# Heat Transfer Operations Questions and Answers – Evaporators – Boiling Point Elevation

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This set of Heat Transfer Operations Multiple Choice Questions & Answers (MCQs) focuses on “Evaporators – Boiling Point Elevation”.

1. How can boiling point elevation be achieved in Evaporators?
a) Increasing pressure
b) Decreasing pressure
c) Increasing Temperature
d) Increasing concentration

Explanation: The boiling point of the solution increases as the water evaporates and the concentration increases, this is known as boiling point elevation.

2. Boiling point of the solution ________ as the evaporation process proceeds.
a) Increases
b) Decreases
c) Remains same
d) Becomes half

Explanation: The boiling point of the solution increases as the water evaporates and the concentration increases, this is known as boiling point elevation. Boiling point, melting point are few properties of liquids that depend on the concentration of solid.

3. Boiling point of a solution is a colligative property.
a) True
b) False

Explanation: The boiling point, melting point and osmotic pressure are few properties of liquids that depend on the concentration of solid in them, this is known as colligative properties.
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4. The boiling point of a solution is a colligative property because?
a) It depends on the temperature of solution
b) It depends on the concentration of solution
c) It depends on the applied pressure on the solution
d) It depends on the heat capacity of the solution

Explanation: The four properties of fluid that are colligative in nature are –
i. Vapour pressure depression
ii. Boiling point elevation
iii. Melting point depression
iv. Osmotic pressure.
Colligative properties are those properties of the liquid which change (either increase or decrease) with increasing the number of solute molecules (concentration) in the solvent.

5. Boiling point elevation causes the driving force of evaporation in the fluid to _________
a) Decrease
b) Increase
c) Remain same
d) Increase tenfold

Explanation: Boiling point elevation causes the boiling point of the solution to increase and hence the temperature now required to evaporate water also increases, thus the driving force decreases.

6. The temperature of water vapour released from the solution after facing boiling point elevation is ________
a) Equal to 100℃
b) Below 100℃
c) Above 100℃
d) Much larger than 100℃

Explanation: After the boiling point elevation has occurred and the solution now boils at a temperature above the normal and hence the steam ejected is also at that temperature, hence the steam is superheated.

7. The vapour released from the feed after being considerably concentrated is superheated steam.
a) True
b) False

Explanation: As boiling point is a colligative property, as the boiling point elevation has occurred and the solution now boils at a temperature above the normal and hence the steam ejected is also at that temperature, hence the steam is superheated.

8. The Boiling point elevation as the evaporation in an evaporator proceeds can be approximately measured by __________
a) Duhring’s rule
b) Arrhenius rule
c) Kozeny Carman rule
d) Sieder tate rule

Explanation: The verified rule known as the Duhring’s rule measures the value of BPE of a solution. The rule states that – the boiling point (BP) of a given solution has a linear function with the boiling point of that same solution’s solvent which is pure water at the same temperature and pressure.

9. To help coping with boiling point elevation in evaporators, which of the following can be done?
a) Increase the feed temperature to its boiling point
b) Use superheated steam at the temperature of maximum boiling point
c) Increase pressure
d) Reduce pressure

Explanation: To cope with the BPE problem, industries mostly use superheated steam with the maximum Boiling point to ensure evaporation till the very end of the operation.

10. Which one the following properties of fluid in not a colligative property?
a) Osmotic pressure
b) Surface tension
c) Boiling point elevation
d) Vapour pressure depression

Explanation: The four properties of fluid that are colligative in nature are –
i. Vapour pressure depression
ii. Boiling point elevation
iii. Melting point depression
iv. Osmotic pressure.

11. The vapour pressure of the solution ________ when the concentration of solutes increases.
a) Increases
b) Decreases
c) Remains same
d) Becomes zero

Explanation: The four properties of fluid that are colligative in nature are –
i. Vapour pressure depression
ii. Boiling point elevation
iii. Melting point depression
v. Osmotic pressure.
Hence the vapour pressure decreases on increasing concentration.

12. To cope with the change in vapour pressure with increasing concentration and considering the sensitivity of the solution to high temperatures, which one of the following step can be undertaken?
a) Increase the feed temperature to its boiling point
b) Use superheated steam at the temperature of maximum boiling point
c) Increase pressure
d) Reduce pressure

Explanation: If the temperature cannot be increased to the maximum considering sensitivity of feed, the only option we have is to reduce pressure as it would bring down the boiling point of the feed.

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