Determining Melting Temperature

Introduction

The melting temperature of a compound is the temperature at which it changes from a solid to a liquid. This is a physical property often used to help identify compounds or to check the purity of a compound. The melting temperature is related to the amount of kinetic energy that one adds to a solid substance to overcome the intermolecular attractions that maintain its solid state under given conditions.

It is very difficult, however, to find an exact melting point. Because it is a thermodynamic process, when a substance begins to melt, a dynamic equilibrium is established within which the substance exists in both solid and liquid form. Because the energy transferred to this system is not used entirely to convert the solid to a liquid, a single temperature value is commonly not reported, but rather a temperature range.

Thus, melting temperatures are usually reported as values with a range of 2–3°C. Melting temperature is not a unique physical property of a substance, but it does help you understand more about the substance. It can also help determine the purity of a substance that you have synthesized.

You will use a Vernier Melt Station to determine the melting temperature of a solid substance. Your sample will be one of several possible pure compounds. Your first trial will help you narrow your possibilities. On subsequent trials you will be able to accurately determine the melting temperature of your sample, thus identifying the compound.

Objectives

In this experiment, you will

• Prepare a solid substance for measuring melting temperature.
• Measure the temperature of a solid substance as it warms to melting.
• Analyze the temperature vs. time graphs to determine the rate of heating and the melting temperature of a sample of a solid organic compound.
• Identify the solid from a list of possible pure compounds.