18.9
相変化
相転移は、熱流の研究において理論的および実践的に重要な役割を果たします。 融解または融合では、固体は液体に変わります。 逆のプロセスはフリーズします。 蒸発では、液体は気体に変わります。 反対のプロセスは凝縮です。
物質は、融点と呼ばれる温度で溶けたり凍ったり、沸点で沸騰したり凝縮したりします。 こ…
主に、物質には固体、液体、気体の3つの異なる相があります。
温度を変更すると、1つの状態が別の状態に変わります。この現象は相変化と呼ばれ、本質的に可逆的です。
加熱すると、固体が液体に変化するこのプロセスは溶融と呼ばれ、その逆は凍結と呼ばれます。
質量「m」の材料について、この遷移で吸収または放出される熱は、相変化で必要または放出される単位質量あたりのエネルギーである融解熱を使用して評価できます。
同様に、液体が加熱されると、それは気体に変換され、このプロセスは気化または沸騰と呼ばれます。冷却時には、気体が液体に変化することを凝縮と呼びます。
液体と気体の間の相変化で必要または放出される単位質量あたりのエネルギーは、気化熱と呼ばれ、Qは吸収または放出される熱です。
ドライアイスのような一部の材料は、昇華と呼ばれる固体から気体への直接遷移を起こすことがあります。
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Q1: What are the three main phases of matter?
The three main phases of matter are solid, liquid, and gas. These states differ in how their molecules are arranged and move. Temperature changes cause matter to transition between these phases, a reversible process called phase change that is fundamental to understanding heat flow and specific heat.
Q2: What is the difference between melting and vaporization?
Melting is the process where a solid converts to a liquid when heated, while vaporization (or boiling) occurs when a liquid converts to a gas. Both are phase transitions driven by temperature increase. The reverse processes are freezing and condensation, respectively.
Q3: How does pressure affect melting and boiling points?
High pressure typically raises both melting and boiling points because it favors denser substance forms. Low pressure lowers these temperatures. For example, water boils at 100°C at 1.00 atm, but at higher pressures the boiling point increases. Water's melting point is an exception to this rule.
Q4: What is heat of fusion and how is it calculated?
Heat of fusion is the energy per unit mass required to change a substance from solid to liquid, or released when changing from liquid to solid. This material constant depends on the number and strength of molecular bonds. It is determined experimentally and varies for different substances.
Q5: What is latent heat and why is it called hidden?
Latent heat is the energy absorbed or released during phase changes without causing temperature change in the system. It is called hidden because the energy enters or leaves without altering the substance's temperature, making it invisible to simple temperature measurements. Both heat of fusion and heat of vaporization are forms of latent heat.
Q6: What is sublimation and which materials undergo this process?
Sublimation is a phase transition where a solid directly converts to a gas without passing through the liquid phase. Dry ice is a common example of a material that undergoes sublimation. This process is reversible and occurs when certain substances are heated or when pressure conditions favor the gaseous state.
Q7: How does the mass of a sample affect the energy required for phase changes?
The energy required for phase changes is proportional to the sample's mass because it depends on the total number of molecular bonds. Larger masses contain more molecules, requiring more total energy for phase transitions. Heat of fusion and heat of vaporization are expressed per unit mass to account for this relationship.