What is a melting point? (with photos)

The melting point of a substance is often the same temperature as the freezing point, exemplified by water that usually melts and freezes at 32°F (0°C).

Most people have seen an object melt, like ice melting into water on a hot day. The melting point of an object is the point at which the solid object turns into a liquid. A more scientific way of saying this is that it is the point at which the solid and liquid states of a pure substance are in equilibrium. This point is unique to pure substances, so scientists can use temperature as a way to identify a specific substance.

Dry ice has no melting point.

When heat is applied to a solid object, the molecules within the solid begin to move faster. As more heat is applied, the molecules in the solid continue to move until the attractive forces holding them together in a compact form are overcome by the amount of energy the molecules have. At this point, the solid melts and the substance becomes a liquid. Even if more heat is applied to an object after it starts to melt, the object will not change temperature until all the solid has become a liquid. No matter how much heat is applied to an ice cube, for example, the ice and water will remain at 0°C (32°F) until all the ice has melted.

Many people are familiar with the melting and boiling points of water. Water usually melts and freezes at 32°F (0°C) and boils at 212°F (100°C). The familiarity of water can make the melting points of other substances extreme in comparison. For example, the temperature at which carbon melts is 6,422°F (3,550°C), while mercury melts at -37.97°F (-38.87°C).

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The melting point of a substance is often equal to the freezing point, but this is not always the case. Some liquids can be super-cooled. Supercooling a liquid is the process by which one is able to cool a liquid beyond its freezing point without it turning into a solid. This liquid needs to be pure because a single crystal, impurity or sometimes just a movement will cause the liquid to crystallize. If the supercooled liquid encounters such an impurity or movement, it will freeze almost instantly.

There are also objects that do not normally have a melting point. One of the most famous and used substances that does not have atmospheric pressure is carbon dioxide. The solid phase of carbon dioxide is commonly called “dry ice”. At -109.3°F (-78.5°C), carbon dioxide jumps from its solid phase to the gas phase in a process called sublimation. Carbon dioxide occurs only as a liquid at pressures that exceed five atmospheres.

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