Raven spreads a table of melting and boiling points across the Bench. "Every pure substance has its own two numbers," she says.
Rocket reads down the list. "Water, zero and one hundred. Iron, over fifteen hundred to melt. Oxygen melts at minus 219?"
"What do you notice about the gap between the two numbers?" Raven asks.
"Between melting and boiling, the stuff is a liquid," Rocket says. "Water is liquid for a hundred degrees. Iron is liquid for over a thousand."
Nova hovers over the row for salt and brightens it. "Your Bench salt melts at 801 degrees," she says. "Why do you never see that happen?"
Rocket laughs. "Because a kitchen never gets that hot. It just sits there, solid."
Raven writes: melting point, boiling point, and the liquid range between.
Each pure substance has characteristic properties that can be used to identify it. Melting point and boiling point are two of them.
The melting point is the temperature at which a solid changes into a liquid. It is the same temperature as the freezing point of the liquid.
At the melting point, the vibrations of the particles overcome the forces holding the solid together. Solid and liquid are then in balance.
The boiling point is the temperature at which bubbles of vapor form all through a liquid and rise to the surface.
Between the two points, a substance is a liquid. Below the melting point it is a solid. Above the boiling point it is a gas.
This table combines melting points in degrees Celsius from LibreTexts chemistry with boiling points from PubChem. PubChem is the National Institutes of Health chemistry database.
PubChem lists boiling points in kelvins. They were changed to Celsius by subtracting 273.15 and rounding to the nearest degree.
| Substance | Melting point (degrees Celsius) | Boiling point (degrees Celsius) | State at room temperature |
|---|---|---|---|
| Hydrogen | -259 | -253 | Gas |
| Oxygen | -219 | -183 | Gas |
| Mercury | -39 | 357 | Liquid |
| Water | 0 | 100 | Liquid |
| Table salt (sodium chloride) | 801 | not listed here | Solid |
| Silver | 961 | 2162 | Solid |
| Gold | 1063 | 2856 | Solid |
| Iron | 1538 | 2861 | Solid |
Water boils at 100 degrees Celsius at sea level. On top of Mount Everest, 8,848 meters up, it boils at only about 70 degrees Celsius.
The air pressure is lower high up, so the liquid boils at a lower temperature. As altitude increases, the boiling point decreases.
That is why a table always says "at sea level" or "at standard pressure" next to a boiling point.
| Statement | True or false? |
|---|---|
| Melting point and boiling point can help identify a pure substance. | ? |
| Hydrogen is a liquid at room temperature. | ? |
| Water boils at a lower temperature on a high mountain than at sea level. | ? |
| Iron melts at a lower temperature than silver. | ? |
| Table salt is a solid in any kitchen because kitchens never reach 801 degrees Celsius. | ? |
Strong data reading. Tomorrow you will find melting, boiling, evaporating and condensing all around your home.