Chemistry Lab: Classifying Unknown Substances

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. Rinse out beaker using water and distilled water

5. Repeat 1-4 four times

6. Repeat 1-5 using compound B and C

Experiment 3 – melting time test

1. Fill deflagrating spoon with compound A using scoopula

2. Hold deflagrating spoon over flame from bunsen burner for 30 seconds or until compound is melted

3. Record the melting time

4. Wash and rinse deflagrating spoon with water and distilled water

5. Repeat 1-4 using compound B and C

Experiment 4 – magnifying test

1. Put some compound A on the spot plate using scoopula

2. Observe with magnifying glass

3. Record if it has a crystalline structure

4. Repeat 1-3 using compound B and C

Observations and Results:

Test one: Solubility

Substance

Trial 1 time (s)

Trial 2 time (s)

Trial 3 time (s)

Trial 4 time (s)

A

30

31

40

32

B

38

46

37

38

C

32

31

25

30

During this test, all the substances seemed to take a similar time to dissolve. However, when stirring, substance A seemed to disperse and have just a few granules left floating around until they dissolved. Substance B dispersed quickly, much like A, but resembled a small hurricane while I stirred, until it was completely dissolved. Substance C was different in the sense that while stirring, all the granules remained clumped together in the center of the beaker until they dissolved.

Test two: Conductivity

Substance

Trial 1 time (s)

Trial 2 time (s)

Trial 3 time (s)

Trial 4 time (s)

A

9

9

9

9

B

1

2

1

1

C

7

6

7

6

During this test, all the substances were dissolved in water. The results were very straight forward. Substance A is very conductive with a conductivity of 9. Substance B was not conductive with a conductivity of 2. Substance C was some what conductive with a conductivity of 6.5.

Test three: Melting Time

Substance

Time to melt (s)

A

Did not melt

B

4

C

10

For this test, melting time was determined by when the substance began to boil. Substance A did not boil after 30 seconds. 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). This means that ionic compounds are soluble in water. Molecular substances usually cannot dissolve in water. A solubility test can help determine whether the substance is ionic or molecular.

Ionic compounds have charged ions that are bonded by attraction in a crystal lattice structure and therefore cannot conduct electricity (which requires mobile ions). In water, the crystal lattice structure breaks down (see description above) and positive and negative ions are released. These ions can conduct electricity, therefore ionic compounds, when dissolved in water, can conduct electricity. Molecular compounds do not break up into ions when dissolved in water, so it does not conduct electricity. A conductivity test with a conductivity tester can help determine whether the substance is ionic or molecular.

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While doing this experiment, we have discovered the type of bonds present in each of the substances. I say with great certainty that substance A is an ionic compound. In the solubility test, it dissolved quickly, although not the quickest, it was the middle ground between the other two, with an average dissolving time of 33.25 seconds. High solubility is a property of ionic bonds, whereas a covalent bond will be less soluble. Also, when dissolved, an ionic compound will be highly conductive compared to a covalent compound. As shown in the results, substance A was the most conductive, with consistent 9/10s. It had a very high melting point, as it didn’t melt within a minute and a half, a property expected of ionic compounds. When it was examined under a magnifying glass, I could see a crystal lattice formation. All of these factors lead me to the conclusion that substance A is ionic.

Substance B, on the other hand, is most likely covalent, or molecular. It took a lot longer on average to dissolve, was hardly conductive, melted almost instantaneously, yet still had a crystal lattice shape. The lower solubility (39.75 seconds in this case) is expected of a covalent or molecular bond, and that leads me one step towards my conclusion. A lack of conductivity is also a property of a covalent bond, which makes me even surer of the identity of the substance. When it melted in only four seconds, I was certain of the identity of this substance as being covalent. 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.