From Heat to Enthalpy: Thermometry

James Richard Fromm

The first half of the eighteenth century marks the real beginning of both the technology and the science of heat. In these fifty years it became clear that heat could be employed to do useful work, replacing that of men, horses, wind, or falling water. These had been the only available means to do significant work throughout human history. Theoretical ideas, which were clearly formulated by the end of the century, began to develop before 1750. The two most important of these were the suggestion that heat might be conserved and the distinction between amount of heat or quantity of heat and quality of heat. Quality of heat is what we now call temperature and the study of temperature is called thermometry. The study of amount of heat is called calorimetry.

The advances in thermometry in the first half of the eighteenth century included the significant work of Guillaume Amontons (d. 1705) on gases. He studied the expansion of gases on heating but did not achieve formulation of Charles' Law. He developed the air thermometer, which measures the increase in pressure of a system of constant volume when the temperature increases, and also made significant studies of the liquid-in-glass thermometer. The liquids used in a thermometer by Amontons, and still used today, are alcohol (with red dye in it, used at low temperatures), linseed oil (for higher temperatures), water, and mercury.

The key question in thermometry at this time was whether substances changed from one state to another at a single fixed temperature or whether they did so over a narrow range of temperatures. By careful experimentation it was shown that a pure substance melts or boils at a single fixed temperature. Mixtures of substances, which include impure substances, melt or boil over a range of temperatures. Organic chemists often take advantage of this in checking the purity of newly prepared compounds.

The temperature of melting of a pure solid substance, the melting point, is the same temperature at which the liquid compound will freeze, the freezing point. This temperature is characteristic of the pure substance. Advantage can be taken of this in the identification of pure substances isolated from a preparative reaction mixture. Data for some common pure substances are given in the table below.

Table: Specific Thermal Properties of Pure Substances
Substance Melting Point (oC) Boiling Point (oC) Specific Heat (J/g K)
Water (H2O) 0.0 100.0 4.2179  (0oC)
Water 0.0 100.0 4.1857 (15oC)
Water 0.0 100.0 4.1794 (25oC)
Water 0.0 100.0 4.2158 (100oC)
Ice 0.0 --- 2.0995 (-2.2oC)
Steam --- 100.0 2.0125 (100oC)
Mercury (Hg) -38.87 356.58 0.1389 (25oC)
Methanol (CH3OH) -93.9 64.96 10.657 (25oC)
Ethanol (C3H5OH) -117.3 78.5 10.389 (25oC)
Copper (Cu) 1083.0 2595.0 0.3849 (25oC)
Iron (Fe) 1535.0 3000.0 0.4510 (25oC)
Platinum (Pt) 1769.0 3827.0 0.1360 (25oC)
Tin (alpha) (Sn) 231.91 2270.0 0.2167 (25oC)
Lead (Pb) 327.3 1744 0.1289 (25oC)

A few substances, of which iodine is an example, convert from solid to gas directly without ever passing through the liquid state, or sublime. The temperature at which this happens is called the sublimation point, and it is also characteristic of the substance.

Melting points of substances do not vary significantly as the pressure changes, but sublimation points do. So does the temperature at which a pure substance turns from liquid to vapor, the boiling point, which is also the condensation point. Considerable confusion originated from this because the effect of normal changes in the atmospheric pressure is measurable and the effect of altitude is quite marked. The barometer and the manometer were already known, however, and eventually it was realized that if (and only if) the pressure were constant then the boiling point would be constant also. Melting points as well as boiling points are now always given at a standard pressure of one atmosphere which is exactly 101325 Pa. They are then called the normal melting point and normal boiling point.

Now if, for example, water becomes liquid (or gas) at a single temperature, is this temperature the same at all places on earth? We now know that it is, but in the eighteenth century this was not a trivial question. It was complicated both by the effect of altitude or pressure on boiling point as well as by reports of anomalous boiling points or other different behavior in inaccessible places such as Peking or Hudson's Bay.

A lesser question related to practical thermometry was whether or not the thermal expansion of gases, liquids, and solids was uniform over a range of temperature. In practice, the thermal expansion of matter is generally found to be approximately uniform over any reasonably narrow temperature range; exceptions occur, mostly with solids, when the crystal structure or other form changes at some particular temperature in the range. This is known as a phase change, as are the transitions from gas to liquid to solid. As a consequence of the reasonably uniform thermal expansion of matter, all but the most precise thermometer scales have units of uniform length, although the size of the units does depend upon the substance used in each particular thermometer.

Copyright 1997 James R. Fromm