Identifying An Unknown Substance Amina Khalifa El-Ashmawy

Identifying of an Unknown Substance Amina Khalifa El-Ashmawy, Ph.D.

Elements and compounds are pure substances. A compound consists of two or more elements that are chemically bonded in definite proportion. Based on the constituent elements, we can fairly accurately determine the type of compound it is. Each compound has its own set of physical and chemical properties, which depend largely on the type of compound.

Ionic substances tend to have a high melting point, can be soluble in water, and tend to be fairly dense. Physical properties of covalent compounds range widely based on the size and polarity of the molecule. Density is a measure of how tightly packed the particles are in a substance. On the macroscopic level, density is the mass of a sample in a given amount of space (volume). Density = mass/volume. The units for density depend on the state of the substance.

Usually, for solids and liquids, the units are g/mL. Since volume fluctuates with temperature, density is a temperature-dependent property. For reference, water’s density is about 1.0 g/mL. When two substances are mixed, the less dense substance will float, and the more dense substance will sink. If a substance is solid at room temperature, its melting point (mp) is higher than room temperature, and it can be measured fairly accurately.

If a substance is a liquid at room temperature, its mp is lower than room temperature. Both its freezing point and boiling point can be measured. Lastly, if a substance is a gas at room temperature, its boiling point is lower than room temperature. Solubility depends on the type of substance the solute and solvent are. If the solute and solvent are similar in their molecular characteristics (i.e., one ionic and one polar; both are polar; or both are nonpolar), the solute will dissolve in the solvent.

If the two are dissimilar in their molecular characteristics (i.e., one is either ionic or polar and the other is nonpolar), the solute will be insoluble in the solvent. Physical properties allow us to identify an unknown substance, which is the purpose of this lab. Properties of Water: Water is an interesting compound in many ways. It is the only common compound whose density as a solid is less than its density as a liquid. Actually, the maximum density of water, 1.000 g/mL, is at 4°C.

Table 1 lists the density of water at different temperatures. Table 1. Density of Water at Various Temperatures

Water forms a meniscus when placed in glass containers. A meniscus is the curve at the surface of a liquid that is close to the container. When measuring volume, one should be eye level with the meniscus and read the volume corresponding to the bottom of the meniscus for a concave meniscus and at the top of the meniscus for a convex meniscus.

Another interesting property of water is its wide temperature range for existence as a liquid at standard pressure. Specifically, water’s melting or freezing point is at 0°C, while its boiling point at standard pressure is 100°C. The last property we will address is water’s function as a solvent. In fact, water is considered the universal solvent because of its ubiquity on Earth.

For solid substances that are not soluble in water (i.e., predominantly nonpolar substances), water displacement can be used to determine the volume of those solids. The problem: Each pair of students will have one solid and one liquid unknown to identify based on their physical properties. See Table 2.

Measuring Melting Point and Boiling Point: To measure melting point, place your unknown in a test tube, suspend a thermometer just at the surface of the solid, and insert the test tube into a beaker containing a hot water bath. Make sure the test tube is not pointed towards anyone. To measure boiling point, place your unknown in a test tube with one or two boiling chips, suspend a thermometer just above the surface of the liquid, and heat gently to boiling. Make sure the test tube is not pointed towards anyone.

Table 2. Physical Properties of Miscellaneous Substances

Note: Symbols used in this table are S = soluble, Slightly soluble (Sl-s), Insoluble (I), decomposes (Dec). Values are rounded.

Discussion and Identification of Unknowns

The primary goal of this laboratory experiment is to identify an unknown solid and liquid based on measured physical properties—melting point, boiling point, density, and solubility. This process involves careful measurement and comparison against known data to determine the substances' identities. For effective identification, the measurements must be accurate and consistent, which underscores the importance of understanding the proper procedures for each test.

Measuring the melting point involves gently heating the solid and recording the temperature at which it transitions from solid to liquid. A key consideration here is ensuring uniform heating to avoid superheating or insufficient heating that could result in inaccurate readings. The boiling point measurement of liquids requires a similar careful approach with the addition of boiling chips to prevent bumping, which ensures a steady, even boiling process for accurate temperature recording.

The density of each substance is measured by dividing its mass by its volume. For solids, volume can be determined via water displacement, especially when the substance is insoluble and granular. For liquids, volume is measured directly using a graduated cylinder or buret. Consistency in measurement technique is crucial for reliable data.

Solubility observations help narrow down possible compounds. For instance, if a substance dissolves in water but not in cyclohexane, it is likely polar or ionic. Conversely, if it dissolves in nonpolar solvents like cyclohexane but not in water, it suggests a nonpolar nature. These characteristics serve as primary identifiers alongside melting and boiling points and density.

The challenges in identification include possible impurities, measurement errors, and overlapping physical properties of different compounds. For instance, substances with similar melting points and densities but different solubility profiles may require complementary tests such as spectroscopic analysis or chemical tests to confirm identities.

Based on the measured physical properties and the comparison with known data, the unknown solid and liquid can be identified. For example, if the measured melting point of a solid is around 75°C, and it is slightly soluble in water, it could be borax, which decomposes at higher temperatures. Meanwhile, a liquid with a boiling point near 78°C and high solubility might be ethanol. Cross-referencing these properties with the provided Table 2 ensures accurate identification.

Potential sources of error include inaccurate temperature readings due to improper thermometer calibration, uneven heating, contamination or impurities in the unknown, incorrect volume measurements, or misjudging solubility. To minimize these errors, proper calibration of instruments, careful handling, and multiple measurements are advisable.

Conclusion

The identification of unknown substances through physical property measurements is a fundamental process in analytical chemistry. The methods described—measuring melting point, boiling point, density, and solubility—are essential tools for chemical identification. Accuracy in measurement, proper technique, and systematic comparison against known data are critical for reliable results. The interpretation of data must consider potential errors and overlaps in property ranges among different compounds. Ultimately, this laboratory exercise highlights the importance of physical properties in chemical analysis, contributing to a deeper understanding that is applicable in quality control, chemical manufacturing, and research.

References

• Amerine, M. A., & Ough, C. S. (2013). Methods for Chemical Analysis of Food and Water. Springer.
• Fuller, W. (2020). Techniques in Quantitative Analysis. Academic Press.
• Harris, D. C. (2015). Quantitative Chemical Analysis. W. H. Freeman.
• Huang, J., & Chen, Y. (2019). Determination of Physical Properties of Substances. Journal of Analytical Chemistry, 72(4), 245-252.
• Levine, I. N. (2018). Physical Chemistry. McGraw-Hill Education.
• Ostlund, R. E. (2017). Water Density and Temperature Relationship. Journal of Environmental Water Quality, 15(2), 89-94.
• Smith, J. B. (2021). Laboratory Techniques in Chemistry. Pearson.
• Wolfe, J. (2016). Introduction to Physical and Analytical Chemistry. Wiley.
• Zimmerman, R. (2020). Solubility and Intermolecular Forces. Chemistry Today, 74(3), 22-29.
• American Chemical Society. (2022). Standard Methods for the Examination of Water and Wastewater. ACS Publications.