Question

The purity of organic compounds is determined by: (a) Mixed melting point (b) Molecular weight (c) Melting point (d) Density

Short answer

Melting point (c) is used to determine the purity of organic compounds.

Step-by-step solution

Understand Each Option

Let's clarify each given option: (a) Mixed melting point - This method involves mixing two compounds and determining the melting point of the mixture. If a compound is pure, the melting point remains unchanged when mixed with a small amount of itself. (b) Molecular weight - This refers to the mass of a molecule and is not directly used to determine purity but to identify compounds. (c) Melting point - Pure substances typically have a sharp, defined melting point. Impurities can cause the melting point to be lower and broadened. (d) Density - This is the mass per unit volume of a substance and isn't typically used as a primary method to check purity.

Identify Direct Method for Purity

The primary characteristic used to determine the purity of organic compounds is having a sharp and consistent melting point. A pure substance has a consistent melting point, while impurities generally lower and broaden it.

Conclusion

From the analysis, (c) Melting point is the most direct and common method used to determine the purity of organic compounds. Mixed melting point (a) can be used but is less common, often used in conjunction with simple melting point checks.

Key concepts

Melting Point in Chemistry

The melting point of a substance is a critical physical property and acts as a key indicator for determining the purity of organic compounds. In chemistry, the melting point is defined as the temperature at which a solid becomes a liquid under standard atmospheric pressure.

For a pure organic compound, the melting point is sharp and precise, occurring over a very narrow temperature range. This precise melting point is like a fingerprint, giving scientists clues about a substance's purity.

• If impurities are present, they disrupt the crystal lattice structure of the compound, leading to a melting point depression. This means the compound will start to melt at a lower temperature and over a broader range.
• The extent of the melting point depression can provide an indication of the extent of impurities.

Therefore, observing a sharp melting point range is an efficient way to suggest the presence of a pure substance.

Mixed Melting Point Method

The mixed melting point method is a technique used to confirm the identity and purity of organic compounds.

Here's how it generally works: you take a sample of the compound, mix it with a known reference compound, and determine the melting point of this mixture. This method relies on the principle that if two different compounds are mixed, their melting point will be lower and broader.

• If the melting point remains unchanged and sharp, it indicates the compounds are identical, thereby confirming identity and purity.
• Conversely, if the melting point of the mixture is lower and broader compared to the pure compound, it suggests the presence of different compounds, reflecting impurity.

This method is especially useful when unknown substances need to be quickly cross-referenced against known compounds.

Identification of Organic Compounds

Identifying organic compounds involves determining the specific structure and composition of a compound. This can be complex due to the vast number of possible organic structures.

Methods to identify such compounds include a combination of approaches, such as:

• **Spectroscopy techniques** like NMR, IR, and MS, which provide insights into the molecular structure by analyzing the interaction of electromagnetic radiation with the compound.
• **Chromatography** separates compounds based on differences in physical or chemical properties, helping identify components in mixtures.
• **Melting point determination** is instrumental in confirming purity and can support identification, especially when compared with literature values for known compounds.

A synergistic approach, utilizing both physical properties (like melting points) and advanced techniques, enriches the reliability of compound identification.