Properties of Pure Substances純物質性質

What you'll be able to do本章學習成果

• Describe the physics of phase-change and read the T–v, P–v, and P–T diagrams.說明相變化的物理機制並讀取 T–v、P–v 與 P–T 圖。
• Determine properties from tables, including quality for two-phase mixtures.由性質表查得性質，包括兩相混合物的乾度。
• Use linear interpolation and the saturated-liquid approximation for compressed liquids.使用線性插値與壓縮液體的飽和液近似。

Key equations重要公式

Phases & phase change相與相變化

A pure substance has a fixed chemical composition throughout. Heat a compressed liquid at constant pressure and it marches through:純物質全體具有固定的化學組成。在定壓下加熱壓縮液體，將依序經歷：

• Compressed (subcooled) liquid — not about to vaporize.壓縮（未飽和）液體——尚未达到永魔。
• Saturated liquid — on the verge of vaporizing.飽和液體——即將永魔。
• Saturated liquid–vapor mixture — both phases coexist; temperature holds constant while latent heat is absorbed.飽和液氣混合物——兩相共存；溫度保持不變而潛熱被吸收。
• Saturated vapor — the last drop has just vaporized.飽和蒸氣——最後一滴剛完全永魔。
• Superheated vapor — heated beyond saturation.過熱蒸氣——已加熱至超過飽和狀態。

The energy absorbed during vaporization is the latent heat of vaporization $h_{fg}$ — for water at 1 atm, about 2257 kJ/kg.永魔過程中吸收的能量為永魔潛熱 $h_{fg}$ ——水在 1 大氣壓下約為 2257 kJ/kg。

Saturation & property diagrams飽和與性質圖

The saturation temperature $T_{sat}$ and saturation pressure $P_{sat}$ trace the saturation curve. On a T–v diagram, the saturated-liquid and saturated-vapor lines meet at the critical point, enclosing the two-phase dome. Above the critical pressure there is no distinct boiling. The triple point (water: 0.01 °C, 0.6117 kPa) is where solid, liquid, and vapor coexist.飽和溫度 $T_{sat}$ 與飽和壓力 $P_{sat}$ 描繪出飽和曲線。在 T–v 圖上，飽和液線與飽和氣線在臨界點交汇，圍成兩相圓頂。在臨界壓力以上則無明顯永魔。三相點（水：0.01 °C、0.6117 kPa）是化固、液體與蒸氣三相共存之處。

Quality of a mixture混合物的乾度

Inside the dome, temperature and pressure are not independent — so we need another property to fix the state: the quality $x$, the vapor mass fraction:在圓頂內，溫度與壓力並非獨立——因此需另一個性質來固定狀態：乾度 $x$，即蒸氣質量分率：

The same lever-rule form gives $u = u_f + x\,u_{fg}$ and $h = h_f + x\,h_{fg}$. Geometrically, the mixture state sits a fraction $x$ of the way from the liquid line to the vapor line.同樣形式給出 $u = u_f + x\,u_{fg}$ 與 $h = h_f + x\,h_{fg}$。在圓頂圖上，混合狀態點位於液線到氣線之間 $x$ 的位置。

Saturation dome & quality lab飽和圓頂與乾度實驗

Pick a saturation temperature, then slide the quality from 0 (saturated liquid, point f) to 1 (saturated vapor, point g). The state point glides along the tie-line, and specific volume and enthalpy follow the lever rule.選擇飽和溫度，再將乾度從 0（飽和液體， f 點）滑至 1（飽和蒸氣， g 點）。狀態點沿結線滑動，比體積與焓遵循桿桿法則。

Saturated-water data (teaching subset). The specific-volume axis is logarithmic to show the full liquid-to-vapor span.

Property tables性質表

Most substances are too complex for simple equations, so properties are tabulated. The steam tables are the canonical example: saturation tables (by T and by p) list $v_f, v_g, u, h, s$; superheated and compressed-liquid tables cover single-phase regions where $T$ and $p$ are independent. When a state falls between entries, use linear interpolation. Tables are built on a reference state (water: 0.01 °C; R-134a: −40 °C) — only changes matter.大多數物質太複雜而難以簡單方程式描述，因此性質均被表格化。蒸汽表是典型範例：飽和表（按 T 與按 p）列出 $v_f, v_g, u, h, s$；過熱蒸氣表與壓縮液體表涵蓋單相區。狀態介於兩項間時，使用線性插値。表格不同性質的參考狀態各不相同（水：0.01 °C；R-134a：−40 °C）——實際上只有變化量才有意義。

Properties of a wet mixture濕混合物的性質