How to Decide Whether a Sample Is Pure from Melting-Point Data

Two white crystalline powders look identical. An authenticated sample of compound R melts between 121.0 °C and 121.5 °C. Unknown sample A melts between 121.2 °C and 121.8 °C, while sample B starts melting at 116.5 °C and does not become completely liquid until 120.0 °C.

Which sample is more likely to be pure?

Sample A. Its melting range is narrow and closely matches the reference. Sample B has a lower, broader melting range, which is typical of a sample containing impurities.

This gives you the basic method for deciding whether a sample is pure from melting-point data: compare the width of its melting range and its position against the accepted melting point of the substance. You need both pieces of information. A narrow range alone does not prove purity.

01The decision: what does the melting range tell you?

A pure crystalline substance has a characteristic melting temperature at a specified pressure. Ideally, it changes from solid to liquid at one temperature.

In a real experiment, however, you usually record a small temperature interval rather than a single value. This is called the melting range.

There are two temperatures you need to identify:

• Onset temperature: The temperature at which the first visible liquid appears among the solid crystals.
• Clear point: The temperature at which the last remaining crystal disappears and the sample becomes completely liquid.

The width of the melting range is calculated using:

\[
\Delta T=T_{\text{clear}}-T_{\text{onset}}
\]

Here, \(\Delta T\) is the melting range width, measured in degrees Celsius.

For example, if a sample begins melting at 118.0 °C and becomes completely liquid at 120.5 °C:

\[
\Delta T=120.5-118.0
\]

\[
\boxed{\Delta T=2.5\ ^\circ\mathrm{C}}
\]

The sample has a melting range of 118.0-120.5 °C, with a width of 2.5 °C.

Notice that you need both endpoints. Reporting only the midpoint would hide useful information about how the sample actually melts.

How to interpret the results

Use the following comparison when analysing experimental data.

The important distinction is between two questions:

1. Does the sample melt over a narrow temperature range?
2. Does that range agree with the expected melting point of the substance?

A sample can melt sharply at the wrong temperature. It might be a different pure compound rather than an impure version of the substance you expected.

There is also no universal numerical cutoff that proves purity. A range of 0.5 °C might be excellent for one experimental setup but less convincing for another. Always consider the apparatus, heating rate, and reference data.

02How is melting-point data collected?

Before interpreting the numbers, you need to understand where they come from.

A melting-point experiment uses a small quantity of solid in a narrow capillary tube. The sample is heated gradually while its temperature and physical appearance are monitored.

The apparatus

The experiment follows a straightforward sequence.

1. Prepare the sample.

A small amount of dry solid is finely crushed into a powder.

The powder is packed into the sealed end of a thin capillary tube, usually to a height of approximately 2-3 mm.

Using a small, compact sample helps it heat relatively evenly. A large, loosely packed sample can develop temperature differences between different regions.

2. Position the capillary tube.

In a traditional oil-bath arrangement, the capillary is attached to a thermometer. The sample sits level with the thermometer bulb so that both experience approximately the same temperature.

The thermometer and capillary are immersed in an oil-filled Thiele tube. Heating the side arm produces convection currents, which circulate the oil and distribute heat around the apparatus.

In a digital melting-point apparatus, the capillary is inserted into a heated block beside a temperature sensor. The instrument controls the heating rate and displays the temperature.

3. Heat the sample gradually.

The heater transfers energy to the oil bath or metal block. Heat then passes into the capillary and the solid sample.

The temperature sensor measures the temperature of its surroundings, which is used to estimate the sample’s temperature.

The sample does not necessarily reach exactly the same temperature as the sensor at every instant. This becomes particularly important if the apparatus is heated too quickly.

4. Observe the melting process.

Watch the sample through the observation window or magnifier.

Record the temperature when the first liquid appears. Continue heating slowly until the final crystal disappears, and record that temperature too.

These are the onset temperature and clear point.

Why must the sample be heated slowly?

Imagine that the heating block is increasing in temperature rapidly.

The temperature sensor may register 130 °C while the sample inside the capillary is still slightly cooler. The sample needs time to absorb energy from its surroundings.

If you record the displayed temperature as soon as the sample starts melting, you may report a value higher than its actual melting temperature.

Rapid heating can therefore produce an artificially high and broader measured melting range.

A better method is to heat relatively quickly while well below the expected melting point, then slow the heating rate as the sample approaches its melting temperature. A rate of approximately 1-2 °C per minute near the transition is a useful experimental guide, subject to the instrument’s instructions.

Repeating the measurement with a fresh portion of sample helps determine whether the result is reproducible.

03Why do impurities change the melting range?

To understand the result, picture a pure crystalline solid.

Its particles are arranged in an organised, repeating structure called a crystal lattice. The particles are held together by attractive forces.

As the solid absorbs thermal energy, its particles gain energy and vibrate more strongly. At the melting temperature, the ordered structure can no longer remain stable, and the particles become free to move past one another as a liquid.

For a pure substance at constant pressure, this transition occurs at a characteristic temperature.

What changes when an impurity is present?

Imagine stacking identical building blocks in a neat arrangement. Each block fits into the repeating structure.

Now introduce blocks with different shapes. They disrupt the arrangement, making some regions less stable than others.

An impurity can have a similar effect on a crystal lattice.

For many common impurities that mix with the substance during melting:

• The regular crystal arrangement is disrupted.
• Some of the solid can begin melting at a lower temperature.
• Different regions of the sample may melt at slightly different temperatures.
• The sample therefore melts over a broader interval.

This produces two common observations:

1. The melting range is depressed, meaning it occurs at a lower temperature than the pure reference.
2. The melting range is broadened, meaning the difference between the onset and clear point is larger.

This is why a lower, broader melting range is useful evidence of impurities.

However, the analogy has limits. Not every contaminant enters the crystal lattice in the same way. An insoluble contaminant, for example, may behave differently from a substance that mixes readily with the liquid.

The usual HSC interpretation is a useful general rule, not a claim that every possible mixture must show identical behaviour.

What would a heating curve look like?

You can also represent melting using a temperature-versus-time graph.

For a pure substance, the temperature rises as the solid absorbs energy. During melting, the temperature remains approximately constant while the solid changes into liquid.

The energy supplied during this interval is used to overcome the attractive forces holding the particles in the solid structure.

An impure sample commonly shows a less distinct transition. Its temperature can continue rising while different parts of the sample melt.

The graph reinforces the same idea: a pure substance has a well-defined melting temperature, while a typical impure sample melts across a wider interval.

A common misconception: a narrow range always means pure

Consider an unknown sample that melts between 95.0 °C and 95.5 °C.

That is a narrow range. Does it prove that the sample is pure?

No.

Suppose the substance you are investigating should melt at 122 °C. The unknown sample’s narrow range is nowhere near the expected value.

It might be a different pure compound. It might also be a different solid form of the same compound, or the experiment might have been affected by an error.

The narrow range tells you that the sample undergoes a relatively sharp melting transition. It does not establish its chemical identity.

Always compare the measured range with a reliable reference value.

An accepted melting point is a property of a particular substance under specified conditions. It is not a general measure of how strongly all solids are held together. If you want to understand why different pure ionic compounds have different melting points, see Why Group 1 and Group 2 Chlorides Have Different Melting Points.

04Worked examples

The following examples use original illustrative teaching data, not measurements from actual laboratory experiments.

05A reliable method for HSC questions

When a question gives you melting-point data, work through these decisions in order.

1. Identify the reference. What melting point should the pure substance have?
2. Find the experimental endpoints. Identify when melting begins and when the last crystal disappears.
3. Calculate the range width. Subtract the onset temperature from the clear point.
4. Compare both features. Is the experimental range narrow, and does it agree with the reference?
5. Check the experimental conditions. Consider heating rate, calibration, pressure, and whether the sample was prepared correctly.
6. State a justified conclusion. Use phrases such as ‘consistent with a pure sample’ or ‘suggests the presence of impurities’.

In an HSC response, avoid claiming that a sample is definitely impure from one broad melting range unless the question provides enough information to rule out experimental error.

06Questions and solutions

The following questions move from straightforward interpretation towards more challenging experimental reasoning. All numerical data are illustrative.

Question 1

A pure reference sample of compound A has a melting range of 72.0-72.6 °C.

An unknown sample of A begins melting at 71.9 °C and becomes completely liquid at 72.5 °C.

(a) Calculate the melting range width of the unknown sample.

(b) Is the unknown sample consistent with being pure? Explain your reasoning.

Solution 1

(a) The melting range width is 0.6 °C.

Using:

\[
\Delta T=T_{\text{clear}}-T_{\text{onset}}
\]

\[
\Delta T=72.5-71.9
\]

\[
\boxed{\Delta T=0.6\ ^\circ\mathrm{C}}
\]

(b) The unknown sample is consistent with being relatively pure compound A.

Its melting range of 71.9-72.5 °C closely matches the reference range of 72.0-72.6 °C. Both ranges have the same width, and their endpoints differ by only 0.1 °C.

These observations support the conclusion that the unknown has a similar melting behaviour to the pure reference.

However, they do not conclusively prove chemical purity. An independent identification method would provide stronger evidence.

Question 2

Three batches of a solid are tested using the same calibrated apparatus, at the same pressure, and with the same slow heating rate.

An authenticated pure sample of compound B melts between 103.2 °C and 103.8 °C.

The results are:

(a) Calculate the melting range width of each batch.

(b) Identify the batch that is most consistent with pure compound B.

(c) Explain why Batch R should not automatically be classified as impure, despite its melting range being different from the reference.

Solution 2

(a) The melting range widths are:

For Batch P:

\[
\Delta T=103.9-103.1
\]

\[
\boxed{\Delta T_P=0.8\ ^\circ\mathrm{C}}
\]

For Batch Q:

\[
\Delta T=103.6-99.0
\]

\[
\boxed{\Delta T_Q=4.6\ ^\circ\mathrm{C}}
\]

For Batch R:

\[
\Delta T=105.4-104.8
\]

\[
\boxed{\Delta T_R=0.6\ ^\circ\mathrm{C}}
\]

(b) Batch P is the most consistent with pure compound B.

Its melting range of 103.1-103.9 °C closely matches the reference range of 103.2-103.8 °C. Its width is also relatively small.

Batch Q has a much broader range and begins melting at a substantially lower temperature. This suggests that it contains impurities.

(c) Batch R has a narrow melting range, but it does not match the reference for compound B.

Its melting range is 104.8-105.4 °C, which is approximately 1.6 °C higher than the reference at both endpoints.

This does not automatically mean that R is impure. It could be a different pure substance, or a different solid form of the intended compound.

The important distinction is that a narrow range indicates a sharp melting transition, whereas agreement with the reference helps establish whether the sample behaves like the expected substance.

Batch R cannot be confidently identified as pure compound B from these results.

Question 3

A student investigates a sample labelled compound C.

An authenticated pure reference sample has a melting range of 121.0-121.5 °C.

The student performs three measurements using a digital melting-point apparatus. Its temperature sensor measures the heating block temperature.

Assume the sample is unchanged between trials, and the heating block heats more quickly than the sample can absorb energy during Trial 1.

(a) Which trials provide the most reliable evidence about the sample’s melting point?

(b) Explain why Trial 1 produces a different result.

(c) What conclusion can be drawn about the purity of the sample?

Solution 3

(a) Trials 2 and 3 provide the most reliable evidence.

Both use a slower heating rate and produce similar results.

Their melting range widths are:

\[
\Delta T_2=122.0-121.2=0.8\ ^\circ\mathrm{C}
\]

\[
\Delta T_3=121.9-121.1=0.8\ ^\circ\mathrm{C}
\]

The results are also close to the authenticated reference range of 121.0-121.5 °C.

(b) Trial 1 produces an artificially high and broad measured range because of thermal lag.

The heating block temperature rises faster than the sample temperature. The sensor therefore reports a temperature that is higher than the actual temperature of the solid.

When the sample begins melting, the displayed temperature is already relatively high. The difference between the block and sample temperatures can also change throughout the transition, producing an exaggerated melting range.

Slower heating allows the sample to absorb energy more evenly and reduces this discrepancy.

(c) The sample is consistent with being relatively pure compound C.

The two slow-heating trials produce reproducible, narrow ranges that closely match the reference.

Trial 1 should not be used as evidence that the sample is impure because its rapid heating rate makes the measurement unreliable.

The results support purity, but they do not prove it conclusively.

Question 4

A student is given an unknown solid, U, which is believed to be compound D.

An authenticated pure sample of D has a melting range of 112.0-112.8 °C.

The student obtains the following results:

All samples are tested using the same calibrated apparatus and a slow heating rate.

(a) Explain why the melting range of U initially suggests that it may be pure D.

(b) Interpret the result obtained when U is mixed with the authenticated reference.

(c) Can the experiment establish whether U is a different pure compound or an impure sample of D? Explain.

Solution 4

(a) The melting range of U initially suggests that it may be pure D because it closely matches the authenticated reference.

The reference melts between 112.0 °C and 112.8 °C, while U melts between 112.1 °C and 112.9 °C.

The widths are also identical:

\[
112.8-112.0=0.8\ ^\circ\mathrm{C}
\]

\[
112.9-112.1=0.8\ ^\circ\mathrm{C}
\]

Based on these results alone, U appears consistent with pure D.

(b) The mixed sample provides evidence that U is not behaving like pure D.

When two samples of the same pure compound are mixed, the mixture should retain approximately the same melting range. Both samples contain the same chemical substance, so combining them does not introduce a new component into the crystal system.

However, the mixture of U and D melts between 106.0 °C and 110.5 °C.

This is both substantially lower and broader than the reference melting range.

The result suggests that U contains a different substance or an impurity that disrupts the melting behaviour of D.

This procedure is called a mixed melting-point test.

(c) No. The experiment cannot distinguish conclusively between a different pure compound and an impure sample of D.

If U is a different pure compound, mixing it with D introduces two different substances. The resulting mixture can have a depressed, broadened melting range.

If U is impure D, its existing impurity can also produce this behaviour when mixed with additional pure D.

Both possibilities explain the observations.

An independent chemical identification method would be needed to distinguish them.

Question 5

Extension challenge

A laboratory receives a white crystalline powder labelled compound X.

Three separate measurements produce the following results:

An authenticated pure sample of X has a certified melting range of 78.0-78.4 °C.

The apparatus is calibrated, the samples are dry, and all trials use slow heating.

However, the laboratory also knows that two other solids, A and B, can form a binary mixture. Their pure melting points are:

• Pure A: 92.0 °C.
• Pure B: 110.0 °C.

A phase diagram for the A-B system shows that a mixture containing exactly 40% A and 60% B has a eutectic composition and melts sharply at 78.2 °C.

A eutectic composition is a particular ratio of two substances at which their mixture melts at a single temperature lower than the melting points of either pure component.

A student argues:

‘The unknown sample must be pure X because it has a narrow melting range, the results are reproducible, and its melting point agrees with the authenticated reference.’

Evaluate this conclusion.

In your answer:

(a) Explain why the student’s evidence initially appears convincing.

(b) Describe the alternative explanation involving A and B.

(c) Determine whether melting-point data alone can establish the purity of X.

(d) Recommend an additional experimental technique that could distinguish the two possibilities.

Solution 5

(a) The student’s evidence is convincing because all three measurements produce narrow, reproducible melting ranges that agree with the authenticated reference.

The measured ranges have widths of:

\[
\Delta T_1=78.3-78.0=0.3\ ^\circ\mathrm{C}
\]

\[
\Delta T_2=78.2-78.0=0.2\ ^\circ\mathrm{C}
\]

\[
\Delta T_3=78.3-78.1=0.2\ ^\circ\mathrm{C}
\]

All three results are close to the certified range of 78.0-78.4 °C.

The repeated measurements also make random experimental error a less convincing explanation for the observed melting behaviour.

If you only considered the standard purity rule, you would reasonably conclude that the sample is consistent with pure X.

However, this is not the only possible explanation.

(b) The alternative explanation is that the unknown sample contains a mixture of A and B in their eutectic proportions.

At the specified eutectic composition of 40% A and 60% B, the mixture melts sharply at 78.2 °C.

This is lower than the melting point of either pure component:

\[
78.2<92.0<110.0
\]

Unlike an ordinary mixture that melts over a broad temperature interval, a mixture at its exact eutectic composition can melt at a single characteristic temperature.

Consequently, the A-B mixture could produce almost exactly the same melting-point data as pure X.

(c) No. Melting-point data alone cannot establish that X is pure.

There are two competing explanations for the observations:

• Model 1: The unknown sample is pure X, which melts at approximately 78.2 °C.
• Model 2: The unknown sample is a 40:60 eutectic mixture of A and B, which also melts at approximately 78.2 °C.

Both models explain the narrow melting range and the reproducibility of the measurements.

Repeating the same experiment reduces uncertainty in the measured melting range, but it does not resolve this ambiguity.

This is the important distinction between obtaining reliable data and obtaining enough evidence to establish chemical identity.

(d) An independent identification technique, such as infrared (IR) spectroscopy, could help distinguish the possibilities.

IR spectroscopy measures the absorption of infrared radiation by chemical bonds in a sample.

If X has a different molecular structure from A and B, its characteristic absorption bands may distinguish it from the mixture.

The laboratory could compare the unknown spectrum with reference spectra for X, A, and B.

Alternatively, chromatography could separate the components of the mixture, provided a suitable method is available.

The key is to use a technique that provides information about chemical composition rather than simply repeating the same melting-point measurement.

This question exposes the main limitation of the purity test: a sample can have a narrow, reproducible melting range without necessarily containing only one chemical substance.

07What should you do after a melting-point test?

A melting-point measurement is a useful first check of sample purity. It tells you whether the solid undergoes a sharp phase transition and whether its melting behaviour agrees with an authenticated reference.

It does not always identify the substance or reveal every possible impurity.

If the melting range is broad and depressed, investigate possible contamination and experimental errors. If the range is narrow but differs from the reference, investigate whether the sample is a different substance or solid form.

If the melting range matches the reference, you have evidence supporting purity. For stronger identification, use an independent analytical technique.

For another example of how chemical structure can be investigated using experimental data, read How to Read and Compare IR Spectra of Petroleum Samples.

The distinction to remember is simple: melting-point data test how a substance behaves during a phase change; they do not, by themselves, prove what the substance contains.