Melting and Boiling Points as a Test of Purity

Introduction

When a chemist makes a new medicine, one of the first questions is simple: is it pure? A tiny amount of an unwanted substance could make a drug less effective or even harmful. One of the quickest and cheapest ways to check purity is to measure a melting point or a boiling point. Pure substances behave in a very tidy way when they change state, while mixtures do not. This page explains how that difference becomes a practical test.

Core explanation

Pure substances have fixed points. Every pure substance melts at one exact temperature and boils at one exact temperature (at a given pressure). Pure water melts at 0 °C and boils at 100 °C at normal atmospheric pressure. Pure ethanol boils at 78 °C. Pure aspirin melts at about 136 °C. Because these values never change for a pure substance, they are like a fingerprint: they can be looked up in data books and compared with a measured value.

Sharp versus gradual. When a pure solid is heated, it stays solid until its melting point, then melts completely at that one temperature. The change is sharp . On a heating curve, you see a flat, level section at exactly the melting point.

Impurities change the picture. If a solid contains even a small amount of another substance, two things happen:

- The melting point is lowered — the solid starts to melt below the pure value. - The melting point is spread out — the solid melts gradually over a range of temperatures instead of at one point.

For boiling, an impurity dissolved in a liquid usually raises the boiling point and makes the liquid boil over a range of temperatures. Salty water, for example, boils a little above 100 °C, and the boiling temperature creeps upwards as water boils away and the salt becomes more concentrated.

Using the test. To judge purity, compare two things with the data-book value: is the measured value the same, and is the change sharp? A sample that melts sharply at exactly the expected temperature is very likely pure. A sample that melts lower and over several degrees contains impurities. The wider the range and the further below the true value, the less pure the sample tends to be.

Mixtures have no single melting point. Alloys, waxes, glass and chocolate are all mixtures. None of them has one fixed melting point; they soften and melt gradually over a range.

Step-by-step reasoning

To decide whether a solid is pure:

1. Look up the melting point of the pure substance. 2. Heat the sample slowly and watch closely. 3. Record the temperature when melting starts and when it finishes. 4. If melting starts and finishes at the data-book value, within about 1 °C, the sample is pure. 5. If melting starts lower and spreads over several degrees, the sample is impure.

Visual explanation

Imagine two heating curves side by side. The pure sample's curve rises, then runs perfectly flat at the melting point, then rises again. The impure sample's curve has no flat section; instead it has a gentle, sloping bend that begins a few degrees lower and stretches over a range of temperatures.

Real-world analogy

Think of a line of dominoes all the same size, evenly spaced: they fall in one smooth, predictable run. Slip in a few odd-sized pieces at random and the pattern breaks down; the fall becomes uneven. Impurities disturb the regular arrangement of particles in a solid in a similar way.

Real-world example

Pharmaceutical companies routinely check the melting point of each batch of a drug such as paracetamol, which melts at about 169 °C when pure. A batch that melts at 163–166 °C would be rejected or purified again, because the low, wide range shows it contains something that should not be there.

Why?

Why do impurities lower the melting point? In a pure solid, identical particles sit in a regular, repeating pattern held by equal forces. Foreign particles disrupt this pattern and weaken the attractions in places. Less energy is needed to break the arrangement apart, so melting begins at a lower temperature, and different regions melt at slightly different temperatures.

Common misconception

"If a substance melts, it must be pure." Mixtures melt too. What matters is where and how it melts: a pure substance melts at a sharp, fixed temperature that matches the data book, while a mixture melts lower and over a range.

Worked example

Question: The pure form of a compound melts at 122 °C. Sample A melts between 121.5 and 122 °C. Sample B melts between 115 and 119 °C. Which sample is purer, and why?

Reasoning: Sample A melts sharply at almost exactly the pure value. Sample B melts several degrees lower and over a 4 °C range, both signs of impurities.

Answer: Sample A is purer; sample B contains impurities that lower and widen its melting range.

Quick check

1. What two signs in melting data show that a solid is impure? Answer: It melts below the pure value, and it melts over a range of temperatures rather than at one point.

Exam focus

Examiners often give a table of melting data and ask which sample is pure. Look for a sharp value matching the known melting point. Remember the direction of the effect: impurities lower the melting point but raise the boiling point. Say "a range" rather than "a different temperature" for impure samples.

Advanced insight

The lowering of the freezing point by dissolved substances is used deliberately. Salt is spread on icy roads because salty water freezes below 0 °C, and antifreeze in car engines lowers the freezing point of the coolant. Chemists call these effects freezing-point depression and boiling-point elevation; their size depends on how many dissolved particles there are.

Summary

A pure substance melts and boils at sharp, fixed temperatures that match data-book values. Impurities lower the melting point and make the solid melt over a range; dissolved impurities raise the boiling point. Measuring whether a sample changes state sharply at the expected temperature is a quick, cheap and widely used test of purity.

Practice questions

1. Pure water boils at 100 °C. A sample of water boils at 102 °C. What does this suggest? Answer: The water contains a dissolved impurity, such as salt, which has raised its boiling point. 2. Why does chocolate soften gradually instead of melting at one temperature? Answer: Chocolate is a mixture of several substances, so it has no single melting point and melts over a range. 3. A student's sample melts sharply at 80 °C. The pure substance melts at 80 °C. What can the student conclude? Answer: The sample is very likely pure, because it melts sharply at the correct temperature. 4. Explain, in terms of particles, why an impurity lowers the melting point of a solid. Answer: Impurity particles disrupt the regular arrangement and weaken the attractions between particles, so less energy is needed to break up the solid.