounds form strong ionic bonds (forms ions and crystal lattices), so they require more energy to break. Therefore, heating unknown substances can help determine whether the substance is ionic or molecular. Ionic compounds form crystal lattices, because positive and negative ions are attracted to each other, and will bond easily. Molecular compounds do not. Observing the structure of a grain of a substance can help determine whether the substance is ionic or molecular. Ionic compounds are polar, so they are able to bond with water (negative oxygen and positive hydrogen atoms). ...read more.

Conclusion

Although the magnifying glass examination casts some doubt on the type of bond, all the other evidence points towards a covalent bond. Substance C was the tricky one. With a high solubility, dissolving in only 29.5 seconds on average, and a decent conductivity averaging 6.5/10, it points towards being ionic. However, the magnifying glass test showed a random structure, rather than a crystal lattice, and the melting point was rather low. It took some thinking to come to the conclusion that this is a covalent bond. My reasoning here is that the structure of the substance is very critical. The majority if not all ionic substances are a crystal lattice formation. To further support this, I also found that in all sources I checked, they all said that MOST covalent compounds had low conductivity and solubility. Not all. This leads me to think that this is one of these exceptions, seeing as the conductivity wasn?t incredible, leaving the only strong indicator of an ionic bond to be the solubility. With only one test supporting ionic and three opposing, I believe it to be a covalent or molecular bond. Errors: 1. - There were people walking by the bunsen burner. The wind their movement created may have affected temperature of flame and results. - Flame emitted from the bunsen burner was inconsistent, it sometimes turned orange (lower temperature). 1. - The deflagrating spoon had unwashable residue on it, and it may have affected the results. ...read more.

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