Unit 3 Properties of Substances and Mixtures
AP Chemistry Spectroscopy
Which kind of radiation causes which change in a molecule, why solutions are colored, and how a spectrophotometer is set up.
Spectroscopy studies how matter absorbs and emits electromagnetic radiation. Topic 3.11 asks one central question: which region of the spectrum causes which kind of change in a particle? The calculations that go with it, E = hν and E = hc/λ, are worked in AP Chemistry photon energy, and measuring concentration from absorbance is the Beer-Lambert law. This page connects the two.
The Electromagnetic Spectrum From Low to High Energy
All electromagnetic radiation travels at the speed of light; the regions differ in wavelength, frequency, and the energy of each photon. Moving from radio waves toward X-rays, wavelength gets shorter while frequency and photon energy increase. Within the visible region, red light (about 700 nm) has the least energy and violet light (about 400 nm) the most.
Which Radiation Causes Which Molecular Change
| Region | What it changes | Example |
|---|---|---|
| Microwave | Molecular rotation | A microwave oven heats food by making water molecules rotate faster |
| Infrared | Molecular vibration: bonds stretching and bending | CO2 absorbs infrared radiation, which is why it is a greenhouse gas |
| Visible and ultraviolet | Electronic transitions: an electron moves to a higher energy level | Colored solutions absorb visible light; ozone absorbs ultraviolet light |
| Higher-energy ultraviolet and X-ray | Breaking bonds or removing electrons | Photoelectron spectroscopy uses high-energy photons to eject electrons |
The pattern follows the size of the energy gaps: rotational levels are closest together, vibrational levels farther apart, and electronic levels farthest apart, so each needs a more energetic photon than the last. Removing electrons entirely is the basis of photoelectron spectroscopy in Unit 1.
Absorption and Emission Spectra
A particle absorbs a photon only when the photon’s energy matches the gap between two of its energy levels. An absorption spectrum records which wavelengths are taken in; an emission spectrum records the wavelengths given off when excited particles drop back to lower levels. Because an atom’s energy levels are fixed, a gas of atoms absorbs and emits only specific wavelengths, seen as lines. Molecules in solution have many closely spaced levels, so their spectra are broad bands instead.
Why Colored Solutions Absorb the Complementary Color
A solution looks colored because it absorbs part of the visible spectrum and lets the rest through. The color you see is roughly the complement of the color absorbed: complementary pairs sit opposite each other on a color wheel, such as orange and blue, red and green, and yellow and violet. A blue copper(II) sulfate solution absorbs mainly orange-red light. A colorless solution absorbs no visible light, though it may still absorb ultraviolet light.
How a Spectrophotometer Measures Absorbance
- A light source passes through a wavelength selector, so only one wavelength reaches the sample.
- The light passes through a cuvette of the solution, and a detector measures how much gets through.
- The instrument compares this with a blank containing only the solvent, and reports absorbance.
Set the wavelength at λmax, the wavelength the solute absorbs most strongly. There, absorbance changes the most for a given change in concentration, so the measurement is most sensitive. Absorbance at λmax is then proportional to concentration through A = εbc. The 2026 AP exam built a whole question on this setup, worked in the 2026 AP Chemistry FRQ answers.
Worked Example 1: Matching Radiation to a Molecular Change
Identify the change each type of radiation causes: (a) microwave radiation absorbed by gaseous H2O, (b) infrared radiation absorbed by CO2, (c) ultraviolet radiation absorbed by O3.
- (a) Molecular rotation. Microwave photons carry just enough energy to change rotational energy.
- (b) Molecular vibration. Infrared photons match the energy needed to stretch and bend the C=O bonds.
- (c) Electronic transitions. Ultraviolet photons have enough energy to promote electrons, and in ozone can break an O–O bond.
Worked Example 2: Choosing the Wavelength for a Blue Solution
A student will use a spectrophotometer to find the concentration of a blue dye. The dye’s absorbance spectrum peaks at 630 nm and is nearly zero at 450 nm. Which wavelength should the student use, and why?
630 nm. It is λmax, where the dye absorbs most strongly, so absorbance changes most with concentration. At 450 nm the dye barely absorbs, so absorbance would hardly change with concentration and the measurement would be unreliable. It makes sense that a blue dye absorbs strongly near 630 nm, in the orange-red part of the spectrum, the complement of blue.
Worked Example 3: Ranking the Energy of Three Transitions
Rank a rotational transition, a vibrational transition, and an electronic transition of the same molecule from lowest to highest energy, and name the region of radiation for each.
Rotational (microwave) < vibrational (infrared) < electronic (visible or ultraviolet). Higher-energy transitions need photons with shorter wavelengths. To put numbers to the photons in each region, see the worked examples in photon energy.
Common Spectroscopy Mistakes
- Saying infrared causes electronic transitions. Infrared changes vibrations; electronic transitions need visible or ultraviolet light.
- Saying a solution absorbs the color it appears. It transmits that color and absorbs roughly the complementary one.
- Choosing a wavelength the sample does not absorb. Absorbance there barely changes with concentration; use λmax.
- Linking longer wavelength with higher energy. Energy is inversely proportional to wavelength.
These ideas tie Units 1 and 3 together, from electron energy levels to concentration measurements. To see how points on questions like these count toward a 1–5 score, use the AP Chem Score Calculator.
Related Resources
- AP Chem Score Calculator
- Unit 3 Review: Properties of Substances and Mixtures
- AP Chemistry Photon Energy
- AP Chemistry Beer-Lambert Law
- AP Chemistry Photoelectron Spectroscopy
- AP Chemistry Solubility
- 2026 AP Chemistry FRQ Answers Explained
Frequently Asked Questions
What does each region of the electromagnetic spectrum do to molecules?
Microwave radiation changes how molecules rotate, infrared radiation changes how bonds vibrate, and visible and ultraviolet light move electrons to higher energy levels (electronic transitions). Higher-energy ultraviolet light can break bonds, and X-rays can remove inner electrons.
What is the order of the electromagnetic spectrum from lowest to highest energy?
Radio waves, microwaves, infrared, visible (red to violet), ultraviolet, X-rays, and gamma rays. Energy and frequency increase in that order while wavelength decreases.
Why does a solution have a color?
It absorbs some wavelengths of visible light and transmits the rest. The color you see is made of the light that is not absorbed, roughly the complementary color of what is absorbed: a blue solution absorbs mainly orange light.
Why do you measure absorbance at the wavelength of maximum absorbance?
At lambda max the absorbance changes the most for a given change in concentration, so measurements are most sensitive, and small errors in the wavelength setting change the reading least.
What is the difference between an absorption spectrum and an emission spectrum?
An absorption spectrum shows which wavelengths a sample takes in as its particles move to higher energy states. An emission spectrum shows the wavelengths given off as excited particles drop to lower energy states. For atoms, both appear as lines at the same wavelengths, because the energy levels are fixed.
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