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Chemistry · Chapter 2: Separation and Purification

Checking Purity: Melting and Boiling Points

Testing purity with melting and boiling points: pure substances melt sharply, impurities lower the melting point and raise the boiling point.

Taught in Class 11O LevelA Level

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01Concept

A pure substance contains only one substance, with nothing mixed in. Purity matters: medicines, food and chemicals used in industry must be pure, because even small amounts of impurities can be harmful.

Melting and boiling points as tests of purity

Pure substanceImpure substance
Melting pointSharp: melts at one fixed temperatureLower, and melts over a range of temperatures
Boiling pointSharp: boils at one fixed temperatureHigher, and boils over a range
ExamplePure ice melts at 0 °C; pure water boils at 100 °C (at sea level)Ice made from salt water melts below 0 °C; salt water boils above 100 °C

So, to check purity, measure the melting or boiling point and compare it with the value in a data book. If it is sharp and matches, the substance is pure.

Measuring a melting point

  1. Put a little of the powdered solid in a thin capillary tube, sealed at one end.
  2. Attach it to a thermometer and heat it slowly in a water or oil bath (or in an electric melting-point apparatus).
  3. Note the temperature when the solid starts to melt and when it has completely melted.
  4. A pure substance melts within about 1 °C; an impure one melts lower and over several degrees.

Other tests

  • Chromatography: a pure substance gives a single spot.
  • Appearance: pure crystals usually have a regular shape; but appearance alone is not a reliable test.

02Virtual Lab

Try this topic for yourself in the virtual lab. It opens on its own page with steps, tasks and questions.

ExperimentMelting Point and PurityCompare pure and impure benzoic acid, and see why you must heat slowly.Open the lab →ExperimentPaper ChromatographySee that a pure dye gives one spot, while a mixture gives several.Open the lab →ExperimentSimple and Fractional DistillationSee that pure water boils at a fixed temperature, while a mixture boils over a range.Open the lab →

03Real World Examples

Medicines

Pharmaceutical companies test batches of medicines for purity, for example by checking melting points and chromatograms, before they are released.

Cooking in the mountains

In the northern areas, water boils below 100 °C because the air pressure is lower, so dal and meat take longer to cook; a pressure cooker helps.

Pure gold

Gold is tested for purity in carats; 24-carat gold is pure. Impurities change its properties, just as impurities change melting points.

04Practice Questions

Try to answer in your own words first, then open the model answer.

Short questions

Q1.What is a pure substance?Show answer

Answer

A pure substance contains only one kind of substance (element or compound), with nothing else mixed in.

Q2.How do impurities affect melting and boiling points?Show answer

Answer

  • They lower the melting point and make it melt over a range.
  • They raise the boiling point and make it boil over a range.
Q3.A sample of a solid melts between 112 °C and 118 °C. The data book value is 122 °C. Is it pure? Explain.Show answer

Answer

No. It melts below the data-book value and over a range of 6 °C. A pure sample would melt sharply at 122 °C.

Q4.Why is salt spread on icy roads?Show answer

Answer

Salt acts as an impurity in the water, lowering its freezing point below 0 °C, so the ice melts at ordinary winter temperatures.

Q5.Give two methods of checking the purity of a substance.Show answer

Answer

  • Measure its melting or boiling point and compare with the known value.
  • Run a chromatogram: a pure substance gives one spot.

Long questions

Q6.Explain how melting and boiling points are used to test purity. Describe how a melting point is measured.Show answer

Answer

A pure substance has a sharp, fixed melting point and boiling point. Impurities lower the melting point (and widen its range) and raise the boiling point. Comparing the measured value with the data-book value shows whether a sample is pure.

  1. Put the powdered solid in a capillary tube sealed at one end.
  2. Fix it beside the bulb of a thermometer in a water or oil bath, or use a melting-point apparatus.
  3. Heat slowly and stir.
  4. Record the temperatures at which melting starts and finishes.
  5. A narrow range at the correct value means the sample is pure.

05Quiz

Question 1 of 7
A pure substance melts:

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