Properties of steam: 18 MCQs with answers
All 18 multiple-choice questions from this kit, free and with no sign-up.
Warm-up · 6 questions
Q1. What does the dryness fraction (x) of a saturated steam–water mixture represent?
BeginnerShow answer & explanation
The mass of dry steam per unit mass of the mixture
Dryness fraction is defined as the ratio of the mass of dry steam (vapor) to the total mass of the mixture (vapor + liquid).
Q2. If the dryness fraction of steam is 0.85, what is the percentage of moisture (liquid water) in the mixture?
BeginnerShow answer & explanation
15%
Moisture fraction = 1 - x = 1 - 0.85 = 0.15, which is 15%.
Q3. For a saturated liquid, what is the value of the dryness fraction (x)?
BeginnerShow answer & explanation
0
A saturated liquid contains no vapor, so the mass of dry steam is zero, giving x = 0.
Q4. Which property is NOT directly used to define the state of a pure substance like steam?
BeginnerShow answer & explanation
Color
Color is not a thermodynamic property; the state is defined by properties like pressure, temperature, and specific volume.
Q5. In the formula for enthalpy of wet steam, $h = h_f + x h_{fg}$, what does $h_{fg}$ represent?
BeginnerShow answer & explanation
Latent heat of vaporization
$h_{fg}$ is the enthalpy of vaporization, i.e., the heat required to convert unit mass of saturated liquid to dry saturated vapor at constant pressure.
Q6. The dryness fraction of a saturated liquid is exactly 1.
BeginnerShow answer & explanation
False
A saturated liquid has no vapor, so the dryness fraction is 0, not 1.
Building up · 11 questions
Q7. The specific volume of a saturated liquid–vapor mixture can be calculated using the formula: $v = v_f + x(v_g - v_f)$. What does $v_g$ represent?
IntermediateShow answer & explanation
Specific volume of saturated vapor
$v_g$ is the specific volume of saturated vapor (dry saturated steam) at the given pressure or temperature.
Q8. What is the enthalpy of a wet steam mixture with dryness fraction x, given $h_f$ (enthalpy of saturated liquid) and $h_{fg}$ (enthalpy of vaporization)?
IntermediateShow answer & explanation
$h = h_f + x h_{fg}$
The enthalpy of a wet mixture is the sum of the enthalpy of saturated liquid and the fraction of latent heat corresponding to the dryness fraction: $h = h_f + x h_{fg}$.
Q9. At a given pressure, the specific volume of dry saturated steam is $v_g$. If the dryness fraction is 0.9, what is the specific volume of the mixture? (Assume $v_f$ is negligible compared to $v_g$.)
IntermediateShow answer & explanation
$0.9 v_g$
Using $v = v_f + x(v_g - v_f)$ and neglecting $v_f$, we get $v \approx x v_g = 0.9 v_g$.
Q10. For a saturated mixture, the specific enthalpy $h$ lies between which two values?
IntermediateShow answer & explanation
$h_f$ and $h_g$
For a wet mixture, $h = h_f + x h_{fg}$, and since $0 \le x \le 1$, $h$ ranges from $h_f$ (x=0) to $h_g = h_f + h_{fg}$ (x=1).
Q11. What is the dryness fraction of superheated steam?
IntermediateShow answer & explanation
1
Superheated steam contains no liquid water, so it is considered dry saturated steam with x = 1 (or beyond, but dryness fraction is defined as 1 for dry steam).
Q12. The specific volume of a wet steam mixture is always ____ the specific volume of dry saturated steam at the same pressure.
IntermediateShow answer & explanation
less than
Since $v = v_f + x(v_g - v_f)$ and $x < 1$ for wet steam, $v$ is less than $v_g$.
Q13. A steam sample has a dryness fraction of 0.95. What is the mass of water vapor in 10 kg of the mixture?
IntermediateShow answer & explanation
9.5 kg
Mass of dry steam = x × total mass = 0.95 × 10 = 9.5 kg.
Q14. If the specific volume of saturated liquid is $v_f$ and that of saturated vapor is $v_g$, what is the specific volume of a mixture with dryness fraction x?
IntermediateShow answer & explanation
$v = v_f + x(v_g - v_f)$
The correct formula is $v = v_f + x(v_g - v_f)$, which is derived from the definition of dryness fraction.
Q15. What is the enthalpy of saturated liquid water at a given pressure if the enthalpy of vaporization is 2000 kJ/kg and the enthalpy of dry saturated steam is 2800 kJ/kg?
IntermediateShow answer & explanation
800 kJ/kg
$h_g = h_f + h_{fg}$, so $h_f = h_g - h_{fg} = 2800 - 2000 = 800$ kJ/kg.
Q16. Which of the following statements is TRUE about the dryness fraction?
IntermediateShow answer & explanation
It is always between 0 and 1 for a saturated mixture.
For a saturated liquid–vapor mixture, x ranges from 0 (saturated liquid) to 1 (saturated vapor). It cannot exceed 1 or be negative.
Q17. The enthalpy of wet steam is always less than the enthalpy of dry saturated steam at the same pressure.
IntermediateShow answer & explanation
True
Since $h = h_f + x h_{fg}$ and $x < 1$ for wet steam, $h$ is less than $h_g = h_f + h_{fg}$.
Topper level · 1 questions
Q18. The specific volume of a saturated mixture is 0.2 m³/kg. If $v_f = 0.001$ m³/kg and $v_g = 0.5$ m³/kg, what is the dryness fraction?
AdvancedShow answer & explanation
0.4
Using $v = v_f + x(v_g - v_f)$, we get $0.2 = 0.001 + x(0.5 - 0.001)$, so $x = (0.2 - 0.001) / 0.499 \approx 0.4$.
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