Aha! Chemistry with Prof Bob
  • HOME
  • NAVIGATION
    • Table of contents
    • Index
    • TALK WITH PROF BOB?
  • LEARNING MODULES
    • Chapter 02 Stuff, matter: What is it? >
      • 0200 Stuff, matter: A theory of atoms
      • 0201 Atoms: The building blocks of all stuff
      • 0202 People classifying stuffs. Why?
    • Chapter 05 Chemical reactions, chemical equations >
      • 0500 Chemical reactions vs. chemical equations. Overview
      • 0501 Chemical amount and its unit of measurement, mole
      • 0502 The Avogadro constant: How many is that?
      • 0503 The Avogadro constant: Why is it that number?
      • 0504 Chemical formulas: What can they tell us??
      • 0505 Chemical equations: What can they tell us?
      • 0506 Limiting reactants: How much reaction can happen?
      • 0507 Balanced chemical equations: What are they?
      • 0508 Chemical reactions as competitions
    • Chapter 09 Aqueous solutions >
      • 0901 What is a solution? And what is not?
      • 0902 Miscibility of liquids in each other
      • 0903 Like dissolves like? Shades of grey
      • 0905 Dissolution of ionic salts in water: A competition
      • 0906 Can we predict solubilities of salts?
      • 0907 Solution concentration
      • 0908 Chemical species, speciation in aqueous solution
      • 0909 Solutes: Electrolytes or non-electrolytes?
      • 0910 Electrolytes - strong or weak?
      • 0911 Concentrated, dilute, strong, weak
      • 0912 Species concentration vs. solution concentration
      • 0913 Weak electrolytes: Getting quantitative
    • Chapter 11: Dynamic chemical equilibrium >
      • 1100 Dynamic chemical equilibrium: Overview
      • 1101 Visualising dynamic chemical equilibrium
      • 1102 The jargon of chemical equilibrium
      • 1103 Equilibrium constants: The law of equilibrium
      • 1104 The law of equilibrium: an analogy
    • Chapter 22 Spectroscopy >
      • 2200 Spectroscopy: Overview and preview
      • 2201 Quantisation of forms of energy
      • 2202 Light: Wave-particle "duality"
      • 2203 Ultraviolet-visible spectroscopy
      • 2204 Beer’s law: How much light is transmitted?
    • Chapter 27 The greenhouse effect, climate change >
      • 2700 The greenhouse effect: overview
      • 2701 Is Earth in energy balance?
      • 2702 CO2 in the atmosphere before 1800
      • 2703 So little CO2! Pffft?
      • 2704 Does CO2 affect Earth's energy balance?
      • 2705 The "greenhouse effect"
      • 2706 Why does CO2 absorb radiation from Earth?
      • 2707 The "enhanced greenhouse effect"
      • 2708 Why doesn't CO2 absorb the radiation from the sun?
      • 2709 Why are N2 and O2 not greenhouse gases?
      • 2710 Doesn't water vapour absorb all the IR?
      • 2711 Carbon dioxide from our cars
      • 2712 The source of energy from combustion
      • 2713 Comparing fuels as energy sources
      • 2714 Methane: How does it compare as a GHG?
      • 2715 Different sorts of pollution of the atmosphere
      • 2716 "Acidification" of seawater
    • Chapter 27 Communicating chemistry >
      • 2700 Overview, preview
      • 2703 The jargon we use
  • TEACHERS' CORNER
    • T01 Communicating chemistry
    • T02 Beer's law
    • T03 Professional amnesia of the chemistry teaching professio
    • T04 Law of equilibrium
    • T05 Visusalizing dynamic chemical equilibrium
    • Information vs. knowledge
  • PERSONAL GALLERY
    • Family
    • Travel
    • Playful dolphins
    • The University of Western Australia
    • Kings Park
    • Perth
    • At work
    • 999 Thermodynamics

Module 0903


Like dissolves like? Shades of grey

Are all polar liquids miscible with all polar liquids?

Are all non-polar liquids miscible with all non-polar liquids?
​
​Are all polar liquids immiscible with all non-polar liquids?

Must it be all-or-nothing? Partial miscibility?


​
​Things (miscibilities) are not as clear-cut as the saying "like dissolves like" suggests ....
Prof Bob clarifies the non-universality of the saying "Like dissolves like"
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KEY IDEAS - Like dissolves like? Shades of grey

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Put your problems of understanding miscibility behind you
There is a commonly used principle for predicting the miscibility of molecular liquids in each other: “Like dissolves like”.

​More specific statements of this generalization are:
  • Substances whose molecules are polar are miscible with other substances whose molecules are polar
  • Substances whose molecules are non-polar are miscible with other substances whose molecules are non-polar
  • Substances whose molecules are non-polar are not miscible with substances whose molecules are polar.

Water and ethanol are miscible.  This observation is consistent with the first category above

Mixtures of water and some alcohols

Water and ethanol are miscible.  This observation is consistent with the first category above (polar liquids and polar liquids)

​Let’s consider the miscibility in water of some other alcohols with just one O-H group. We’ll focus on the number of carbon atoms in the chain attached to the –O-H group.

1-propanol  [C-C-C-O-H, not showing the H atoms on the C atoms]  Its molecules have a polar group. And indeed it IS miscible with water. Did you predict that, using “like dissolves like”?

1-octanol [C-C-C-C-C-C-C-C-O-H]  Its molecules have a polar group. But it is NOT miscible with water. If we shake up some octanol and water, it will remain as two layers – the less dense octanol being the upper layer. Did you predict that?

​Unthinking use of the saying “Like dissolves like” leads us to the wrong prediction. Furthermore, it give us no basis for understanding why 1-octanol and water are immiscible.


To make sense of this observation, we can use the more rational approach based on the two different driving forces of entropy and energy discussed in Module 0902: Miscibility of liquids in each other.
​
This approach uses Prof Bob’s law of miscibility of liquids in each other, which views miscibility/immiscibility as the result of a competition between the  "driving forces" of increase of entropy and decrease of energy. There is a natural tendency for the molecules of different liquids to intermix among each other (entropy factor) unless there are energy requirements preventing them from doing so.


For example, in the case of 1-propanol and water, which are miscible, the net energy requirement is the result of
  • the energy needed to separate the water molecules (A strong network of hydrogen bonding between water molecules).
  • the energy needed to separate the propanol molecules (Some hydrogen bonding between the molecules because of the highly polar O-H group in each molecule).
  • compensation of energy due to attractions between propanol molecules and water molecules if they are dispersed among each other.

This approach allows us to explain observed miscibilities, rather than to predict.

For example, the observation of propanol-water miscibility tells us that even if the net energy demand is unfavourable to mixing, it is not enough to cancel out the natural tendency to mix.
​

How can we make sense of that? We can imagine that a large factor in the energy demand is due to the fact that if the octanol and water molecules to disperse among each other, the long C-C-C-C-C-C-C-C- chain of each octanol molecule will need to push apart several water molecules from each other – against the strong forces of attraction due to hydrogen bonding.
​
This requires so much work to be done (energy to be input) that it is not sufficiently compensated by interactions between octanol and water molecules to allow scrambling.
​
Miscibility in all proportions
If we shake up some 1-propanol and water together, regardless of whether we make a mixture 99% propanol, or only 1% propanol, it remains as only one layer – a solution of these two substances in each other. They are said to be miscible in all proportions.
​
Immiscibility
If we shake up some 1-octanol and water together, no matter how little octanol we add to some water, nor how little water we add to some octanol, the liquids separate out into two layers (less dense octanol on top). These are immiscible.

Partial miscibility
1-propanol (with its 3-carbon atom chain) is miscible with water, but 1-octanol (8 carbon atom chain) is not. What about 1-butanol (4 C atoms), 1-pentanol (5 C atoms), 1-hexanol (6 C atoms), and 1-heptanol (7 C atoms)? There must be a switch from miscibile with water to immiscible somewhere along this series.

Suppose that we add some 1-butanol to 100 g of water.
If we add any amount of butanol up to 8g, it all dissolves, and there is only one layer. Are 1-butanol and water miscible? Wait …

Any small additional amount over 8g does not dissolve: another layer forms on top. And the more butanol that we add, the larger does the upper layer become. (Just like adding salt to water in increments – you come to a point where no more can dissolve - a saturated solution)

So, 1-butanol is miscible with water only up to a point.
​
And what’s more, the two layers that are present beyond that point are not pure water and pure butanol: we can think of the lower layer as a solution of 1-butanol in water, and the upper layer as a solution of water in 1-butanol.

At 20 °C, the compositions of the two layers are the following:
            Upper layer: 20% water, 80% butanol
            Lower layer: 8 % 1-butanol, 92% water

Pairs of liquids such as 1-butanol and water are said to be partially miscible.
​

So you want to know about 1-pentanol, 1-hexanol, and 1-pentanol? You probably won’t be surprised to learn that the larger the hydrocarbon section of the molecules, the less soluble is the alcohol in water.

The solubilities at 25 °C (in mol of alcohol per L of water) of the C4 to C7 alcohols are listed here:
                                                1-butanol          1.1
                                                1-pentanol        0.03
                                                1-hexanol         0.006
                                                1 heptanol        0.0008
Butanol and pentanol can be regarded as partially miscible with water.
For all practial purposes, we would probably classify 1-heptanol as immiscible with water – although no two liquids are absolutely immiscible: there is always at least a tiny amount of A in the layer of B, and some B in the layer of A.
​

Solubilities in non-polar solvents.

Oppositely to the relative solubilities in water, 1-octanol is more soluble in hexane than methanol is.
 
And we can use that same reasoning to explain relative solubilities in non-polar solvents: We can also use Prof Bob’s law of solubilities of molecular liquids in each other to make sense of such observations.
​

 
Aha!

We may not be able to predict miscibilities with certainty, but we can make sense of observations sensibly.
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SELF CHECK: Some thinking tasks
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Is nothing black-and-white? Check your certainty about uncertainty .......
1.            1-Octanol is immiscible with water, but miscible with hexane. Which of the following is/are correct in relation to the immiscibility of 1-octanol with water?
A             1-octanol molecules are polar, and so are water molecules, so this observation is consistent with the “Like dissolves like” rule.
B             There is a natural tendency (increase of entropy) for 1-octanol molecules and hexane molecules to disperse among each other, but the net energy demand of doing so is large enough to prevent this from happening.
C             There is only one hydrogen bond in 1-octanol.
D             The reason why the net energy demand of scrambling 1-octanol and water molecules among each other is so high is that the long 1-octanol molecules would separate a lot of water molecules from each other against the strong forces of attraction (hydrogen bonding) between them, without much compensation of energy from forces of attraction between 1-octanol molecules and water molecules.
​

2.            1-Octanol is immiscible with water, but miscible with hexane. Which of the following is/are correct in relation to the miscibility of 1-octanol with hexane?
A             1-octanol molecules are polar, but hexane molecules are not, so this observation is not consistent with the “Like dissolves like” rule.
B             There is a natural tendency (increase of entropy) for 1-octanol molecules and hexane molecules to disperse among each other, and the net energy demand of doing so is not too large to prevent this from happening.
C             The reason why the net energy demand of scrambling 1-octanol and hexane molecules among each other is not too high is that (i) although a lot of energy is need to separate 1-octanol molecules from each other during scrambling, and (ii) a lot of energy is needed to separate hexane molecules from each other during scrambling, (iii) there is large energy compensation due to the strong forces of attraction between 1-octanol molecules and hexane molecules when dispersed among each other.
D             The reason why the net energy demand of scrambling 1-octanol and hexane molecules among each other is not too high is that (i) not a lot of energy is need to separate 1-octanol molecules from each other during scrambling, and (ii) not a lot of energy is needed to separate hexane molecules from each other during scrambling, (iii) the energy compensation due to the dispersion forces of attraction between 1-octanol molecules and hexane molecules when dispersed among each other is similar to the energy requirements of (i) and (ii).
​

3.            Which of the following do you think is/are correct?
A             Acetic acid is miscible with water, consistent with the “Like dissolves like” principle.
B             Decanoic acid is miscible with water, consistent with the “Like dissolves like” principle.
C             Decanoic acid is immiscible with water, consistent with the “Like dissolves like” principle.
D             Decanoic acid is immiscible with water, inconsistent with the “Like dissolves like” principle.


4.            Liquid A is added incrementally to 10 mL of liquid B. Here are the experimental observations, after shaking on each addition:
Picture

​Which of the following is correct?
A             A and B are miscible in all proportions.
B             A and B are immiscible.
C             (i) A and B are partially miscible, (ii) after addition of 4 mL of A, the two layers are pure A and pure B, and (iii) A is less dense than B.
D             (i) A and B are partially miscible, (ii) after addition of 4 mL of A, the two layers are pure A and pure B, and (iii) A is more dense than B.
E             (i) A and B are partially miscible, (ii) after addition of 4 mL of A, the the lower layer is a solution of A in B, and the upper layer is a solution of B dissolved in A, (iii) A is less dense than B


Answers
1.  B and D
2.  A, B, D
3.  A and D
4.  E

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  LEARNING CHEMISTRY FOR UNDERSTANDING

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  • HOME
  • NAVIGATION
    • Table of contents
    • Index
    • TALK WITH PROF BOB?
  • LEARNING MODULES
    • Chapter 02 Stuff, matter: What is it? >
      • 0200 Stuff, matter: A theory of atoms
      • 0201 Atoms: The building blocks of all stuff
      • 0202 People classifying stuffs. Why?
    • Chapter 05 Chemical reactions, chemical equations >
      • 0500 Chemical reactions vs. chemical equations. Overview
      • 0501 Chemical amount and its unit of measurement, mole
      • 0502 The Avogadro constant: How many is that?
      • 0503 The Avogadro constant: Why is it that number?
      • 0504 Chemical formulas: What can they tell us??
      • 0505 Chemical equations: What can they tell us?
      • 0506 Limiting reactants: How much reaction can happen?
      • 0507 Balanced chemical equations: What are they?
      • 0508 Chemical reactions as competitions
    • Chapter 09 Aqueous solutions >
      • 0901 What is a solution? And what is not?
      • 0902 Miscibility of liquids in each other
      • 0903 Like dissolves like? Shades of grey
      • 0905 Dissolution of ionic salts in water: A competition
      • 0906 Can we predict solubilities of salts?
      • 0907 Solution concentration
      • 0908 Chemical species, speciation in aqueous solution
      • 0909 Solutes: Electrolytes or non-electrolytes?
      • 0910 Electrolytes - strong or weak?
      • 0911 Concentrated, dilute, strong, weak
      • 0912 Species concentration vs. solution concentration
      • 0913 Weak electrolytes: Getting quantitative
    • Chapter 11: Dynamic chemical equilibrium >
      • 1100 Dynamic chemical equilibrium: Overview
      • 1101 Visualising dynamic chemical equilibrium
      • 1102 The jargon of chemical equilibrium
      • 1103 Equilibrium constants: The law of equilibrium
      • 1104 The law of equilibrium: an analogy
    • Chapter 22 Spectroscopy >
      • 2200 Spectroscopy: Overview and preview
      • 2201 Quantisation of forms of energy
      • 2202 Light: Wave-particle "duality"
      • 2203 Ultraviolet-visible spectroscopy
      • 2204 Beer’s law: How much light is transmitted?
    • Chapter 27 The greenhouse effect, climate change >
      • 2700 The greenhouse effect: overview
      • 2701 Is Earth in energy balance?
      • 2702 CO2 in the atmosphere before 1800
      • 2703 So little CO2! Pffft?
      • 2704 Does CO2 affect Earth's energy balance?
      • 2705 The "greenhouse effect"
      • 2706 Why does CO2 absorb radiation from Earth?
      • 2707 The "enhanced greenhouse effect"
      • 2708 Why doesn't CO2 absorb the radiation from the sun?
      • 2709 Why are N2 and O2 not greenhouse gases?
      • 2710 Doesn't water vapour absorb all the IR?
      • 2711 Carbon dioxide from our cars
      • 2712 The source of energy from combustion
      • 2713 Comparing fuels as energy sources
      • 2714 Methane: How does it compare as a GHG?
      • 2715 Different sorts of pollution of the atmosphere
      • 2716 "Acidification" of seawater
    • Chapter 27 Communicating chemistry >
      • 2700 Overview, preview
      • 2703 The jargon we use
  • TEACHERS' CORNER
    • T01 Communicating chemistry
    • T02 Beer's law
    • T03 Professional amnesia of the chemistry teaching professio
    • T04 Law of equilibrium
    • T05 Visusalizing dynamic chemical equilibrium
    • Information vs. knowledge
  • PERSONAL GALLERY
    • Family
    • Travel
    • Playful dolphins
    • The University of Western Australia
    • Kings Park
    • Perth
    • At work
    • 999 Thermodynamics