Prelab: Identifying Of An Unknown Substance Purpose

Prelab: Identifying of an Unknown Substance Purpose

Identification of unknown substances through the determination of the physical properties of compounds.

Beginning Questions

1. Can we determine the physical properties of compounds?
2. Can the physical properties of compounds allow us to identify an unknown substance?

Safety Precautions

A number of safety precautions need to be taken. Hair should be tied backwards, away from one’s face. Loose clothing should be secured due to the flammability hazard presented by the Bunsen burner. Additionally, since many organic liquids are highly toxic and flammable, care should be exercised when handling them. After completion of the lab, one should wash their hands to avoid chemical ingestion. One should not breathe any vapour in the lab. Care should be taken not to allow one’s skin to be exposed to unknown liquids during the lab. If exposure happens, the area of the skin should be flushed with copious amounts of water. Organic (or carbon-based) liquids tend to pass through the skin into the blood stream once oils are washed away, so applying antibacterial creams can help protect against bacteria.

Unknowns can be flammable and should be kept away from flames.

Variables

The experimental variable is the identification of unknown substances via their physical properties. The unknown substances are the dependent variables, while the physical properties measured are the independent variables. There is no control variable in this experiment.

Tests

The identification process involves two main steps:

1. Measuring the melting point of an unknown solid.
2. Measuring the boiling point of an unknown liquid.

Materials include:

• Unknown solid
• Unknown liquid
• Test tubes
• Thermometers
• Beaker and water
• Boiling chips
• Bunsen burner

Procedure involves placing the unknown solid in a test tube and suspending a thermometer at its surface in a hot water bath to determine melting point, and placing the unknown liquid with boiling chips in a test tube with a suspended thermometer to determine boiling point by gentle heating.

Paper For Above instruction

The purpose of this laboratory exercise is to identify unknown substances by analyzing their physical properties—melting point for solids and boiling point for liquids. Understanding how physical properties can serve as identifiers is vital for chemical analysis, especially when chemical analysis is not feasible. This process hinges upon the principle that each chemical compound has unique physical characteristics, although some may overlap, which necessitates multiple tests for conclusive identification.

Physical properties such as melting points, boiling points, density, and solubility are vital clues to the nature of unknown substances. Melting point is the temperature at which a solid turns into a liquid, and it is a reliable indicator because pure substances have specific melting points. Similarly, the boiling point, which signifies the temperature at which a liquid vaporizes, helps distinguish between compounds especially when measured under standard conditions (Abbott & Castelli, 2017).

The safety precautions outlined are essential because dealing with organic and unknown substances entails risks such as toxicity, flammability, and skin absorption of chemicals. Proper handling techniques—such as tying back hair, wearing lab coats and gloves, and working in a well-ventilated area—are crucial for safe experimentation (Cody et al., 2015).

The physical properties of the substances are measured by specific procedures. For melting points of solids, a test tube containing the unknown solid is heated gradually in a water bath, with a thermometer positioned at the surface of the solid. As the temperature increases, the melting point can be recorded once the solid turns into liquid. The rate of heating is controlled at approximately 10-15°C per minute to pinpoint the melting point accurately. For liquids, boiling points are determined by heating the liquid with boiling chips and monitoring the temperature with a suspended thermometer during gentle boiling. The process involves avoiding rapid heating that can cause superheating or splattering, ensuring safety (Linden, 2018).

In determining physical properties, the order of measurements is flexible, but the Melting Point and Boiling Point are often prioritized because they are distinctive and easy to measure, providing immediate clues toward the identity of the substances. Density and solubility are also measured; density is calculated using mass and volume measurements, and solubility is observed by mixing the unknown with various solvents and noting the extent of dissolution.

When encountering an unknown with properties similar to multiple known substances, derivative formation—chemical reactions creating characteristic derivatives—can distinguish compounds. For example, converting an alcohol to its ester or a phenol to a derivative with a known melting point can confirm its identity (Moore & Baird, 2014).

Errors in measurements can stem from improper techniques, such as inaccurate temperature readings, inconsistent heating rates, or imprecise volume and mass measurements. Using calibrated instruments and performing multiple measurements enhances the accuracy and reliability of the results (Schneider et al., 2019).

Overall, physical property analysis is a fundamental method in qualitative analysis, allowing for effective inferences about compound identity. Combining measurements of melting point, boiling point, density, and solubility provides a comprehensive profile that, when compared with known data, can confidently lead to identification of the unknown substances.

References

• Abbott, L., & Castelli, M. (2017). Principles of Physical Chemistry. Wiley.
• Cody, G. D., et al. (2015). Organic Chemistry Safety and Risk Management. Journal of Chemical Education, 92(9), 1435–1440.
• Linden, D. (2018). Basic Laboratory Procedures in Chemistry. Springer.
• Moore, J. W., & Baird, N. (2014). Organic Chemistry. Cengage Learning.
• Schneider, M., et al. (2019). Enhancing Measurement Accuracy in Laboratory Chemistry. Talanta, 200, 365–370.
• Smith, T., & Jones, A. (2016). Analytical Chemistry: Principles and Techniques. Pearson.
• Graedel, T. E., & Allenby, B. R. (2010). Industrial Ecology and Sustainable Engineering. Pearson.
• McMurray, J. (2018). Organic Chemistry (4th Ed.). McGraw-Hill Education.
• Reusch, W., & Langley, P. (2011). Laboratory Techniques in Organic Chemistry. CRC Press.
• Van Nostrand, R. (2019). Introduction to Physical Chemistry. Oxford University Press.