Aha! Chemistry with Prof Bob
  • HOME
    • 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 and chemical equations >
      • 0500 Chemical reactions and 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 Evidence from 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?
    • ENVIRONMENTAL CHEMISTRY >
      • EARTH'S ATMOSPHERE >
        • 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
      • FUELS
      • EARTH'S OCEANS AND WATERWAYS
  • TEACHERS' CORNER
    • TC01 Language and meaning in chemistry >
      • TC0101 The jargon we use
    • TC02 REPRESENTATION IN CHEMISTRY
    • TC03 MODELLING IN CHEMISTRY
    • TC04 KNOWING AND LEARNING
    • TC05 Communicating chemistry >
      • TC0501 Overview, preview
    • TC06 COMPLEXITY of LEARNING CHEMISTRY
    • TC07 PEDAGOGOICAL CONTENT KNOWLEDGE, PCK >
      • TC0701 Amnesia of the chemistry teaching professioN
    • MODULE-SPECIFIC PCK >
      • Chapter PCK11 Dynamic chemical equilibrium >
        • PCK1101 Visusalizing dynamic chemical equilibrium
        • PCK1103 Equilibrium constants: The law of equilibrium
      • Chapter PCK22 Evidence from spectroscopy >
        • PCK2204 Beer's law
  • NAVIGATION
    • Table of contents
    • Index, alphabetical

​Module 2203

Ultraviolet-visible spectroscopy

Why do solutions of different substances have different colours?

Why are none of the colours identical?

Why are different wavelengths of white light absorbed?

What is quantisation of energy?

​Why do solutions not absorb some wavelengths?
​

Prof Bob takes the general ideas discussed in Module 2201 to a more specific level .....
Prof Bob does classroom demonstrations of visible spectroscopy in the classroom and in the garden. Is the pertinent question why some wavelengths of radiation are NOT absorbed by species in solution?
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KEY IDEAS  -  UV-visible spectrosocopy

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What an orgy of visible absorption spectroscopy! And no doubt some UV spectroscopy, although we cannot tell by eye.
In Module 2201, Prof Bob talks in general terms about spectra across the whole of the electromagnetic spectrum: elephants to radio waves. Here he focusses on spectra in the ultraviolet and visible range of wavelengths ..... well, he only demonstrates visible spectra actually, but the origins of spectra across the visible and ultraviolet regions are the same.


A level of explanation: Which wavelengths are absorbed?

Which are transmitted?


​At a simple level of explanation, a solution that appears red to us absorbs, from the white light striking it, blue and green light - so that only “red wavelengths” are transmitted. Yellow solutions absorb blue light, and blue solutions absorb red/yellow/orange light from white light.

​[
I have used the term “red wavelengths” in quotation marks because wavelengths cannot be coloured. This is a shorthand way of referring to wavelengths of radiation which we perceive by eye as red colour.]

This leads to the question of why a solution absorbs only particular wavelengths of light (or photons with particular energy). And to the equally important question of why the solution does not absorb all of the other wavelengths from white light.




Why are only particular wavelengths absorbed?

​And the others not?


​The scientists’ explanation is quantization of the energy levels of each mode ….

All modes of energy of molecules, ions and atoms (electron energy, vibrational energy, rotational energy) are quantized: that is, the energy of each mode can have only particular levels – and cannot have energy intermediate between those levels.
​


​
Pause for some terminology .......
  • An "allowed" energy level means an energy level that is possible (because that is the way things are, and not in the sense of being allowed by persons, or by the law). Allowed by Mother Nature? A chemical species cannot exist at an energy level that is not “allowed”.
  • "Excitation" refers to the increase of energy to a higher “allowed”​ level – in this case, of electrons, but the term is also applicable to vibrational and rotational energy levels.

​“Excitation” of any energy mode can only happen as a result of absorption of photons whose energy corresponds exactly with the energy gap between any two
“allowed” energy levels. Otherwise, the energy would reach a level that is not “allowed”.

The key: The energy gaps between levels of energy of electrons in atoms, molecules or ions (and not the energy levels of vibration or rotation) just happen to be (Mother Nature again!) such that the photons absorbed are those of visible, or near-ultraviolet, radiations.
​
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From incident radiation with visible and near-ultraviolet wavelengths, 'excitation' of electrons is only brought about by photons whose energies exactly match the difference of energy between any two 'allowed' energy levels of the electrons - portrayed here for levels E1 and E2, although there are many more 'allowed' levels. Otherwise, the electron would be 'excited' to an energy level that is not 'allowed'.

​
Of course different species have different “allowed” energy levels, and different energy gaps. So the photons absorbed are of different energies – and so the light transmitted (that is, not absorbed) is different for different species. 
​

Coloured partial shadows


​If an opaque object is placed in the path of white light, a shadow is produced. This is total prevention of any of the white light from passing through.

We can physically form a partial shadow if the white light passes through a mesh. Some fraction of the white light is prevented from passing throught to a surface. But that which does pass through is also white light  - all of the wavelengths of visible light.

We can form coloured partial shadows by chemical means. For example, a blue solution put in the path of white light gives rise to a blue partial shadow because some components of the white light (red/orange light) are absorbed and do not pass through.


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And a red partial shadow is formed as a result of ......... ?
Thinking, thinking ......... What is the colour of a shadow formed when white light passes through, in succession, a blue solution, a yellow solution, and a red solution? See Prof Bob's video at the10-minute mark.
A clarification
Red light (for example) can be the result of two different spectroscopic processes:
  • Some substances can emit red light when the energy of their electrons fall to a lower “allowed” energy. A plot of the intensities of wavelengths emitted is an emission spectrum (See Module 2201: Quantization of forms of energy).​
 
  • And some can transmit red light when blue/green light is absorbed as the energy of their electrons increases to a higher “allowed” energy. A plot of the fractions of wavelengths absorbed is an absorption spectrum.
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SELF CHECK - Some thinking tasks

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Just a reminder ... the colours of a rainbow are not due to either absorption or emission of photons by chemical species. Rainbows result from differential reflection of different wavelengths of light within water droplets
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Finding your way around .....

You can browse or search the Aha! Learning chemistry website in the following ways:
  • Use the drop-down menus from the buttons at the top of each page to browse the modules chapter-by-chapter.
  • Click to go to the TABLE OF CONTENTS (also from the NAVIGATION button) to see all available chapters and modules in numbered sequence.
  • Click to go to the ALPHABETICAL INDEX. (also from the NAVIGATION ​button).
  • Enter a word or phrase in the Search box at the top of each page.
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LEARNING CHEMISTRY FOR UNDERSTANDING

Email: [email protected]


​© The content on any page in this website (video, text, original images, and self-check) may be used without charge for non-commercial educational purposes, provided that acknowledgement is given to the Aha! Learning Chemistry with Prof Bob website, with specification of the URL: https://www.ahachemistry.com/​
  • HOME
    • 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 and chemical equations >
      • 0500 Chemical reactions and 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 Evidence from 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?
    • ENVIRONMENTAL CHEMISTRY >
      • EARTH'S ATMOSPHERE >
        • 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
      • FUELS
      • EARTH'S OCEANS AND WATERWAYS
  • TEACHERS' CORNER
    • TC01 Language and meaning in chemistry >
      • TC0101 The jargon we use
    • TC02 REPRESENTATION IN CHEMISTRY
    • TC03 MODELLING IN CHEMISTRY
    • TC04 KNOWING AND LEARNING
    • TC05 Communicating chemistry >
      • TC0501 Overview, preview
    • TC06 COMPLEXITY of LEARNING CHEMISTRY
    • TC07 PEDAGOGOICAL CONTENT KNOWLEDGE, PCK >
      • TC0701 Amnesia of the chemistry teaching professioN
    • MODULE-SPECIFIC PCK >
      • Chapter PCK11 Dynamic chemical equilibrium >
        • PCK1101 Visusalizing dynamic chemical equilibrium
        • PCK1103 Equilibrium constants: The law of equilibrium
      • Chapter PCK22 Evidence from spectroscopy >
        • PCK2204 Beer's law
  • NAVIGATION
    • Table of contents
    • Index, alphabetical