What Are Melting and Boiling Points?
Every pure substance changes state at specific, predictable temperatures:
- The melting point is the temperature at which a solid turns into a liquid (or a liquid turns back into a solid).
- The boiling point is the temperature at which a liquid turns into a gas (boils throughout), not just evaporating at the surface.
These temperatures are fixed for any given pure substance and are useful for identifying it.
Pure water melts at exactly 0 °C and boils at exactly 100 °C at standard atmospheric pressure (sea level). Scientists use these reference points to define the Celsius temperature scale.
Why Different Substances Have Different Values
The melting and boiling points of a substance depend on how strongly its particles attract each other:
- Strong forces between particles → high melting/boiling point (more energy needed to overcome the forces).
- Weak forces between particles → low melting/boiling point.
Iron has a very high melting point (1538 °C) because its metal particles are held very strongly. Oxygen, by contrast, only liquefies at −183 °C because its molecules have very weak attractions.
Some Common Melting and Boiling Points
| Substance | Melting Point | Boiling Point |
|---|---|---|
| Water (H₂O) | 0 °C | 100 °C |
| Ethanol (alcohol) | −114 °C | 78 °C |
| Iron (Fe) | 1538 °C | 2862 °C |
| Oxygen (O₂) | −219 °C | −183 °C |
| Table salt (NaCl) | 801 °C | 1413 °C |
Using Melting and Boiling Points to Identify Substances
Because a pure substance always melts and boils at the same temperature, scientists use these values as a "fingerprint" to identify unknown materials. If a white powder melts at 801 °C, it is very likely to be table salt.
Impurities in a substance lower its melting point and make the melting range broader instead of sharp. Salt spread on icy roads works by dissolving into the surface water, lowering its freezing point below 0 °C so the ice melts even in cold weather.
Effect of Pressure on Boiling Points
The boiling point of a liquid depends on the pressure above it. At high altitudes (like on top of a mountain), the atmospheric pressure is lower, so water boils at a lower temperature — around 90 °C at 3000 m elevation. This is why cooking times are longer at high altitude: the water is not as hot when it boils.
In a pressure cooker, the pressure is raised above atmospheric, so water can be heated above 100 °C before it boils, cooking food faster.
Real world: Gritting roads in winter
Salt (sodium chloride) is spread on icy roads in winter because it lowers the freezing point of water below 0 °C. This is an application of melting point depression — impurities reduce the temperature at which ice forms. The same principle is used in antifreeze in car radiators (ethylene glycol lowers the freezing point of coolant).
Real world: Identifying fake gold
Jewellers can test whether a metal is real gold using its melting point. Pure gold melts at 1064 °C. A fake "gold" item made of cheaper metals like brass (which melts at a lower temperature) would behave very differently when heated. Melting and boiling points act like a "fingerprint" for substances.
Real world: Pressure cookers and mountain cooking
At high altitude (e.g. in the Alps), reduced air pressure means water boils below 100 °C — around 90 °C at 3000 m. Food takes longer to cook because the boiling water isn't as hot. A pressure cooker does the opposite: it raises pressure, pushing the boiling point above 100 °C so food cooks faster.
Where do you see this in real life? Notice water boiling at 100 °C when you cook pasta. Salt added to the cooking water slightly raises the boiling point — and flavours the food. Pressure cookers and altitude hiking both involve the same boiling point principle.
Practice Problems
A student heats an unknown liquid and finds it boils at exactly 78 °C. What might this liquid be, and how would she confirm her answer?
Show answer
Why does water on top of a high mountain boil at a lower temperature than at sea level?