Infrared spectroscopy is one of those IB Chemistry topics that feels like you’re supposed to “just see it.” A jagged line, a few dips, and suddenly you’re expected to announce: alcohol! carbonyl! carboxylic acid!
But IR isn’t magic. It’s closer to listening than looking: molecules absorb infrared light when their bonds vibrate at matching frequencies. Once you learn a small set of signals, IB Chemistry IR questions become predictable -- even calming.
What infrared spectroscopy actually measures (IB Chemistry)
In IB Chemistry, infrared spectroscopy (IR) is an analytical technique that measures which wavenumbers (cm⁻¹) a molecule absorbs. Those absorptions correspond to bond vibrations.
Think of covalent bonds like springs. When IR radiation hits a molecule:
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Bonds stretch (change bond length)
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Bonds bend (change bond angle)
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Different bonds absorb different frequencies
The instrument plots an IR spectrum as % transmittance (y-axis) vs wavenumber (x-axis). A downward “peak” (dip) means absorption.
If you want the exact ranges the exam expects, keep the RevisionDojo data booklet open while you practice: Chemistry Data Booklet (Infrared Data).
The two regions you must use in IB Chemistry IR
Most IB Chemistry exam interpretation follows the same rhythm: scan the functional group region, then use the fingerprint region only for confirmation.
Functional group region (4000--1500 cm⁻¹)
This is where the “big identity clues” live: O--H, N--H, C=O, C=C, C--H.
Fingerprint region (<1500 cm⁻¹)
This region is messy but unique. You typically don’t assign every peak. Instead, you use it to compare against a known sample or check if your product “matches.”
A quick checklist to interpret an IR spectrum (exam-ready)
Use this fast checklist in IB Chemistry Paper 2 style questions:
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Look for a broad O--H (alcohol vs carboxylic acid)
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Look for a strong C=O near ~1700 cm⁻¹
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Check for C=C if unsaturation is relevant
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Use C--H stretches to support your story
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Mention fingerprint region only as a “confirm identity” tool
For targeted syllabus practice, the cleanest place to drill this is RevisionDojo’s topic hub: S3.2.9 Infrared spectra (HL) and the companion S3.2.9 Notes.
The common peaks you actually need to memorise (IB Chemistry)
You don’t need to memorise the whole data booklet. In IB Chemistry, these show up constantly:
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Alcohol O--H: broad, 3200--3600 cm⁻¹
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Carboxylic acid O--H: very broad, ~2500--3300 cm⁻¹, plus C=O
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Carbonyl C=O (aldehyde/ketone/acid/ester): strong, ~1700 cm⁻¹
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Alkene C=C: ~1640--1680 cm⁻¹
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C--H stretches: ~2850--3100 cm⁻¹
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Amine N--H: ~3300--3500 cm⁻¹
A good way to make these automatic is spaced repetition. RevisionDojo has dedicated decks, like Spectroscopic identification flashcards and Functional groups flashcards.
What IR spectroscopy is used for in IB Chemistry questions
IR is mainly about functional groups, but exam questions usually frame it as decision-making.
Identifying functional groups
IR can quickly show whether an unknown contains an O--H, C=O, N--H, or C=C.
Checking whether a reaction happened
In IB Chemistry, you’ll often be told a transformation and asked what changes you’d expect:
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O--H disappears: alcohol consumed
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C=O appears: carbonyl formed
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C=C disappears: addition reaction likely occurred
Comparing an unknown to a reference
You can say: “The fingerprint region is unique, so it can confirm identity when compared with a known spectrum.”
To connect IR to bigger structure questions, pair it with the structural analysis unit and practice: S3.2.12 Structural analysis questionbank and S3.2.12 Notes.
Common IB Chemistry misunderstandings (and how to fix them)
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“IR gives the molecular formula.” No. IR gives evidence for functional groups. You need other methods (and good reasoning) for full structures.
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“Bigger peak = higher concentration.” Not reliably. Peak intensity depends heavily on bond polarity and how strongly that vibration interacts with IR.
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“Fingerprint region is useless.” It’s not for memorising -- it’s for matching and confirmation.
How to revise infrared spectroscopy with RevisionDojo
If you’re revising IB Chemistry, the goal is not to read about IR once -- it’s to see enough spectra that your brain starts predicting the answer.
A simple loop:
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Review the concept in the IB Chemistry topic pages
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Drill exam-style interpretation in the IB Chemistry Questionbank
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Use AI Chat to explain why each peak matters (especially when you were “almost right”)
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Build mini Mock Exams with the exam builder, then use grading tools to spot patterns
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When you’re close to exams, practise with IB Chemistry Predicted Papers
FAQ: Infrared spectroscopy in IB Chemistry
Can IR spectroscopy distinguish aldehydes and ketones in IB Chemistry?
Not reliably by the carbonyl peak alone, because both aldehydes and ketones show a strong C=O absorption around the ~1700 cm⁻¹ region. In an IB Chemistry exam, you should say that IR is excellent for confirming “a carbonyl is present,” but weaker for telling which carbonyl it is. Sometimes additional clues exist (like small aldehyde C--H features near ~2720--2820 cm⁻¹), but those details are not always clear in simplified exam spectra. The safest approach is to combine IR evidence with reaction context and other data provided in the question. If the prompt includes multiple spectra or additional information, use IR to narrow functional groups, then finish the identification logically. In RevisionDojo’s Questionbank, explanations train you to write that reasoning in markscheme language.
Can IR spectroscopy show isomers for IB Chemistry?
IR can sometimes distinguish functional group isomers because they contain different key bonds (for example, an alcohol vs an ether). In that case, the functional group region gives different headline peaks, and the answer is straightforward. However, IR usually cannot reliably distinguish stereoisomers, because they share the same set of functional groups. Even structural isomers with the same functional groups may look similar in the functional group region, which is why the fingerprint region becomes important for comparison. In IB Chemistry, you should state that the fingerprint region is unique and can help confirm identity against a reference. But you typically won’t “decode” the fingerprint region peak-by-peak under time pressure. Use IR as one piece of the puzzle rather than the whole solution.
Does IR spectroscopy work on all compounds in IB Chemistry?
IR works best for covalent molecules where vibrations produce a measurable change in dipole moment. Many organic compounds fit this perfectly, which is why IB Chemistry uses IR heavily in organic identification. Some substances may produce spectra that are harder to interpret, especially if peaks overlap or if strong hydrogen bonding broadens signals. Ionic compounds often don’t give the kind of functional-group information students expect from IR in the organic context. In exam questions, you’re usually dealing with an organic unknown, so IR is appropriate and informative. The bigger limitation is not “whether it works,” but what it can and cannot conclude: functional groups, yes; complete structure, no. That’s why RevisionDojo pairs IR with structural analysis practice and exam-style marking.
Conclusion: treat IR like a small vocabulary, not a giant picture
Infrared spectroscopy in IB Chemistry is less about artistic interpretation and more about recognising a handful of high-value patterns: broad O--H, strong C=O, signs of C=C, and the idea that the fingerprint region confirms identity. Learn those signals, practise them in exam conditions, and your confidence rises quickly.
If you want the fastest path, use RevisionDojo to cycle between Study Notes, Flashcards, and the Questionbank, then test yourself with Mock Exams and Predicted Papers. That repetition is what turns an IR spectrum from a scary squiggle into a straightforward mark-collector for IB Chemistry.