Pure, crystalline solids have a characteristic melting point, thetemperature at which the solid melts to become a liquid. The transition between the solidand the liquid is so sharp for small samples of a pure substance that melting points canbe measured to 0.1oC. The melting point of solid oxygen, for example, is-218.4oC.
Liquids have a characteristic temperature at which they turn into solids, known astheir freezing point. In theory, the melting point of a solid should bethe same as the freezing point of the liquid. In practice, small differences between thesequantities can be observed.
It is difficult, if not impossible, to heat a solid above its melting point because theheat that enters the solid at its melting point is used to convert the solid into aliquid. It is possible, however, to cool some liquids to temperatures below their freezingpoints without forming a solid. When this is done, the liquid is said to be supercooled.
An example of a supercooled liquid can be made by heating solid sodium acetatetrihydrate (NaCH3CO2 3 H2O). When this solid melts, thesodium acetate dissolves in the water that was trapped in the crystal to form a solution.When the solution cools to room temperature, it should solidify. But it often doesn"t. Ifa small crystal of sodium acetate trihydrate is added to the liquid, however, the contentsof the flask solidify within seconds.
A liquid can become supercooled because the particles in a solid are packed in aregular structure that is characteristic of that particular substance. Some of thesesolids form very easily; others do not. Some need a particle of dust, or a seed crystal,to act as a site on which the crystal can grow. In order to form crystals of sodiumacetate trihydrate, Na+ ions, CH3CO2- ions,and water molecules must come together in the proper orientation. It is difficult forthese particles to organize themselves, but a seed crystal can provide the framework onwhich the proper arrangement of ions and water molecules can grow.
Because it is difficult to heat solids to temperatures above their melting points, andbecause pure solids tend to melt over a very small temperature range, melting points areoften used to help identify compounds. We can distinguish between the three sugars knownas glucose (MP = 150oC), fructose (MP =103-105oC), and sucrose (MP = 185-186oC), forexample, by determining the melting point of a small sample.
Measurements of the melting point of a solid can also provide information about thepurity of the substance. Pure, crystalline solids melt over a very narrow range oftemperatures, whereas mixtures melt over a broad temperature range. Mixtures also tend tomelt at temperatures below the melting points of the pure solids.
When a liquid is heated, it eventually reaches a temperature at which the vaporpressure is large enough that bubbles form inside the body of the liquid. This temperatureis called the boiling point. Once the liquid starts to boil, thetemperature remains constant until all of the liquid has been converted to a gas.
The normal boiling point of water is 100oC. But if you try to cook an egg inboiling water while camping in the Rocky Mountains at an elevation of 10,000 feet, youwill find that it takes longer for the egg to cook because water boils at only 90oCat this elevation.
In theory, you shouldn"t be able to heat a liquid to temperatures above its normalboiling point. Before microwave ovens became popular, however, pressure cookers were usedto decrease the amount of time it took to cook food. In a typical pressure cooker, watercan remain a liquid at temperatures as high as 120oC, and food cooks in aslittle as one-third the normal time.
To explain why water boils at 90oC in the mountains and 120oC ina pressure cooker, even though the normal boiling point of water is 100oC, wehave to understand why a liquid boils. By definition, a liquid boils when the vaporpressure of the gas escaping from the liquid is equal to the pressure exerted on theliquid by its surroundings, as shown in the figure below.
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|Liquids boil when their vapor pressure is equal to the pressure exerted on the liquid by its surroundings.|
The normal boiling point of water is 100oC because this is the temperatureat which the vapor pressure of water is 760 mmHg, or 1 atm. Under normal conditions, whenthe pressure of the atmosphere is approximately 760 mmHg, water boils at 100oC.At 10,000 feet above sea level, the pressure of the atmosphere is only 526 mmHg. At theseelevations, water boils when its vapor pressure is 526 mmHg, which occurs at a temperatureof 90oC.
Pressure cookers are equipped with a valve that lets gas escape when the pressureinside the pot exceeds some fixed value. This valve is often set at 15 psi, which meansthat the water vapor inside the pot must reach a pressure of 2 atm before it can escape.Because water doesn"t reach a vapor pressure of 2 atm until the temperature is 120oC,it boils in this container at 120oC.
Liquids often boil in an uneven fashion, or bump. They tend to bump when therearen"t any scratches on the walls of the container where bubbles can form. Bumping iseasily prevented by adding a few boiling chips to the liquid, which provide a roughsurface upon which bubbles can form.
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When boiling chips are used, essentially all of thebubbles that rise through the solution form on the surface of these chips.