Phase Change of a Pure Substance MCQ Quiz in मल्याळम - Objective Question with Answer for Phase Change of a Pure Substance - സൗജന്യ PDF ഡൗൺലോഡ് ചെയ്യുക

latent heat of vaporisation refers to the amount of heat energy required to transform a substance from the liquid phase into the vapor phase at a constant temperature and pressure. With that definition, let's evaluate all the options:

• Heat required for complete conversion of saturated liquid into dry saturated vapour: This statement essentially describes latent heat of vaporization. It refers to the necessary heat input to convert a substance from the liquid state (saturated liquid) into the gaseous state (dry saturated vapour), without a change in temperature.
• Heat required for complete conversion of ice into water: This does not describe latent heat of vaporisation. Instead, it illustrates the concept of the latent heat of fusion, which is the amount of heat required to convert a solid (in this case, ice) into a liquid (water) at its melting point.
• Heat added at a constant temperature of 100°C to convert water into steam: This statement also describes the latent heat of vaporisation. At a constant temperature of 100°C (at standard atmospheric pressure), water changes from its liquid phase to its vapor phase -- a process that calls for input of heat energy, i.e., the latent heat of vaporisation.
• Sum of internal latent heat and external work of evaporation: This is a more systematic look at the latent heat of vaporisation. The energy needed for the phase transformation from liquid to vapor can be divided into two components: the internal latent heat, which is the heat energy used to break the intermolecular forces within the liquid state; and the external work of evaporation, which is the energy used to expel the vapor created and make space for it (essentially, the work done against the atmospheric pressure).

In conclusion, the statement that does not correlate correctly with latent heat of vaporisation is option 2, "heat required for complete conversion of ice into water," as that defines latent heat of fusion, not vaporisation.

Explanation:

At critical point saturated liquid and saturated vapor point coincide. No distinct phase can be made in two phases. The latent heat of vaporization is zero at a critical point.

P_cr = 220.6 bar = 22.06 MPa

T_cr = 373.95^oC

v_cr = 0.005155 m³/kg

At a critical point, the liquid is directly converted into vapor without having a two-phase transition. So, enthalpy of vaporization at a critical point is zero. 

The figure below represents the P-V diagram for a pure substance (water).

Concept:

Every single phase of a pure substance is separated by saturation lines.

• The sublimation line separates the solid and vapor regions
• The vaporization line separates the liquid and vapour regions
• The fusion line separates the solid and liquid regions

The slopes of sublimation and the vaporization curves for all substances are positive.

The slope of the fusion curve, however, may be positive or negative. The fusion curve of most substances has a positive slope. Water is one of the important exceptions. The slope of the fusion curve for water is negative.

Matching the curves with the given figure,

Vaporisation - 3, Fusion - 2, Sublimation - 1;

Explanation:

Triple Point:

• The Triple Point is a line on the P-V diagram where all the three phases solid, liquid and gases exist in equilibrium.
• At a pressure below the triple point line, the substance cannot exist in the liquid phase and the substance when heated, transforms from solid to vapour by absorbing the latent heat of sublimation from the surroundings.
• The triple point is merely the point of intersection of the sublimation and vaporization curves.
• It has been found that on a ‘P-T’ diagram the triple point is represented by a point and on a ‘P-V’ diagram it is a line, and on a ‘U-V’ diagram it is a triangle.
• In the case of ordinary water, the triple point is at a pressure of 4.58 mm Hg and a temperature of 0.01°C.

Explanation:

Enthalpy–entropy chart  or Mollier diagram:

• An enthalpy–entropy chart, also known as the h–s chart or Mollier diagram, plots the total heat against entropy,
• In a two-phase region, the lines of constant pressure and temperature on the diagram coincide since constant pressure and temperature lines are plotted there.
• In an isobaric process, the pressure remains constant, so the heat interaction is the change in enthalpy. Enthalpy is constant in an isenthalpic process. An isenthalpic process is shown in the diagram as a horizontal line.
• A vertical line in the h–s chart represents an isentropic process.
• Lines of constant dryness fraction (x), sometimes called the quality, are drawn in the wet region, and lines of constant temperature are drawn in the superheated region.

• The equation which forms the basis of the Mollier diagram is Tds = dh - vdp.
• Although the T dS equations are derived by considering an internally reversible process, an entropy change obtained by integrating these equations is the change for any process, reversible or irreversible, between two equilibrium states of a system
• Because entropy is a property, the change in entropy between two states is independent of the details of the process linking the states
• To show the use of the T dS equations, consider a change in phase from saturated liquid to saturated vapor at constant temperature and pressure. Since pressure is constant. 
• \(\frac{dh}{ds}=T\)

Concept:

Water has maximum density at 4°C.

So, Specific volume = 1/Density

So the density first increases then it reaches a maximum at 4°C and then decreases.

CONCEPT:

• Triple point: The temperature and pressure at which the solid, liquid, and vapor phase of a pure substance coexists in equilibrium are called the triple point.​

• At a pressure below the triple point line, the substance cannot exist in the liquid phase, and the substance when heated transforms from solid to vapor by absorbing the latent heat of sublimation from the surroundings.

EXPLANATION:

• The Triple point is a point on the PT diagram where all the three phases solid, liquid, and gases co-exist in equilibrium. So option 4 is correct.
• The matter solidifies on the left side of the triple point, not at the triple point.
• The matter liquifies on the upper side of the triple point, not at the triple point.
• The matter sublimates on the right side of the triple point, not at the triple point.

Additional Information

• The temperature at which any material changes its phase from solid to liquid is called the fusion point or melting point.
• The temperature at which any material changes its phase from liquid to vapor is called the boiling point.
• The endpoint of the phase equilibrium of a material is called the critical point.

The correct answer is Options (A) (C) and (D).

Key Points

• Mole
  • The amount of any substance that has the same basic unit (molecule/atom/ion) as the atom in 12 grams of carbon-12" is called a mole. 
  • It is equal to the gram atomic mass of the substance, equal to the gram molecular weight and 6.022x10²³ molecules of the substance. Hence, statements 1, 2, and 4 are correct and Statement 2 is not correct.
  • It is also known as Avogadro Constant and is denoted by N_A.
  • SI unit of Mole is mol.
  • The word mole was coined by Wilhelm Ostwald, was a German chemist.
  • In 1909 Ostwald was awarded the Nobel Prize for Chemistry for his work on catalysis, chemical equilibria, and reaction velocities.

Concept:

The heat supplied will broke into two parts.

1. Latent heat: In latent heat, only phase changes and there is no charge is temperature.
2. Sensible heat: This heat causes the charge in temperature.

Now,

– 10°C  → 0° C → sensible heat

0°C → 0°C → Latent heat (Ice melts to liquid water)

0°C → 100°C → Sensible heat

100°C → 100°C → Latent heat to form vapors

Explanation:

Pure substance: A substance that is made up of one kind of particle and contains a fixed or constant structure is called a pure substance.

Sub-cooled liquid: If a liquid exists at a temperature below the normal boiling temperature then it is termed a sub-cooled liquid.

Saturated liquid: The liquid which is about to vapourize for a small change in temperature is called saturated liquid.

Saturated liquid-vapor: The state at which the liquid and vapor phases coexist in equilibrium.

Saturated vapor: A vapor that is about to condense for a small decrease in temperature is called saturated vapor.

Superheated vapor: The vapor which cannot be condensed easily and the liquid will be at a temperature above the boiling temperature is called superheated vapor 
The above-mentioned regions are represented in the given P-V curve:

From the above dome G-H-D:

• G-H = saturated liquid line
• H-D = saturated vapor line 
• H = critical point where saturated liquid and saturated vapor lines meet
• The region to the left of the G-H curve is called the sub-cooled region
• The region to the right of the H-D curve is called the super heated region 

​∴  The water which is about to vaporize is called Saturated liquid.