Testing Purity with Melting Points

Introduction

How can a chemist tell whether a newly made white powder is pure? You cannot see impurities with the naked eye, but you can measure them indirectly. One of the quickest and most widely used checks is the melting point. A pure solid melts sharply at a known temperature; an impure one melts lower and over a range. This simple idea is used every day in pharmaceutical and research laboratories.

Core explanation

Pure solids melt sharply. Every pure crystalline solid has its own melting point , the same for every sample at normal pressure. Pure ice melts at 0 °C, pure benzoic acid at 122 °C and pure aspirin at about 136 °C. Because all the particles are identical and held by the same forces, the whole solid melts over a very narrow range, usually within 1 °C. This is called a sharp melting point .

Impurities lower the melting point. When another substance is present, its particles disrupt the regular arrangement of the crystal. The forces holding the structure together are weakened in places, so melting begins at a lower temperature than for the pure substance. This effect is called melting point depression .

Impurities widen the melting range. In an impure sample, some regions melt first, and as the solid melts the composition of the remaining solid and liquid changes. The sample therefore melts gradually over several degrees. A sample of aspirin that starts to melt at 128 °C and finishes at 133 °C is clearly impure.

Reading the evidence.

Observation Conclusion --- --- Sharp melt at the data-book value Probably pure Melts below the data-book value Impure Melts over a range of several degrees Impure Sharp melt at a different value Probably a different pure substance

How it is measured. A tiny amount of powdered solid is packed into a thin glass capillary tube and heated slowly in an electric melting point apparatus or a heated oil bath, while the temperature is read from a thermometer or digital sensor. Heating must be slow near the melting point so that the thermometer reading keeps pace with the sample. Two temperatures are recorded: when the first drop of liquid appears and when the last crystal disappears.

Mixed melting point. To confirm the identity of a substance, chemists grind a little of the unknown with a pure known sample. If the melting point stays sharp and unchanged, the two are the same substance. If it drops and widens, they are different, because each acts as an impurity in the other.

Step-by-step reasoning

To judge purity from melting point data:

1. Find the data-book melting point of the pure substance. 2. Compare the start of melting with that value. Lower means impurities. 3. Look at the width of the range. More than about 1–2 °C suggests impurity. 4. State a conclusion and give both pieces of evidence.

Visual explanation

Picture a neat wall of identical Lego bricks in perfect rows: every row is held equally firmly and the wall collapses all at once when shaken hard enough. Now replace a few bricks with odd-shaped pieces that do not fit well. Those weak spots give way first and collapse spreads gradually. The odd pieces are like impurity particles in a crystal.

Real-world analogy

Imagine a line of dancers holding hands. If everyone is the same height, the line is strong and breaks at one moment. Mix in a few people of very different heights and the grip is awkward in places, so the line starts to break earlier and falls apart bit by bit.

Real-world example

Pharmaceutical companies test every batch of a drug before it is sold. A melting point that is too low or too broad signals that the product contains leftover starting materials or by-products and must be purified again, for example by recrystallisation, before it can be used in medicines.

Why?

Why do impurities lower the melting point? In a pure crystal the particles fit together perfectly, maximising the attractions between them. Foreign particles do not fit the pattern, so they weaken the structure around them. Less energy is needed to break the weakened structure, so melting starts at a lower temperature.

Common misconception

"Impurities make a solid harder to melt, so the melting point goes up." For almost all ordinary impure solids, the melting point goes down, not up. Road salt melts ice precisely because the impurity (salt) lowers the melting point of the ice below 0 °C.

Worked example

Question: Pure benzoic acid melts at 122 °C. Sample A melts at 121.5–122 °C. Sample B melts at 115–119 °C. Which is purer?

Reasoning: Sample A melts sharply, within 0.5 °C, at the expected value. Sample B melts 3–7 °C too low and over a 4 °C range.

Answer: Sample A is purer; Sample B contains impurities.

Quick check

1. State two ways in which an impurity affects the melting of a solid. Answer: It lowers the temperature at which melting starts and widens the melting range.

Exam focus

Use two pieces of evidence in answers: the value (compared with the data book) and the sharpness (a range rather than a single temperature). Remember that impurities lower melting points; exam questions often test whether you confuse this with boiling points, which impurities raise.

Advanced insight

Some mixtures of two solids have a special composition, called the eutectic, that melts sharply at a temperature lower than either pure component. Traditional tin–lead solder is close to a eutectic mixture, which is why it melts at one low temperature. A sharp melting point is therefore strong but not absolute proof of purity.

Summary

A pure solid melts sharply at a fixed temperature, its melting point. Impurities disrupt the crystal structure, which lowers the melting point and spreads melting over a range. Comparing a measured melting point with a data-book value, and checking how sharp it is, gives a quick and reliable test of purity.

Practice questions

1. What is meant by a sharp melting point? Answer: Melting completely at a fixed temperature, over a very narrow range of about 1 °C or less. 2. A sample of aspirin melts between 129 °C and 133 °C. Pure aspirin melts at about 136 °C. What can you conclude? Give two reasons. Answer: The sample is impure, because it melts below the pure value and over a range of 4 °C. 3. Explain, using particles, why impurities lower the melting point of a solid. Answer: Impurity particles disrupt the regular arrangement, weakening the forces holding the structure, so less energy is needed to melt it. 4. Why must the sample be heated slowly near its melting point when the measurement is made? Answer: So that the thermometer reading keeps up with the sample's temperature and the melting temperatures are recorded accurately.