Q&A - PSLE Science

[starlight1968sg]

I have a question to ask:

We have a cup of pure water and a cup of salt solution. During a hot day, which one will evaporate faster?

[atutor2001]

starlight1968sg:

I have a question to ask:

We have a cup of pure water and a cup of salt solution. During a hot day, which one will evaporate faster?

My guess is that pure water will evaporate faster.

I think the explanation is beyond the Pr sch syllabus.

[trytry]

starlight1968sg:

I have a question to ask:

We have a cup of pure water and a cup of salt solution. During a hot day, which one will evaporate faster?

Pure water will evaporate first.
Comparing pure water and salt water, salt water has a higher boling point, and therefore takes longer time to boil.

(not necessary to know)
This is caused by the solute (salt) in the salt water, higher level of energy is required to convert the solute component to gaseous state.

[starlight1968sg]

trytry:

Pure water will evaporate first.

Comparing pure water and salt water, salt water has a higher boling point, and therefore takes longer time to boil.
(not necessary to know)
This is caused by the solute (salt) in the salt water, higher level of energy is required to convert the solute component to gaseous state.

Thanks trytry.
With these 2 cups in the freezer, the pure water will freeze faster than the salt solution?

[CoffeeCat]

starlight1968sg:

trytry:

Pure water will evaporate first.

Comparing pure water and salt water, salt water has a higher boling point, and therefore takes longer time to boil.
(not necessary to know)
This is caused by the solute (salt) in the salt water, higher level of energy is required to convert the solute component to gaseous state.

Thanks trytry.
With these 2 cups in the freezer, the pure water will freeze faster than the salt solution?

nope the presence of salt will lower the freezing point & raise the boiling point. That's why not all sea water freeze at colder areas.

[starlight1968sg]

CoffeeCat:

nope the presence of salt will lower the freezing point & raise the boiling point. That's why not all sea water freeze at colder areas.

A bit confusing here; does the pure water take a shorter time to freeze than the salt solution since you mentioned that the freezing point of the salt solution is lower?

This translates to pure water freezes faster than the salt solution.

[CoffeeCat]

yea pure water takes a shorter time to freeze. assuming all else equal.

[James Ang]

atutor2001:

starlight1968sg:

I have a question to ask:

We have a cup of pure water and a cup of salt solution. During a hot day, which one will evaporate faster?

My guess is that pure water will evaporate faster.

I think the explanation is beyond the Pr sch syllabus.

I wonder if the question is a real question or hypothetical question.

Students at P5 are taught that the rate of evaporation is affected by 4 factors, namely (and summarised as W.H.A.T);

W - Wind
H - Humidity
A - Area of exposed surface
T - Temperature

So the mentioned variable such as type of water (whether pure or containing salt) is not part of the 4 factors listed.

In secondary syllabus, in addition to the 4 factors, the rate of evaporation is also affected by another factor - Pressure of atmosphere. Still the syllabus does not mention about the concentration of a solution although there is a fact that impurities like salt lowers the melting and increases the boiling point of a solution, but that should be mixed up with evaporation.

[atutor2001]

To explain why pure water will evaporate faster than salt solution, we need to understand "vapour pressure" which I can’t remember if it is covered in JC or secondary.

[James Ang]

CoffeeCat:

nope the presence of salt will lower the freezing point & raise the boiling point. That's why not all sea water freeze at colder areas.

The fact that water does not freeze completely at colder areas is also due to the anomalous nature of water which expands when its temperature drops to below 4 degree celsius and floats when it becomes ice and thus form a layer of ice above the water that protects the bottom layer of water from further freezing and which can still support life such as fishes. That's why people can still do fishing on frozen lakes by making a hole on the frozen ice to reach the water below the frozen surface.