Boiling Point Determination

For solids, the melting point is the go-to identity and purity check. For liquids, the equivalent is the boiling point. It’s a little trickier to measure well, because boiling depends on the pressure of the air above the liquid, but with the right technique it gives valuable information from very little sample.

This guide explains what boiling actually is, two practical methods, and how to interpret the result.

What is boiling?

Molecules at the surface of any liquid are constantly escaping into the gas phase. In a closed space, this creates a vapour pressure, which rises as the temperature rises.

A liquid boils when its vapour pressure equals the pressure of the surroundings. At that point, bubbles of vapour can form inside the liquid, not just evaporate from the surface.

This leads to two important consequences:

1. The boiling point depends on external pressure. At the top of a high mountain, water boils below 100 °C; in a pressure cooker it boils above 100 °C.
2. The normal boiling point is defined at standard atmospheric pressure, 101.325 kPa (1 atm, 760 mmHg). Literature values are quoted at this pressure.

The boiling point reflects the strength of the intermolecular forces: stronger forces (such as hydrogen bonding) mean higher boiling points.

Boiling point and purity

For a pure liquid, the temperature stays constant throughout boiling. Impurities change the picture:

• A non-volatile impurity (such as a dissolved solid) raises the boiling point.
• A volatile impurity makes the liquid boil over a range, as the more volatile component distils off first.

So a steady boiling temperature close to the literature value suggests a pure compound.

Safety

• Wear eye protection and work in a fume cupboard; many organic liquids are flammable and give off harmful vapours.
• Use an electric heating mantle, hot plate or oil bath, not a Bunsen flame, for flammable liquids.
• Never heat a closed system. Pressure builds and the apparatus can burst.
• Always add anti-bumping granules before heating a liquid in a flask. Without them, the liquid can superheat and then boil violently (“bump”). Never add them to a hot liquid, which can erupt.
• Check the safety data sheet for each liquid. See lab safety rules.

Method 1: Simple distillation (larger samples)

This method suits samples of about 5 cm³ or more, and purifies the liquid at the same time.

1. Put the liquid and a few anti-bumping granules into a round-bottomed flask (no more than half full).
2. Fit a still head with a thermometer. The bulb must sit just below the side arm leading to the condenser, so that it’s bathed in vapour that’s about to condense.
3. Connect the condenser, with cooling water entering at the bottom and leaving at the top, so the jacket stays full.
4. Heat gently. Vapour rises, surrounds the thermometer bulb and condenses into the receiver.
5. Record the temperature once it becomes steady while liquid drips from the condenser at a steady rate (about one drop per second).

That steady temperature is the boiling point. If it rises continuously during distillation, the liquid is a mixture.

Why the thermometer position matters: if the bulb is too high, it isn’t fully in the vapour and reads too low. If it’s in the liquid, it may read the temperature of superheated liquid instead of the equilibrium vapour.

Method 2: The micro method (Siwoloboff method)

This uses only a few drops of liquid.

1. Put about 0.5 cm³ of the liquid in a small ignition tube (or a narrow test tube).
2. Drop in a capillary tube sealed at one end, open end down.
3. Attach the tube to a thermometer with a rubber band (kept well above the heating liquid) so the sample is next to the bulb.
4. Heat the assembly gently in an oil bath, Thiele tube or heated block.
5. As the temperature rises, air in the capillary expands and a few bubbles escape. When the boiling point is passed, a rapid, continuous stream of bubbles comes out of the capillary.
6. Stop heating and let it cool slowly while watching carefully.
7. The bubbles slow and stop. At the moment the liquid is drawn back up into the capillary, record the temperature. That is the boiling point.

Why this works: while the stream of bubbles emerges, the capillary is full of the liquid’s vapour, and its vapour pressure is higher than atmospheric. As it cools, the moment the vapour pressure falls just below atmospheric pressure, the outside air pressure pushes liquid into the capillary. At that instant, the vapour pressure equals atmospheric pressure: the definition of the boiling point.

Taking the reading on cooling, not heating, avoids errors from superheating.

Correcting for pressure

Lab air pressure varies with weather and altitude. At 1–2 kPa below standard pressure, a liquid boils slightly lower than its normal boiling point. Record the barometric pressure with every boiling point.

A useful rule of thumb for many organic liquids: near standard pressure, the boiling point changes by roughly 0.3 °C for every 1 kPa change in pressure (about 0.04 °C per mmHg). For accurate work, more precise corrections are used, or the value is simply reported with the pressure, for example “b.p. 78 °C at 100.5 kPa”.

Worked interpretation

A student distils a liquid thought to be ethanol (normal boiling point 78.4 °C). The lab pressure is 100.0 kPa.

• Expected boiling point at 100.0 kPa: about 1.3 kPa below standard, so roughly 0.35 °C lower, around 78.0 °C.
• Observed: a steady 77.9–78.1 °C for most of the distillation.

Conclusion: consistent with ethanol. (A small water content wouldn’t show a big change, because ethanol and water form an azeotrope boiling at about 78.2 °C. That’s why boiling point alone can’t prove purity: spectroscopy or density measurements would confirm it.)

Common errors and fixes

Error	Effect	Fix
thermometer bulb too high in the still head	reading too low	bulb just below the side arm
no anti-bumping granules	bumping, superheating	add granules before heating
heating too fast	reading unsteady, superheated vapour	heat steadily; about one drop per second
micro method read on heating	reading too high	read the temperature as liquid enters the capillary on cooling
pressure not recorded	result can’t be compared with literature	always note the barometric pressure

See experimental errors.

Boiling points in context

Boiling points are used alongside other tests to identify liquids, such as gas chromatography and infrared spectroscopy. They also explain why some elements are gases, liquids or solids at room temperature: see highest boiling point element and states of matter. For solids, see the companion guide on melting point determination.

Key takeaways

• A liquid boils when its vapour pressure equals the external pressure; normal boiling points are quoted at 101.325 kPa.
• A pure liquid boils at a constant temperature; impurities raise it or spread it over a range.
• In distillation, the thermometer bulb goes just below the side arm, and anti-bumping granules prevent bumping.
• In the micro (Siwoloboff) method, the boiling point is the temperature at which liquid re-enters the capillary on cooling.
• Always record atmospheric pressure, and never heat a closed system.