Test: Pure Substances - 2


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30 Questions MCQ Test GATE Mechanical (ME) 2023 Mock Test Series | Test: Pure Substances - 2

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Test: Pure Substances - 2 - Question 1

In the figure shown, the system is a pure substance kept in a piston-cylinder arrangement. The system is initially a two-phase mixture containing 1 kg of liquid and 0.03 kg of vapour at a pressure of 100 kPa. Initially, the piston rests on a set of stops, as shown in the figure. A pressure of 200 kPa is required to exactly balance the weight of the piston and the outside atmospheric pressure. Heat transfer takes place into the system until its volume increases by 50%. Heat transfer to the system occurs in such a manner that the piston, when allowed to move, does so in a very slow (quasi-static I quasi-equilibrium) process. The thermal reservoir from which heat is transferred to the system has a temperature of 400°C. Average temperature of the system boundary can be taken as 17°C. The heat transfer to the system is I kJ, during which its entropy increases by 10 J/K. Atmospheric pressure.

Specific volumes of liquid (vf) and vapour (vg) phases, as well as values of saturation temperatures, are given in the table below. 

The work done by the system during the process is:

Detailed Solution for Test: Pure Substances - 2 - Question 1

Work done = first constant volume heating

Test: Pure Substances - 2 - Question 2

A thermodynamic cycle with an ideal gas as working fluid is shown below.

Q.

The above cycle is represented on T-S plane by   

Test: Pure Substances - 2 - Question 3

The following table of properties was printed out for saturated liquid and saturated vapour of ammonia. The titles for only the first two columns are available. All that we know is that the other columns (columns 3 to 8) contain data on specific properties, namely, internal energy (kJ/kg), enthalpy (kJ/kg) and entropy (kJ/kgK)

When saturated liquid at 40°C is throttled to -20°C, the quality at exit will be

Detailed Solution for Test: Pure Substances - 2 - Question 3

Test: Pure Substances - 2 - Question 4

When an ideal gas with constant specific heats is throttled adiabatically, with negligible changes in kinetic and potential energies

Where h, T and S represent respectively, enthalpy, temperature and entropy, temperature and entropy

Detailed Solution for Test: Pure Substances - 2 - Question 4

Test: Pure Substances - 2 - Question 5

Assertion (A): Water is not a pure substance.

Reason (R): The term pure substance designates a substance which is homogeneous and has the same chemical composition in all phases.

Detailed Solution for Test: Pure Substances - 2 - Question 5

Water for all practical purpose can be considered as pure substance because it is homogeneous and has same chemical composition under all phases.

Test: Pure Substances - 2 - Question 6

The ordinate and abscissa in the given figure showing the saturated liquid and vapour regions of a pure substance represent:

Detailed Solution for Test: Pure Substances - 2 - Question 6

The ordinate and abscissa in given figure are pressure and enthalpy. Such diagram is common in vapour compression refrigeration systems.

Test: Pure Substances - 2 - Question 7

Consider the following statements for a throttling process:

1. It is an adiabatic process.

2. There is no work transfer in the process.

3. Entropy increases in throttling process

Which of these statements are correct ?

Test: Pure Substances - 2 - Question 8

Consider the following:

1. Air

2. Gaseous combustion products

3. Steam

Which of these are pure substances, assuming there is no phase change?

Detailed Solution for Test: Pure Substances - 2 - Question 8

A pure substance is one whose chemical composition does not change during thermodynamic processes.

  •  Pure Substance is one with uniform and invariant chemical composition.
  •  Eg: Elements and chemical compounds are pure substances. (water, stainless steel)
  •  Mixtures are not pure substances. (eg: Humid air)
  •  Exception!! Air is treated as a pure substance though it is a mixture of gases.

Gaseous combustion products are a mixture of gases and not a pure substance.

Test: Pure Substances - 2 - Question 9

Considers the following properties of vapour:

1. Pressure

2. Temperature

3. Dryness fraction

4. Specific volume

Which of these two properties alone are not sufficient to specify the condition of a vapour?

Test: Pure Substances - 2 - Question 10

Which p–v diagram for steam illustrates correctly the isothermal process undergone by wet steam till it becomes superheated?

Detailed Solution for Test: Pure Substances - 2 - Question 10

Up to saturation point pressure must be constant. After saturation its slope will be –ive, as pv = RT or pv = const. or vdp + pdv = 0

Test: Pure Substances - 2 - Question 11

Consider the phase diagram of a certain substance as shown in the given figure. Match List-I (Process) with List-II (Curves/lines) and select the correct answer using the codes given below the lists:

Test: Pure Substances - 2 - Question 12

The conversion of water from 40°C to steam at 200°C pressure of 1 bar is best represented as

Test: Pure Substances - 2 - Question 13

The p-v-T surface of a pure substance is shown in the given p figure. The two-phase regions are labelled as:

Test: Pure Substances - 2 - Question 14

Which one of the following is correct?

At critical point the enthalpy of vaporization is

Detailed Solution for Test: Pure Substances - 2 - Question 14

Characteristics of the critical point
1. It is the highest temperature at which the liquid and vapour phases can coexist.
2. At the critical point hfg, ufg and vfg are zero.
3. Liquid vapour meniscus will disappear.
4. Specific heat at constant pressure is infinite.

Test: Pure Substances - 2 - Question 15

Which one of the following statements is correct when saturation pressure of a vapour increases?

Test: Pure Substances - 2 - Question 16

Which one of the following represents the condensation of a mixture of saturated liquid and saturated vapour on the enthalpy-entropy diagram?  

Detailed Solution for Test: Pure Substances - 2 - Question 16

The slope of the isobar on the h–s diagram is equal to the absolute temp, for condensation T is cost so slope is const, but not zero so it is inclined line.

Test: Pure Substances - 2 - Question 17

Consider the following statements regarding the throttling process of wet steam:   

1. The steam pressure and temperature decrease but enthalpy remains constant.

2. The steam pressure decreases, the temperature increases but enthalpy remains constant.

3. The entropy, specific volume, and dryness fraction increase.

4. The entropy increases but the volume and dryness fraction decrease.

Which of the above statements are correct?

Test: Pure Substances - 2 - Question 18

Consider the following statements:

When dry saturated steam is throttled from a higher pressure to a lower pressure, the

1. Pressure decreases and the volume increases

2. Temperature decreases and the steam becomes superheated

3. Temperature and the dryness fraction increase

4. Entropy increases without any change in enthalpy

Which of these statements are correct?

Detailed Solution for Test: Pure Substances - 2 - Question 18

Temperature decreases and the
steam becomes superheated.

Test: Pure Substances - 2 - Question 19

A fluid flowing along a pipe line undergoes a throttling process from 10 bar to 1 Bar in passing through a partially open valve. Before throttling, the specific volume of the fluid is 0.5 m3 /kg and after throttling is 2.0 m3 /kg. What is the Change in specific internal energy during the throttling process?

Detailed Solution for Test: Pure Substances - 2 - Question 19

Throttling is a isenthalpic process

h1 = h2 or u1 + p1v1 = u2 + p2v2 or u2 – u1 = p1v1 – p2v2 = 1000 × 0.5 – 100 × 2 = 300 kJ/kg

Test: Pure Substances - 2 - Question 20

In the figure shown, throttling process is represented by

Test: Pure Substances - 2 - Question 21

Assertion (A): Air, a mixture of O2 and N2, is a pure substance.

Reason(R): Air is homogeneous in composition and uniform in chemical aggregation.

Detailed Solution for Test: Pure Substances - 2 - Question 21

A pure substance is a substance of constant chemical composition throughout its mass.

Test: Pure Substances - 2 - Question 22

Assertion (A): Air is a pure substance but a mixture of air and liquid air in a cylinder is not a pure substance.

.Reason (R): Air is homogeneous in composition but a mixture of air and liquid air is heterogeneous.

Test: Pure Substances - 2 - Question 23

Two-phase regions in the given pressure-volume diagram of a pure substance are represented by

Test: Pure Substances - 2 - Question 24

Triple point temperature of water is:

Detailed Solution for Test: Pure Substances - 2 - Question 24

Remember: Triple point temperature of water = 273.16 K = 0.01°C

Test: Pure Substances - 2 - Question 25

Entropy of a saturated liquid at 227°C is 2.6 kJ/kgK. Its latent heat of vaporization is 1800 kJ/kg; then the entropy of saturated vapour at 227°C would be:

Detailed Solution for Test: Pure Substances - 2 - Question 25

Test: Pure Substances - 2 - Question 26

A piston –cylinder device contains 0.06m3 of saturated water vapour at 350 kPa pressure. Determine the temperature and mass of the vapour inside the cylinder.

Detailed Solution for Test: Pure Substances - 2 - Question 26

T = Tsat@350kPa = 138.86°C
v = vg@350kPa = 0.52422 m3/kg
m = V/v = 0.06 m3/0.52422 m3/kg = 0.114 kg.

Test: Pure Substances - 2 - Question 27

Constant pressure lines in the superheated region of the Mollier diagram have what type of slope?

Detailed Solution for Test: Pure Substances - 2 - Question 27

Mollier diagram is a h-s plot.

T is always + ive so slope always +ive. Not only this if 

Test: Pure Substances - 2 - Question 28

Dryness fraction of steam means the mass ratio of

Test: Pure Substances - 2 - Question 29

In the following P-T diagram of water showing phase equilibrium lines, the sublimation line is:

Detailed Solution for Test: Pure Substances - 2 - Question 29

(a) p (b) q (c) r (d) S

Test: Pure Substances - 2 - Question 30

Assertion (A): In Mollier chart for steam, the constant pressure lines are straight lines in wet region.

Reason (R): The slope of constant pressure lines in wet region is equal to T. 

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