Melting, boiling and freezing points, and temperature scales

Water freezes at 0 °C and boils at 100 °C, but only under certain conditions. Change the pressure, or dissolve something in the water, and both figures move. This guide explains what melting, boiling and freezing points are, what shifts them, how a melting point can show whether a solid is pure, and how the Celsius, kelvin and Fahrenheit scales relate.

Terms are defined in the glossary.

What is a melting point?

The melting point is the temperature at which a solid turns into a liquid. At that temperature, solid and liquid can exist together in balance (OpenStax).

The freezing point is the same temperature approached from the other direction, as a liquid turns solid. For a pure substance, the two are the same number. Ice melts at 0 °C and water freezes at 0 °C.

Heat added at the melting point goes into breaking the solid's structure, not into raising the temperature. That is why a mix of ice and water stays at 0 °C until all the ice has melted. A heating curve shows this as a flat step:

What is a boiling point?

A liquid gives off vapour at any temperature, which is why puddles evaporate. The pressure that vapour creates is the vapour pressure, and it rises as the liquid warms. A liquid boils when its vapour pressure equals the pressure of the air around it (OpenStax).

The normal boiling point is the boiling point at standard atmospheric pressure (1 atm, 101.3 kPa). For water it is 100 °C.

What is the boiling point of water?

Water boils at 100 °C, 212 °F or 373.15 K at standard atmospheric pressure (1 atm). Because boiling depends on the surrounding pressure, the boiling point changes with it:

Altitude figures are from the Engineering ToolBox, the pressure cooker figure (250 °F) from Penn State Extension, and the autoclave temperatures from NHS Scotland, SHTM 01-01 Part C.

• Lower pressure, lower boiling point. At high altitude the air pressure is lower, so water boils below 100 °C. In a town 3,100 m (10,200 ft) up, at about 68 kPa, water boils at about 90 °C (OpenStax).
• Higher pressure, higher boiling point. A pressure cooker or an autoclave raises the pressure, so water stays liquid well above 100 °C.

Labs use this deliberately. A rotary evaporator lowers the pressure so a solvent boils away at a gentle temperature. Freeze-drying goes further: at very low pressure, ice turns straight to vapour without melting. See what is lyophilisation.

For the boiling points of common lab solvents, see the solvent miscibility and polarity chart.

Freezing point of water, and why solutions freeze lower

Pure water freezes at 0 °C. Dissolving anything in it lowers that point. This is called freezing-point depression (OpenStax).

The size of the drop depends on how many dissolved particles there are, not on what they are:

ΔTf = Kf × m

where Kf for water is 1.86 °C per mol/kg, and m is the number of moles of dissolved particles per kilogram of water.

These are ideal figures. Real salt solutions depress the freezing point a little less, because the ions don't behave fully independently.

This is why a buffer or salt solution doesn't freeze at exactly 0 °C. It also explains why a solution freezes unevenly. As ice forms, the solutes are pushed into the remaining liquid, which gets more concentrated and freezes lower still. A freezer must be well below 0 °C to freeze a solution solid. See freezer temperatures.

The same effect raises the boiling point. For water, it rises by 0.512 °C for each mole of particles per kilogram (OpenStax).

Melting point as a purity check

A pure crystalline solid melts sharply, over a narrow range of a degree or so. Impurities lower the melting point and spread it over a wider range (LibreTexts, Uses of Melting Points).

That makes melting point a quick, rough check on a crystalline sample. It isn't proof of purity or identity on its own, because different compounds can melt at similar temperatures. It is one piece of evidence alongside others.

Why freeze-dried peptides have no sharp melting point

Melting points apply to crystals. Freeze-dried (lyophilised) peptides are usually amorphous: their molecules have no regular packing, like a glass. Instead of melting sharply, an amorphous solid softens gradually above a temperature called its glass transition. See crystalline or amorphous? in the freeze-drying guide.

So a melting point isn't a useful purity test for a peptide. Purity is measured by HPLC instead, and identity by mass spectrometry.

Temperature scales: Celsius, kelvin and Fahrenheit

Celsius

The Celsius scale puts water's freezing point at 0 °C and its normal boiling point at about 100 °C (NIST). It is the scale used in UK labs and on most storage labels.

The kelvin scale

The kelvin (K) is the SI unit of temperature (BIPM). It starts at absolute zero, 0 K, the lowest possible temperature. A kelvin is the same size as a degree Celsius, so converting is a matter of adding a fixed number:

K = °C + 273.15

Kelvin temperatures are written without a degree sign: 298.15 K, not 298.15 °K. Because the scale never goes negative, it is the one used in equations such as the Arrhenius equation, which wouldn't work with Celsius values below zero.

Fahrenheit

To convert Celsius to Fahrenheit, multiply by 1.8 and add 32. To go back, subtract 32 and divide by 1.8 (NIST).

Common lab temperatures on each scale

For more on each storage range, see room temperature, fridge temperatures and freezer temperatures.

How to convert Celsius to kelvin

Add 273.15 to the Celsius temperature:

K = °C + 273.15

• 25 °C: 25 + 273.15 = 298.15 K.
• 4 °C: 4 + 273.15 = 277.15 K.
• −20 °C: −20 + 273.15 = 253.15 K.

To go from kelvin back to Celsius, subtract 273.15: 300 K is 26.85 °C. For rough work, 273 is often used instead of 273.15.

Related guides

For how temperature changes the speed of breakdown, see the Arrhenius equation and accelerated stability testing. For solvents and solutes, see solvents, solutes and solutions. For units in general, see SI units and prefixes.