Melting and Boiling Points as Tests of Purity

Introduction

In everyday language, "pure" orange juice means juice with nothing added. In chemistry, pure has a stricter meaning: a sample containing only one substance. Chemists who make medicines, food additives or electronic materials must be able to check purity quickly. One of the simplest and most reliable checks uses melting and boiling points.

Core explanation

Scientific purity. A pure substance contains only one element or one compound. Pure water contains only water molecules; tap water is not pure in this sense, because it contains dissolved minerals and gases. Orange juice, however "pure" on the label, is a mixture of water, sugars, acids and many other substances.

Pure substances have sharp melting and boiling points. A pure solid melts at one exact temperature, and a pure liquid boils at one exact temperature (at a given pressure). These values match those in data tables. Pure water melts at 0 °C and boils at 100 °C at standard pressure.

Impurities change the melting point. An impure solid: - melts at a lower temperature than the pure substance, and - melts over a range of temperatures rather than at one point.

For example, a sample of an organic compound whose pure melting point is 122 °C might melt gradually between 115 °C and 119 °C if it contains impurities. The bigger the impurity content, the lower and wider the melting range tends to be.

Impurities change the boiling point. Dissolved impurities such as salt raise the boiling point of a liquid, and the liquid boils over a range of temperatures as its composition changes. Salty water boils slightly above 100 °C.

Melting-point tests in practice. A few crystals are packed into a thin glass capillary tube and heated slowly in a melting-point apparatus while the temperature is watched. The temperature at which melting starts and finishes is recorded. A sharp melting point matching the data book shows high purity.

Salt on roads. Adding salt to ice lowers the temperature at which it melts, so salted ice melts even when the air is below 0 °C. This everyday effect is the same one used in purity testing.

Step-by-step reasoning

To decide whether a sample of aspirin (pure melting point 136 °C) is pure:

1. Measure the melting range carefully: sample melts from 128 °C to 133 °C. 2. Compare with the data book: 136 °C. 3. The sample melts below the true value and over a 5 °C range. 4. Conclusion: the sample is impure and needs further purification (for example, recrystallisation).

Visual explanation

Two heating curves on the same axes: a pure solid shows a perfectly flat plateau at its melting point; an impure solid shows a sloping section starting at a lower temperature, because melting happens gradually over a range. A thermometer icon marks "sharp" on the first and "range" on the second.

Real-world analogy

A well-drilled marching band steps off at exactly the same moment when the whistle blows. A band with some untrained members mixed in starts raggedly — some early, some late. Impurity particles disrupt the orderly structure of a solid, so it "starts marching" (melting) unevenly and earlier.

Real-world example

Pharmaceutical companies test every batch of a medicine's active ingredient for purity. A melting-point check is one of the first tests: if the melting range is lower or wider than specified, the batch is rejected before more expensive tests are carried out, protecting patients from impure medicines.

Why?

Why do impurities lower the melting point? In a pure solid, identical particles are arranged in a regular pattern with strong, even attractions. Impurity particles disrupt this regular arrangement, weakening the structure in places. Less energy is needed to break it down, so melting starts at a lower temperature and proceeds gradually.

Common misconception

"Mineral water is purer than distilled water because it is natural." In the chemical sense, distilled water is much purer: it is almost entirely water molecules. Mineral water contains dissolved minerals — it is a mixture, however healthy it may be to drink.

Worked example

Question: Three samples of a compound (pure mp 80 °C) melt at: A, 80 °C exactly; B, 72–77 °C; C, 76–79 °C. Rank them from purest to least pure.

Reasoning: A matches the pure value sharply. C melts close to 80 °C over a 3 °C range. B melts furthest below 80 °C over a 5 °C range.

Answer: A (purest), then C, then B (least pure).

Quick check

1. How does an impurity affect the melting point of a solid? Answer: It lowers the melting point and makes the solid melt over a range of temperatures.

Exam focus

Remember the pattern: pure = sharp point matching data; impure solid = lower melting point over a range; impure liquid = higher boiling point over a range. Be ready to interpret data tables and to explain what "pure" means in chemistry versus everyday advertising.

Advanced insight

The lowering of the freezing point by a dissolved substance depends on the number of dissolved particles, not their identity. This is a colligative property, and chemists can use the size of the lowering to calculate the molar mass of an unknown dissolved substance.

Summary

A pure substance contains a single element or compound. Pure substances melt and boil at sharp, fixed temperatures that match data tables. Impurities lower and broaden melting points and raise boiling points. Measuring melting and boiling points is a quick, widely used test of purity.

Practice questions

1. A liquid boils at exactly 100.0 °C at standard pressure. What might it be, and what does the result suggest? Answer: Water; the sharp value matching the data suggests it is pure. 2. Why is salt spread on icy roads? Answer: It lowers the melting point of the ice, so the ice melts at temperatures below 0 °C. 3. A student's product melts between 108 °C and 114 °C; the pure compound melts at 118 °C. What can be concluded? Answer: The product is impure, because it melts below the pure value and over a range. 4. Explain why orange juice is not a pure substance in the scientific sense. Answer: It contains many different substances (water, sugars, acids and others), so it is a mixture.