Boiling point elevation

Japanese: 沸点上昇 - ふってんじょうしょう（英語表記）elevation of boiling point

This phenomenon occurs when the boiling point of a solution containing a non-volatile substance rises above the boiling point of the solvent. It is abbreviated as BPR. In the case of melting point, the melting point of the solution becomes lower than the melting point of the solvent (this is called freezing point depression), but both can be explained by the difference in the vapor pressure curves of the solution and the solvent. In other words, if the vapor pressure of the solvent is p ₁ and that of the solution is p ₂ , then

Here, n ₁ and n ₂ are the number of moles of the solvent and solute, respectively, and the right-hand side of the equation gives the molar fraction of the solute. The vapor pressure of the solvent decreases when non-volatile substances dissolve. The boiling point is the temperature when the vapor pressure becomes atmospheric pressure, so it is T ₁ for the solvent and T ₂ for the solution. The difference between T ₂ and T ₁ is the boiling point rise. As shown in the equation, the rate of this rise is proportional to the concentration of the solute: Δ T = K _b n . Here, K _b is called the molar rise, and is a value specific to the solvent when 1 mole of solute is dissolved in 1000 grams of solvent.

where _Lv is the heat of vaporization and M0 is the molecular weight. For example, the _Kb of water is 0.515, that of benzene is 2.67, and that _of acetone is 1.19.

[Takashi Shimozawa]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

不揮発性物質を溶かした溶液の沸点が溶媒の沸点より上昇する現象をいう。略号BPR。融点の場合には、溶液の融点が溶媒の融点より低くなる（これを凝固点降下という）が、両者とも溶液と溶媒の蒸気圧曲線の差から説明できる。すなわち、溶媒の蒸気圧をp₁、溶液のそれをp₂とすると

ここで、n₁、n₂はそれぞれ溶媒・溶質のモル数であり、式の右辺は溶質のモル分率を与える。溶媒の蒸気圧は不揮発性物質が溶解することによって低下する。沸点は蒸気圧が大気圧になったときの温度であるから、溶媒ではT₁、溶液ではT₂となる。T₂とT₁との差が沸点上昇になる。式に示したように、この上昇の割合は溶質の濃度に比例する。ΔT＝K_bn。ここでK_bを分子上昇といい、溶媒1000グラムに溶質1モルを溶かしたときの溶媒に固有な値である。