JSSC- JE Mechanical Engg. Question Bank 3 ((Chapter – 3) Properties of Pure Substances)

Mechanical

What happens to the saturation temperature of water when pressure increases?

• It increases
• It decreases
• It remains constant
• It first decreases and then increases

Explanation:

• The saturation temperature of water increases with increasing pressure.

What is the significance of the critical point in a p-v diagram?

• Liquid and vapour phases become indistinguishable
• Water freezes completely
• Steam becomes completely dry at zero pressure
• Liquid volume becomes zero

Explanation:

• At the critical point, the properties of saturated liquid and saturated vapour become identical and the distinction between the two phases disappears.

What is the critical point of water approximately?

• 22.06 MPa and 374°C
• 1 MPa and 100°C
• 10 MPa and 250°C
• 0.1 MPa and 50°C

Explanation:

• Water has a critical pressure of approximately 22.06 MPa and a critical temperature of approximately 374°C.

What does the P-T diagram of a pure substance show?

• Pressure-temperature relationship
• Pressure-volume relationship
• Enthalpy-entropy relationship
• Temperature-volume relationship only

Explanation:

• A P-T diagram represents the equilibrium relationship between pressure and temperature of a pure substance.

Which curves meet at the triple point in a P-T diagram?

• Solid-liquid, liquid-vapour and solid-vapour curves
• Only liquid-vapour curves
• Only solid-liquid curves
• Only solid-vapour curves

Explanation:

• At the triple point, solid, liquid and vapour phases coexist in equilibrium, so the three phase-boundary curves meet there.

What is the triple point of water approximately?

• 0.01°C and 0.611 kPa
• 100°C and 101.325 kPa
• 374°C and 22.06 MPa
• 0°C and 101.325 kPa

Explanation:

• Water exists simultaneously as solid, liquid and vapour at approximately 0.01°C and 0.611 kPa.

What is the region above the critical temperature called in relation to ordinary liquid-vapour distinction?

• Supercritical region
• Wet region
• Saturated region
• Subcooled region

Explanation:

• Above the critical temperature, liquid and vapour cannot coexist as distinct phases under equilibrium conditions, and the fluid can exist in a supercritical state.

On a p-v diagram, the area under a quasi-equilibrium process curve represents what?

• Boundary work
• Heat transfer only
• Entropy
• Enthalpy

Explanation:

• For a quasi-equilibrium process, boundary work is given by the integral of p dv, represented by the area under the process curve on a p-v diagram.

Which region of the water p-v diagram contains both liquid and vapour?

• Two-phase or wet region
• Superheated region
• Compressed liquid region
• Solid region

Explanation:

• The wet region contains a mixture of saturated liquid and saturated vapour.

What happens to the width of the saturation dome as the critical point is approached?

• It decreases and becomes zero at the critical point
• It increases indefinitely
• It remains constant
• It becomes rectangular

Explanation:

• The saturated liquid and saturated vapour states approach each other as the critical point is approached, and the dome terminates at the critical point.

Which diagram is particularly useful for showing the phase boundaries of a pure substance?

• P-T diagram
• p-v diagram only
• h-s diagram only
• F-T diagram

Explanation:

• The P-T diagram clearly shows the equilibrium boundaries between solid, liquid and vapour phases.

For water, what happens to the saturation pressure when saturation temperature increases?

• Saturation pressure increases
• Saturation pressure decreases
• It becomes zero
• It remains unchanged

Explanation:

• Saturation pressure and saturation temperature are directly related for water.

What is a pure substance in thermodynamics?

• A substance having uniform and invariable chemical composition
• A substance existing only as a solid
• A substance containing only water vapour
• A substance with changing chemical composition

Explanation:

• A pure substance has a chemically homogeneous and fixed composition, although it may exist in more than one phase.

What is the first stage in the heating of water at constant pressure from a subcooled liquid state?

• Sensible heating of liquid water
• Superheating of steam
• Condensation
• Freezing

Explanation:

• When water is heated from a subcooled or compressed-liquid state at constant pressure, its temperature rises until it reaches the saturation temperature.

What is a steam table?

• A table containing thermodynamic properties of water and steam
• A table containing only boiler dimensions
• A table containing only pressure losses
• A table containing only fuel properties

Explanation:

• Steam tables provide properties such as pressure, temperature, specific volume, enthalpy and entropy for water and steam.

Which property is commonly obtained from saturated steam tables?

• Specific enthalpy
• Electrical resistance
• Viscosity of steel
• Density of air only

Explanation:

• Saturated steam tables provide thermodynamic properties such as specific volume, enthalpy and entropy at saturation conditions.

What is the first thermodynamic stage in steam generation from water?

• Heating water up to saturation temperature
• Superheating immediately
• Condensing steam
• Freezing water

Explanation:

• Water is first heated until it reaches the saturation temperature corresponding to the operating pressure.

What happens after saturated liquid water reaches the saturation temperature during steam generation?

• Vaporization begins
• Freezing begins
• Superheating begins immediately
• Pressure becomes zero

Explanation:

• Once saturated liquid is reached, further heat addition at constant pressure causes vaporization.

During vaporization at constant pressure, what happens to the temperature of pure water?

• It remains constant
• It continuously increases
• It continuously decreases
• It becomes zero

Explanation:

• During phase change at a fixed pressure, heat supplied is used primarily as latent heat, so the temperature remains at the saturation temperature.

What is the heat required to convert saturated liquid into saturated vapour at constant pressure called?

• Latent heat of vaporization
• Sensible heat
• Specific heat of solidification
• Heat of compression

Explanation:

• The energy required for phase change from saturated liquid to saturated vapour at constant pressure is the latent heat of vaporization.

What is the final stage when saturated steam is heated beyond the saturation temperature at constant pressure?

• Superheating
• Condensation
• Freezing
• Subcooling

Explanation:

• After all liquid has evaporated, further heat addition raises the steam temperature above its saturation temperature, producing superheated steam.

Which steam table is used when both pressure and temperature are known for superheated steam?

• Superheated steam table
• Saturated liquid table only
• Ice table
• Triple-point table only

Explanation:

• Superheated steam tables provide properties at temperatures above the saturation temperature for specified pressures.

Which properties are commonly listed in steam tables?

• v, u, h and s
• Only pressure
• Only temperature
• Only density of air

Explanation:

• Steam tables commonly provide specific volume, internal energy, enthalpy and entropy along with pressure and temperature.

What does h_f generally denote in saturated steam tables?

• Specific enthalpy of saturated liquid
• Specific enthalpy of dry saturated steam
• Latent heat only
• Specific entropy of steam

Explanation:

• In standard steam-table notation, h_f represents the specific enthalpy of saturated liquid.

What does h_fg represent in steam tables?

• Latent heat of vaporization per unit mass
• Enthalpy of compressed liquid
• Entropy of superheated steam
• Specific volume of saturated liquid

Explanation:

• h_fg is the difference between saturated-vapour and saturated-liquid enthalpy, h_g − h_f.

What does h_g represent?

• Specific enthalpy of saturated vapour
• Specific enthalpy of saturated liquid
• Latent heat only
• Specific volume of wet steam

Explanation:

• h_g is the specific enthalpy of saturated vapour or dry saturated steam.

Which property is particularly important for calculating the energy content of steam?

• Specific enthalpy
• Colour
• Electrical conductivity only
• Surface roughness

Explanation:

• Specific enthalpy is widely used in steam-cycle energy calculations because it represents the energy content relevant to flow processes.

What is the purpose of superheating steam in a steam power cycle?

• To increase steam temperature above saturation temperature
• To convert steam into water
• To reduce steam temperature below saturation
• To freeze the working fluid

Explanation:

• Superheating raises steam temperature above the saturation temperature at a given pressure and can improve cycle performance and reduce moisture during expansion.

Which steam condition is obtained immediately after complete vaporization without additional heating?

• Dry saturated steam
• Superheated steam
• Subcooled water
• Compressed liquid

Explanation:

• When all liquid has just vaporized and no additional heat has been supplied, the steam is dry saturated.

What is saturation?

• A condition of phase equilibrium between liquid and vapour
• A condition where pressure is zero
• A condition where water is frozen only
• A condition where steam has no energy

Explanation:

• At saturation, liquid and vapour can coexist in thermodynamic equilibrium at a corresponding saturation pressure and temperature.

What is saturated liquid?

• Liquid at saturation temperature that is about to vaporize
• Liquid below saturation temperature only
• Steam above saturation temperature
• Steam containing no energy

Explanation:

• Saturated liquid is at the saturation condition and is just ready to undergo vaporization.

What is dry saturated steam?

• Steam at saturation temperature containing no liquid water
• Steam containing a large amount of liquid
• Steam below saturation temperature
• Water below saturation temperature

Explanation:

• Dry saturated steam is completely vaporized steam at the saturation temperature and contains no suspended liquid water.

What is wet steam?

• A mixture of saturated liquid and saturated vapour
• Pure superheated vapour
• Compressed liquid only
• Dry saturated vapour only

Explanation:

• Wet steam contains both liquid water and vapour and exists within the saturation dome.

What is superheated steam?

• Steam whose temperature is above the saturation temperature at a given pressure
• Steam at exactly zero pressure
• Liquid water below saturation temperature
• A mixture of ice and water

Explanation:

• When dry saturated steam receives additional heat at the same pressure, its temperature rises above saturation temperature and it becomes superheated.

What is compressed or subcooled liquid?

• Liquid whose temperature is below saturation temperature at the given pressure
• Steam above saturation temperature
• Dry saturated steam
• Wet steam with 100% vapour

Explanation:

• A compressed or subcooled liquid has a temperature lower than the saturation temperature corresponding to its pressure.

What is the dryness fraction of wet steam?

• The ratio of mass of dry saturated vapour to total mass of wet steam
• The ratio of liquid volume to vapour volume
• The ratio of pressure to temperature
• The ratio of heat to pressure

Explanation:

• Dryness fraction x is defined as the mass of dry saturated vapour divided by the total mass of the wet-steam mixture.

What is the dryness fraction of dry saturated steam?

• 1
• 0
• 0.5
• Greater than 1

Explanation:

• Dry saturated steam contains only vapour, so its vapour mass equals the total mass and x = 1.

What is the dryness fraction of saturated liquid?

• 0
• 1
• 0.5
• Greater than 1

Explanation:

• Saturated liquid contains no vapour, so its vapour mass is zero and the dryness fraction is zero.

What is the possible range of dryness fraction for wet steam?

• 0 < x < 1
• x > 1
• x < 0 only
• x = 2 only

Explanation:

• Wet steam contains both liquid and vapour, so its dryness fraction lies between 0 and 1.

If the dryness fraction of steam is 0.8, what does it mean?

• 80% of the total mass is dry saturated vapour
• 80% of the volume is liquid
• 20% of the total mass is vapour
• Steam is 80°C superheated

Explanation:

• A dryness fraction of 0.8 means that 80% of the total mass is vapour and the remaining 20% is liquid water.

Which expression gives the specific enthalpy of wet steam?

• h = h_f + xh_fg
• h = h_f + h_g
• h = xh_f
• h = h_fg/x

Explanation:

• For wet steam, specific enthalpy is h = h_f + xh_fg.

Which expression gives the specific volume of wet steam?

• v = v_f + xv_fg
• v = xv_f
• v = v_g/x
• v = v_fv_g

Explanation:

• The specific volume of wet steam is obtained by v = v_f + x(v_g − v_f), or v = v_f + xv_fg.

Which expression gives the entropy of wet steam?

• s = s_f + xs_fg
• s = xs_f
• s = s_g/x
• s = s_fs_g

Explanation:

• The entropy of wet steam is given by s = s_f + xs_fg.

If the mass of dry steam is 8 kg and the mass of wet steam is 10 kg, what is the dryness fraction?

• 0.8
• 0.2
• 1.25
• 8

Explanation:

• x = mass of dry vapour / total mass = 8/10 = 0.8.

If the dryness fraction is 0.9, what fraction of the total mass is liquid water?

• 0.1
• 0.9
• 1.9
• 0.01

Explanation:

• The liquid fraction is 1 − x = 1 − 0.9 = 0.1, or 10% of the total mass.

Which instrument is commonly used for determining the dryness fraction of steam?

• Calorimeter
• Barometer
• Manometer only
• Thermocouple only

Explanation:

• Steam calorimeters are used to determine the dryness fraction of wet steam.

What is the dryness fraction if wet steam contains equal masses of liquid and vapour?

• 0.5
• 1
• 0
• 2

Explanation:

• If liquid and vapour masses are equal, vapour mass is half of the total mass, so x = 0.5.

Which condition corresponds to a dryness fraction of 0.95?

• Wet steam with 95% vapour by mass
• Dry saturated steam
• Saturated liquid
• Superheated steam

Explanation:

• A dryness fraction of 0.95 means 95% of the total mass is vapour and 5% is liquid.

Can dryness fraction be directly used to describe superheated steam?

• No
• Yes, always
• Only when pressure is zero
• Only at the critical point

Explanation:

• Dryness fraction is defined for a saturated liquid-vapour mixture, not for a single-phase superheated vapour.

What happens to dryness fraction during evaporation at constant pressure?

• It increases from 0 to 1
• It decreases from 1 to 0
• It remains zero
• It becomes greater than 1

Explanation:

• During evaporation, the proportion of vapour increases, so dryness fraction rises from 0 for saturated liquid to 1 for dry saturated steam.

What is the degree of superheat of steam?

• The difference between actual steam temperature and saturation temperature at the same pressure
• The difference between pressure and temperature
• The ratio of enthalpy to entropy
• The difference between liquid and vapour mass

Explanation:

• Degree of superheat = actual temperature of superheated steam − saturation temperature at the same pressure.

If steam temperature is 250°C and saturation temperature at the same pressure is 200°C, what is the degree of superheat?

• 50°C
• 450°C
• 200°C
• 25°C

Explanation:

• Degree of superheat = 250 − 200 = 50°C.

What is the degree of superheat of dry saturated steam?

• 0°C
• 100°C
• 1°C
• Greater than 100°C

Explanation:

• Dry saturated steam is exactly at the saturation temperature, so its degree of superheat is zero.

For superheated steam, the actual temperature is how compared with the saturation temperature at the same pressure?

• Higher
• Lower
• Equal
• Always zero

Explanation:

• Superheated steam has a temperature higher than its saturation temperature at the same pressure.

If the degree of superheat is 75°C and saturation temperature is 180°C, what is the actual steam temperature?

• 255°C
• 105°C
• 180°C
• 75°C

Explanation:

• Actual temperature = saturation temperature + degree of superheat = 180 + 75 = 255°C.

If the actual temperature of steam equals its saturation temperature at the same pressure, what is the steam condition?

• Saturated steam
• Superheated steam
• Subcooled liquid
• Compressed vapour

Explanation:

• When actual temperature equals saturation temperature at a given pressure, the substance is at saturation rather than in the superheated region.

Which steam property is required to determine the degree of superheat?

• Actual temperature and corresponding saturation temperature
• Only specific volume
• Only entropy
• Only dryness fraction

Explanation:

• The degree of superheat is calculated from the difference between actual steam temperature and saturation temperature at the same pressure.

What happens to the degree of superheat if steam temperature increases while pressure remains constant?

• It increases
• It decreases
• It remains constant
• It becomes zero

Explanation:

• At constant pressure, the saturation temperature remains fixed, so an increase in actual steam temperature increases the degree of superheat.

What happens to superheated steam if it is cooled at constant pressure until it reaches saturation temperature?

• It becomes dry saturated steam
• It becomes saturated liquid immediately
• It becomes ice
• Its pressure becomes zero

Explanation:

• Cooling superheated steam at constant pressure until its temperature reaches saturation temperature brings it to the dry saturated state.

Which statement is correct for superheated steam?

• Its temperature is above the saturation temperature at its pressure
• Its temperature is below saturation temperature
• Its dryness fraction is always 0.5
• It always contains liquid water

Explanation:

• Superheated steam is a single-phase vapour with temperature above the saturation temperature corresponding to its pressure.

If the saturation temperature decreases while actual steam temperature remains constant, what happens to the degree of superheat?

• It increases
• It decreases
• It remains unchanged
• It becomes zero

Explanation:

• Since degree of superheat is the difference between actual and saturation temperature, a decrease in saturation temperature increases this difference.

Which chart is also known as the Mollier chart?

• h-s chart
• p-v chart
• P-T chart
• T-v chart

Explanation:

• The Mollier chart is an enthalpy-entropy (h-s) chart used extensively for steam and gas-flow calculations.

What quantities are plotted on the axes of a Mollier chart?

• Enthalpy and entropy
• Pressure and volume
• Pressure and temperature
• Temperature and volume

Explanation:

• The Mollier chart plots specific enthalpy h against specific entropy s.

Which quantity is generally plotted on the vertical axis of an h-s chart?

• Specific enthalpy
• Specific volume
• Pressure
• Temperature only

Explanation:

• In a conventional Mollier chart, specific enthalpy is plotted vertically and specific entropy horizontally.

Which quantity is generally plotted on the horizontal axis of an h-s chart?

• Specific entropy
• Specific enthalpy
• Pressure
• Specific volume

Explanation:

• The horizontal axis of the Mollier chart represents specific entropy.

What is the main use of the Mollier chart for steam?

• Determining steam properties and analyzing steam processes
• Measuring boiler diameter
• Determining pipe colour
• Measuring fuel density only

Explanation:

• The h-s chart provides a convenient graphical method for obtaining steam properties and analyzing processes such as expansion and compression.

Which process is represented approximately by a vertical line on an h-s diagram?

• Isentropic process
• Constant-pressure process
• Constant-volume process
• Constant-temperature process

Explanation:

• Since entropy is on the horizontal axis, constant entropy is represented by a vertical line on the h-s chart.

In an ideal turbine, the expansion process is commonly assumed to be what?

• Isentropic
• Isothermal
• Isochoric
• Isobaric

Explanation:

• For an ideal turbine, expansion is assumed to occur without entropy generation, making it an isentropic process.

What happens to enthalpy during an ideal adiabatic turbine expansion?

• It decreases
• It increases indefinitely
• It remains exactly constant
• It becomes zero in every case

Explanation:

• In an ideal turbine, the decrease in specific enthalpy is converted into useful work output.

What do constant-pressure lines on an h-s chart represent?

• States having the same pressure
• States having the same entropy
• States having the same enthalpy
• States having the same specific volume only

Explanation:

• Constant-pressure lines connect states at equal pressure on the Mollier chart.

What do constant-temperature lines on a Mollier chart represent?

• States having the same temperature
• States having the same entropy
• States having the same pressure only
• States having the same mass

Explanation:

• Constant-temperature lines join points having equal temperature on the h-s chart.

What do constant dryness-fraction lines indicate on the wet region of a Mollier chart?

• States having the same dryness fraction
• States having the same pressure only
• States having the same temperature only
• States having the same enthalpy only

Explanation:

• In the wet region, lines of constant dryness fraction indicate the percentage of vapour by mass in the mixture.

What is the entropy change during an ideal isentropic process?

• Zero
• Positive only
• Negative only
• Infinite

Explanation:

• By definition, an isentropic process occurs at constant entropy, so the entropy change is zero.

Which process on an h-s chart is associated with a horizontal movement?

• A process involving change in entropy at constant enthalpy
• An isentropic process
• A constant-entropy process only
• A process at zero pressure

Explanation:

• Since enthalpy is plotted vertically, a horizontal line represents constant enthalpy while entropy changes.

What is the main advantage of the Mollier chart over repeated steam-table calculations?

• It provides a graphical and convenient method for obtaining properties and process changes
• It eliminates all thermodynamic laws
• It directly measures steam mass
• It is used only for boiler design dimensions

Explanation:

• The Mollier chart allows many steam properties and process changes to be estimated graphically without repeatedly performing detailed table interpolation.

Which property difference can be directly read as a vertical distance on an h-s chart?

• Enthalpy difference
• Entropy difference
• Pressure difference only
• Specific-volume difference only

Explanation:

• Because enthalpy is represented on the vertical axis, the vertical difference between two states corresponds to their enthalpy difference.

Which property difference can be directly represented by horizontal distance on an h-s chart?

• Entropy difference
• Enthalpy difference
• Pressure difference only
• Temperature difference only

Explanation:

• Entropy is represented along the horizontal axis, so horizontal separation corresponds to entropy difference.

Why is the h-s chart useful in turbine analysis?

• It helps determine enthalpy drop during expansion
• It directly measures turbine speed
• It measures blade diameter
• It determines fuel composition

Explanation:

• The enthalpy drop between turbine inlet and outlet is closely related to turbine work, and the Mollier chart provides a convenient graphical way to determine it.

What does the wet region of a Mollier chart represent?

• A mixture of saturated liquid and saturated vapour
• Only superheated steam
• Only compressed liquid
• Only solid water

Explanation:

• The wet region represents two-phase liquid-vapour mixtures and contains lines of constant dryness fraction.

What does the superheated region on a Mollier chart represent?

• Superheated vapour states
• Only saturated liquid states
• Only wet steam states
• Solid states only

Explanation:

• The region beyond the saturated vapour boundary represents superheated steam.