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I had a conversation with somebody who had somewhat confused ideas about food. He thought that "all food contains sugar" and when I gave a bottle of oil as the counterexample, he said the glycerol is the sugar there.

Now let's forget for a minute that glycerol is not a sugar. The point is, glycerol is indeed a "building block" of a triglyceride, and glucose is a "building block" of glucose. But still, if you are talking about a process which needs sugar, starch won't work unless you cleave it off first. Just like starch won't spontaneously conduct electricity or emit beta radiation even though it's chock full of electrons.

Given that we have somebody with a very basic level of knowledge, and who is already trying to wrap his head around several confusing concepts at once, what would be an intuitive way to get him to really grok that, just because a given atom or group is present in the structure of a molecule, it does not mean that a substance "contains" this atom or group in the same way that a wet sponge contains water? And that without going off on a long tangent of theory?

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  • $\begingroup$ "Sugar is Carbohydrate, right? Is everything you eat Carbohydrate?!" And then a link to Wikipedia's Carbohydrate page. Yep. That's what I leave them to study. :) $\endgroup$ – M.A.R. ಠ_ಠ Apr 17 '15 at 16:32
  • $\begingroup$ @MARamezani it was more complicated. He thought that everything including proteins and fats gets converted into sugars before the body can use it for energy. But I'm not asking about that part, I'm asking how to explain that even though starch does get converted into a sugar after being eaten, you still cannot expect starch slurry to behave like glucose syrup during cooking - and how to tick that box off quickly, before moving on to digestive pathways. $\endgroup$ – rumtscho Apr 17 '15 at 16:42
  • $\begingroup$ Well, behavior on the macroscopic scale is dependent on many factors. Usually, the interactions between molecules are way more important roles in play than what molecules are. Oh, Curt just answered this, and I perfectly agree with his answer. $\endgroup$ – M.A.R. ಠ_ಠ Apr 17 '15 at 16:50
  • $\begingroup$ @Klaus that seems to me like retelling a bad memory/experience. $\endgroup$ – M.A.R. ಠ_ಠ Apr 17 '15 at 17:43
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    $\begingroup$ @KlausWarzecha there is a very big difference between somebody who learned something wrong, and somebody who'll defend his ignorance with his last breath. Both exist, but luckily, this guy wasn't aggressively arguing, he was genuinely curious. $\endgroup$ – rumtscho Apr 17 '15 at 17:58
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I would say something like this:

All matter -- that is, all substances -- are made of atoms. The properties of a given substance are determined by (a) the type and (b) the arrangement of the atoms in that material.

Chemists refer to the "types" of atoms as "elements". For example, vegetable oil and table sugar (sucrose) are both composed of three types of atoms only: carbon, hydrogen, and oxygen.

The arrangement of the atoms in the two materials differs drastically. (You could refer to structure diagrams at this point if your friend is interested.) First, oil has much less oxygen and slightly more hydrogen per carbon atom. Second, the way the atoms are connected -- their arrangement -- is very different.

Chemists sometimes give a name to a group of similar arrangements of atoms. For example, "sugars" are compounds that (a) have carbon, hydrogen, and oxygen atoms in ratios that are approximately $\ce{CH2O}$, and (b) often form ring structures, as seen in table sugar, glucose, and corn starch. But the key words are "approximately" and "often" because some molecules that chemists call sugars don't satisfy these rules. Biochemists interested in metabolism sometimes refer to molecules that can "easily" be converted into "true" sugars as sugars.

Naming in chemistry is complicated by the fact that chemists use words for different purposes: to indicate metabolic similarity, to indicate structural similarity, or to indicate similarity in reactivity. Those different purposes sometimes lead situation where chemist B says that what chemist A calls a "sugar" isn't really a sugar. But that's just an argument about humans' imperfect way of naming things, not about chemistry. There is a lot of wiggle room in the words "approxiately", "often", "similar", and "easily". So if we focus on metabolism only, the glycerol part of the vegetable oil arrangement (i.e. structure) is a bit like a sugar. But it is dissimilar from sugars in terms of both structure and reactivity.

Again, what ultimately matters is (a) the type and (b) the arrangement of the atoms in a substance. Those two things are together necessary and sufficient for a complete description of all substances. Whether to call particular arrangements of particular atom types "sugars" or not is something people can argue about.

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  • $\begingroup$ As a side note, my pencil is made of "carbon". Diamond is also made of "carbon". That doesn't mean I can write with diamonds. $\endgroup$ – M.A.R. ಠ_ಠ Apr 17 '15 at 16:51
  • $\begingroup$ (However, I do know that graphite and diamond aren't molecules) $\endgroup$ – M.A.R. ಠ_ಠ Apr 17 '15 at 16:52
  • $\begingroup$ I avoided even mentioning "molecules" because I think it is a bit jargon-y. For example we could argue over whether a single crystal of diamond is a "molecule" or not. Diamond and graphite certainly have very different arrangements of atoms. "Arrangement" is a term that in my mind at least includes both connectivity as well as spatial geometry. $\endgroup$ – Curt F. Apr 17 '15 at 16:54
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    $\begingroup$ I will upvote your answer, but it's not what I was looking for, sorry. Of course this explanation is correct from a chemistry point of view, but all my intuition of explaining things to people tells me that I'll lose the audience along the way :( I hope somebody will have some more succinct idea. But thank you for this answer, it contains interesting points about sugars I hadn't realized myself. $\endgroup$ – rumtscho Apr 17 '15 at 18:02
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    $\begingroup$ Thanks for the upvote and sorry I wasn't able to help! I hope folks chime in with more answers; this is a great question (+1) and deserves several answers from different perspectives. My perspective is that lay people often have problems distinguishing between arguments about names and arguments about reality, but I can certainly see how a discourse on this subject could bore some folks. $\endgroup$ – Curt F. Apr 17 '15 at 18:32
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In his excellent answer, Curt F. wrote:

The properties of a given substance are determined by (a) the type and (b) the arrangement of the atoms in that material.

Let me add some examples to illustrate how subtle differences lead to substantial property changes any layman can experience.

Containing the same number of atoms means nothing

enter image description here

$\alpha$-pinene (1) and camphene (2) have the same sum formula ($\ce{C10H16}$), but a very different constitution and a very different smell, just as their names suggest.

Containing the same number of atoms and having almost the same constitution

enter image description here

Thymene (3) and carvacrol (4) are regioisomers and only differ in the position of the $\ce{OH}$ group. However, thymene has a thyme odour, while carvacrol smells like oregano.

Mirror, mirror

enter image description here

Compounds 5 and 6 have the same constitution and identical boiling points. They only differ in that one group is above/below the ring plane (the world isn't flat). These are (just) mirror images: enantiomers of limonene. However the interested layman will recognize they are not identical: 5 smells like lemon, while 6 smells like orange.


Containing isn't being

A layman might know that chlorine is a toxic gas and was used in the chemical warfare of WWI. Some might even know that fluorine isn't any better.

These unhealthy properties are not inherited by compounds that "contain" these elements!

  • Table salt ($\ce{NaCl}$) "contains chlorine" (admittedly, writing it that way makes me shudder) but is isn't as toxic as the element. Neither is fluoridated tooth-paste.

enter image description here

  • Each molecule of diazepam (7) contains one chlorine atom, each molecule of fluoxetine (8) contains three (!) fluorine atoms. Even so, these compounds are not part of modern chemical warfare, but on the WHO List of Essential Medicines.
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  • $\begingroup$ This is a great answer that I suspect you will be able to point to (or at least copy and paste) in response to many future chem.se questions. $\endgroup$ – Curt F. Apr 17 '15 at 23:37
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For a layman you could simply mention that airplanes contain aluminium, as do rollerskate rails.

The arrangement of the aluminium is a critical factor determining which vehicle could be used for skating around the neighbourhood and which vehicle could be used for intercontinental transport.

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