18.9
Phase Changes
Phase transitions play an important theoretical and practical role in the study of heat flow. In melting or fusion, a solid turns into a liquid; the o…
Primarily there are three different phases of matter: solid, liquid, and gas.
On changing the temperature, one state changes into another. This phenomenon is called phase change, which is reversible in nature.
On heating, a solid changes to liquid, this process is called melting, while vice-versa is called freezing.
The heat absorbed or released in this transition for a material of mass 'm' can be evaluated using the heat of fusion, which is the energy per unit mass required or released in phase change.
Similarly, if a liquid is heated, it converts to a gas, and the process is called vaporization or boiling. On cooling, the change of a gas into a liquid is called condensation.
Energy per unit mass required or released in phase change between liquid and gas is called heat of vaporization, where Q is the heat absorbed or released.
Some of the materials like dry ice can directly have a transition from solid to gas, which is called sublimation.
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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.