What is the difference between the melting point of a pure substance and that of a mixture?

Answer 1

Different substances usually have different melting points, so determining the melting point of a substance can narrow down the possibilities for the identity of the substance. Likewise different substances usually differ in their boiling points, so boiling point can narrow down the possible identities of a substance. Although two substances can have nearly identical melting points or have nearly identical boiling points, it is extremely rare for two different pure substances to have BOTH the same melting point AND the same boiling point, so taking the two together is usually enough to uniquely identify a substance, or at the very least narrow it down to a very small list.

Also, surprisingly, if you mix two substances with the same melting point, the mixture will NOT melt at the expected temperature, so when you have a shortlist based on melting point you take your unknown and mix it with one of the suspects. If it melts at the right temperature, you have identified it. If not, you try the next one.

Answer 2

The boiling point of at mixture depends on the constituents. Sometimes it is higher than both constituents, sometimes it is lower than both constituents. This depends on the interactions between the molecules. Alloys always melts at a lower temperature than the pure metals, often over a temperature range. The temperature at which melting begins is called the solidus, and the temperature when melting is just complete is called the liquidus.

Answer 3

The melting point of a pure substance is a sharp, specific temperature at which the substance changes from a solid to a liquid phase. In contrast, the melting point of a mixture is a range of temperatures over which the components of the mixture melt, due to the presence of different substances with varying melting points.

Answer 4

The melting point of a mixture will be lower and more broad than a pure substance. One of the main characteristics of a molecule's melting point is it's symmetry because this influences the molecule's ability to packed together well. The more packed together the molecule, the tighter the molecules are together, and therefore the higher the melting point. So if you add something into your pure mixture, the ability of it to pack together well is more difficult, which would decrease the melting point.

Answer 5

A pure substance generally has fixed melting and boiling points. Note it must melt or boil without decomposition. The measured values can be used as a guide to identification. Remembering that impurities depress the melting point and elevate the boiling point the "sharpness" of the melting/boiling point is an indication of purity.

Answer 6

There are three common "phases" of matter, solid, liquid or gas. They differ in how easily the the individual molecules that make up the matter can move. In solids, the molecules are bound into a specific location in the solid. The only way to move them is to break the solid. When enough heat is added, the molecules vibrate with enough energy that they break free from their local bonds and can move around short distances, although they remain weakly bonded to other molecules nearby. This is melting and the resulting phase is a liquid. If even more heat is added, the molecules eventually have enough energy to break free from the other molecules more completely and become a gas. This is called vaporization (i.e., boiling). A glass is a special case intermediate between solids and liquids. Finally, if even more heat is added, the molecules and atoms break apart into a gas made up of atomic fragments, which typically have a positive or negative charge. This phase is called a plasma. Some molecules (such as carbon dioxide--dry ice) can skip the liquid state and go directly from solid to gas when they are heated.

Answer 7

The melting point of a substance, at a specified pressure, is the temperature at which it changes phase - from solid to liquid. The boiling point is the temperature at which the phase changes from liquid to gas.

Answer 8

Melting point: change of phase from solid to liquid.
Boiling point: change of phase from liquid to gas.

Answer 9

They're both phase-transition temperatures. The melting point is the solid-liquid phase transition temperature. The boiling point is the liquid-gas phase transition temperature.

Answer 10

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