What makes things melt? Why do some substances melt all at once, while others seem to take forever? And how do you measure the exact temperature at which something melts? We'll answer these questions as we explore an important physical property of a compound: its melting point.
Phase Changes vs. Phase Diagrams
Before we get into the details of melting, let's take a closer look at the most common forms, or phases, of matter: solids, liquids, and gases. Scientists have determined the state of matter for lots of different compounds at different pressures and temperatures. A graph of these results of matter states is called a phase diagram and may look something like this:
A phase diagram shows states of matter at different pressures and temperatures.
On a phase diagram, the solid boundary shows the pressure and temperature at which the substance exists in both phases simultaneously. That's because phase changes are equilibrium processes. This can be an equilibrium between solid and liquid, or between liquid and gas.
So what temperature is required to get this compound to melt? It depends! Notice that the upward pointing line in the middle leans a little to the right. This tells us that at a pressure of 260 atmospheres, which is the upper horizontal dotted line, this compound melts at about 200 Kelvin. But when the pressure is dropped, the compound melts at a lower temperature. Can the melting temperature really change? Yes!
Melting Point Definitions
Now that we know something about phase diagrams, it will make more sense why scientists define melting point the way they do. First, in everyday terms, the melting point is the temperature at which a substance melts or freezes. Did you catch that? The melting point is the same as the freezing point! In other words, it goes from a solid to a liquid at the same temperature that it goes from a liquid to a solid.
Let's take a closer look at the everyday part of this definition. In science, everyday working conditions are usually defined as one atmosphere of barometric pressure. When we look up known values for the melting point of various substances, they're almost always defined at one atmosphere of pressure. Thus, in science, a substance's melting point is the temperature at which the solid becomes a liquid at one atmosphere of pressure.
What Happens During Melting
As a solid substance is heated, or absorbs heat from the environment, the molecules begin to have enough energy to overcome the intermolecular forces, or forces that hold the molecules together in a rigid fashion, like the water molecules in ice, shown here.
Hydrogen bonds between water molecules help maintain the solid structure of ice.
The molecules on the outside of a sample start to melt, even as the inside stays cool enough to remain solid. You've seen this before: think about an ice cube. It melts from the outside in, instead of turning into a puddle all at once. So there are really two processes involved in melting: absorbing heat energy, then using that energy to break apart the orderly lattice structure of the solid.
Melting Point Ranges
The short answer to this is human limitations. Measurement of melting ranges requires human observation, and it requires observation of a sample large enough to observe, for most practical applications anyway. A compound's melting range starts with the temperature where the crystals first begin to liquefy to the temperature at which the entire sample is liquid. Most pure compounds have a melting range of 1-2 degrees Celsius. In the lab, a narrow range like that is obtained only when the sample is heated slowly.
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Melting point, or more accurately, melting range, is an important physical property of a compound. In the laboratory, it's one of the pieces of evidence to help you identify an unknown sample. By comparing a sample's melting range to that of a set of known compounds, from a reference book or reliable source on the Internet, you can identify your unknown sample.
Melting range is also a great tool for assessing the purity of an unknown sample. Pure substances melt over a narrow temperature range of just one or two degrees. Impure samples will have a much broader melting range as well as a lower melting point, which is also true even when the impurity has a higher melting point than the sample of interest. The melting range of impure samples often starts at a lower temperature. For example, consider a known compound with a melting range of 54-56 degrees Celsius. An impure sample of this compound might have a melting range of 51-55 degrees Celsius, which is a broader range with a lower starting temperature. It's important to know that only impurities that interfere with a compound's solid crystalline structure will affect the melting point. If, for example, you get a tiny shard of glass in your melting point capillary tube, the melting point won't be affected because the glass doesn't combine chemically with your sample.
Let's take a moment to review the key points of this lesson on melting point. First, we really shouldn't talk about melting points. The everyday definition is the temperature at which a substance melts into liquid and the scientific definition is the temperature at which the solid becomes a liquid at one atmosphere of pressure. As a solid substance is heated, the molecules begin to have enough energy to overcome the intermolecular forces, or forces that hold the molecules together in a rigid fashion. But a better way to phrase this is melting range, which can be observed on phase diagrams, which are graphs of matter states results. Standard values for melting range are available, and are normally defined as being measured at one atmosphere of pressure. Pure substances have narrow melting ranges, whereas impure substances may have broad melting ranges. Matching the melting range of an unknown to that of a set of known compounds is an important tool for identifying an unknown compound and assessing its purity.
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