Overview
This lecture covers how to measure melting points of compounds, the importance of sample purity, and how impurities affect melting point and its range using caffeine and benzoic acid as examples.
Melting Point Basics
- Melting point is the temperature range at which a solid becomes a liquid.
- Pure compounds have a sharp, narrow melting point range (often 2–4°C).
- Impure compounds melt over a broader and lower temperature range.
Sample Preparation
- Grind sample into a fine powder using a mortar and pestle.
- Load 1–3 mm of packed sample into a closed-end melting point capillary tube.
- Proper packing prevents air pockets that could alter results.
- Tap the tube to pack the sample; some apparatuses have a "tube tapper" feature.
Using the Melting Point Apparatus
- Digital apparatus heats the sample at a set ramp rate (°C per minute) from a start to a stop temperature.
- Start temperature is set below the expected melting point for accuracy.
- Typical ramp rate for analysis is 5°C per minute; faster rates are used for an initial rough estimate.
- Record the temperature when first liquid appears and when all solid disappears.
Observations and Results
- Pure caffeine melts at around 237°C with a narrow range, indicating high purity.
- Pure benzoic acid melts around 122–124°C, also with a narrow range.
- Impure benzoic acid (mixed with caffeine) melts at a significantly lower temperature and over a broader range.
- Even if the impurity has a higher melting point, it still depresses the melting point and broadens the range.
Effects of Impurities
- Impurities disrupt intermolecular forces, causing melting point depression and range broadening.
- The more impurity present, the lower and wider the melting range.
- This principle is similar to salt lowering the freezing point of ice.
Lab Technique Tips
- Always wear goggles for lab safety.
- Record all observations and corresponding temperatures.
- Identify phenomena like shrinking, sweating (release of solvent), or decomposition for accurate interpretation.
Key Terms & Definitions
- Melting Point — The temperature range where a solid turns into a liquid.
- Melting Point Range — The span from the first sign of melting to complete liquefaction.
- Melting Point Depression — Lowering and broadening of the melting range due to impurities.
- Sweating — Release of trapped solvent, sometimes mistaken for melting.
- Shrinking — Solid pulling away from the glass, not actual melting.
Action Items / Next Steps
- Practice preparing and loading melting point tubes with pure and impure samples.
- Run multiple trials and record melting point ranges for comparison.
- Note all temperature and physical changes during melting for accurate lab records.