Melting point is when a solid turns into a liquid due to heat. It is unique to pure substances and can be used to identify them. Water melts and freezes at 32°F (0°C), while other substances have extreme melting points. Some liquids can be supercooled, and some objects lack a melting point, like carbon dioxide which undergoes sublimation.
Most people have seen a melting object, like ice melting in 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 putting it 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 particular substance.
When heat is applied to a solid object, the molecules within the solid start moving faster. As more heat is applied, the molecules in the solid continue to move until the attractive forces holding the molecules in a compact shape are overcome by the amount of energy the molecules have. At that point the solid dissolves and the substance becomes liquid. Even if more heat is applied to an object once it has begun to melt, the object will not change temperature until all of the solid has become liquid. No matter how much heat is applied to an ice cube, for example, ice and water will remain at 32°F (0°C) until all of the ice has melted.
Most 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 by 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).
The melting point of a substance is often the same as its freezing point, but this is not always the case. Some liquids can be supercooled. Supercooling a liquid is the process by which someone is able to cool a liquid past its freezing point without it turning into a solid. Such liquid must be pure because a single crystal, impurities, or sometimes even just 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 commonly lack a melting point. One of the most famous and well-used substances that has none at 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 changes from its solid phase to its gaseous phase in a process called sublimation. Carbon dioxide occurs as a liquid only at pressures exceeding five atmospheres.
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