Where chemistry starts β and why almost every practical question about an oil turns out to be a question about matter.
~11 min read
Matter is anything that has mass and occupies space. That is the whole definition. It covers the bottle in your hand, the oil inside it, and the air above the oil β which has mass and volume even though you cannot see it. Much of what counts in chemistry is imperceptible, which is why we measure.
βChemicalβ has picked up a second, unhelpful meaning β synthetic, suspect, to be avoided. In its real meaning it covers everything. Water is a chemical, and so is the limonene that makes lemon peel smell of lemon. The question is never whether something contains chemicals, but which ones, in what amounts, and from where.
Press a thumb against a table and it stops, so it is natural to conclude the table is solid the way it feels. It is not. An atom is overwhelmingly empty: almost all its mass sits in a nucleus about one hundred-thousandth of the width of the atom around it.
Diagram to come
Two panels side by side. Left: an atom drawn the way textbooks draw it β large central nucleus, a thin ring of electrons β labelled as the picture we are given. Right: the same atom at honest proportion, nucleus reduced to a single dot with a callout line pointing to it, the electron region filling the entire panel, and a scale bar reading nucleus 1 to atom 100,000. The reader must be able to tell at a glance that the left panel is a convenience and the right one is the fact.
What stops your thumb is force, not substance β electrons at your skin refusing electrons at the table. Solidity is a behaviour rather than a material, and the rest of this course is about forces between particles. Before asking what an essential oil is, you need to be able to ask what a liquid is.
Solid, liquid and gas are three behaviours of the same particles, separated by energy. Add energy to a solid and it becomes a liquid, add more and it becomes a gas, take it away and every step runs backwards. Ice, water and steam are one substance in three moods.
Temperature measures the average energy of motion of those particles, not how much heat something holds. A bath at 40 degrees holds more energy in total than a cup of tea at 80; the tea is hotter because its molecules move faster. Warm a bottle between your palms and you hand energy to its molecules β thinner oil, faster evaporation.
| State | What the particles are doing | Where you meet it |
|---|---|---|
| Solid | Locked in position β they vibrate but do not move past one another. | Beeswax, a salt scrub, dried plant material before the still |
| Liquid | Enough energy to slide past one another, not enough to escape. | Essential oils at room temperature, carrier oils, hydrosols |
| Gas | Enough energy to break free entirely, filling whatever space it is given. | Oil vapour leaving a diffuser β why you smell it across a room |
The third row is the one to hold on to. Nothing that stays liquid has a smell: an essential oil lives at the boundary of the second row and the third.
Put a thermometer into a pan of water on a hot ring. The reading climbs to 100 degrees at sea level and then stops. The ring is still delivering energy at the same rate and the water is boiling hard, but the number does not move until the water has gone.
The energy is being spent on something other than speed. Water molecules attract one another unusually strongly, and to leave the liquid a molecule must break every one of those attachments at once. Warming a gram of water by one degree takes about 4.2 joules; turning that gram into vapour at the same temperature takes roughly 2,260 β some five hundred times as much, and none of it raises the reading. Chemists call it latent heat.
A still is that principle, arranged. Steam driven through plant material loosens the aromatic compounds out of the glands holding them and carries them to a cooled coil, where the mixture condenses and separates. The plateau is what makes it controllable: while water is boiling, the distiller works against a fixed ceiling.
A second piece of physics works alongside it. Many aromatic constituents boil well above 200 degrees alone, hot enough to be damaged. They come over at around 100 anyway, because two liquids that do not dissolve in one another each add their own vapour pressure to the total, so the pair boils below the boiling point of either. Steam does not heat the oil out so much as offer it a ride, which is why the method of extraction changes what ends up in the bottle.
Volatile means ready to become a gas. It is the property that defines an essential oil, separates it from every other oil in the kitchen, and decides most of how one behaves. If one idea here stays with you, make it this one.
Particles in a liquid do not all move at the same speed; temperature is an average, and an average conceals a spread. A fast molecule reaching the surface with enough energy to break free of its neighbours leaves. That is evaporation, it happens at every temperature, and boiling is the same escape made general.
Two things decide how readily a molecule leaves: its mass, and how strongly it is attracted to its neighbours. Small and slippery goes early. Large and sticky stays.
Lemon is cold pressed from the peel of Citrus limon and runs 60β75 per cent limonene β a monoterpene, ten carbons and their hydrogens, nothing for a neighbour to grip. Gone from a surface in minutes. Wild Orange, at 80β97 per cent limonene, is briefer still. These are top notes.
The other end of the shelf runs on fifteen-carbon sesquiterpenes rather than ten-carbon monoterpenes β half as heavy again, and the alcohols among them carry an βOH that grips the βOH beside it. A drop of vetiver is still detectable on a wrist the next day. These are base notes.
Diagram to come
A vertical ladder of constituents ordered by how long they stay. Top rung light and fast: limonene, ten carbons, Lemon and Wild Orange, minutes. Middle: linalool and Ξ±-pinene, ten carbons, hours. Bottom rung heavy and slow: santalol and patchouli alcohol, fifteen carbons plus an βOH group drawn explicitly, days. Every rung must carry its carbon count and a plain time estimate, so the reader can see the ordering is caused by size and grip rather than by perfumery tradition.
Why some things have no smell at all
To smell something, part of it has to reach you as a gas. Sugar, salt and glass have no aroma because nothing leaves them; fractionated coconut oil almost none. That is the real line between a carrier and an essential oil: one stays, the other leaves.
Two habits follow. Keep caps on: an open bottle empties itself of the compounds you paid for, lightest first, shifting the balance of what remains. And keep bottles cool and out of direct light β an oil on a sunny windowsill does not only weaken, it becomes a different mixture.
Sandalwood is steam distilled from the wood of Santalum paniculatum and built mainly from alpha-santalol and beta-santalol; vetiver from the root of Vetiveria zizanioides, carrying isovalencenol, khusimol and vetiselinenol. Both are sesquiterpene alcohols β fifteen carbons rather than ten, half as heavy again, each with an βOH group that hydrogen-bonds to its neighbours. Weight and grip together, which is why they stay.
The resins sit between. Frankincense carries monoterpenes such as Ξ±-pinene and limonene alongside much heavier sesquiterpenes, so it opens bright and then settles and stays. Myrrh, from Commiphora myrrha, is largely furanoid sesquiterpenes and barely moves. Put a drop of each on paper and return in an hour: the lemon has gone, the myrrh has not.
When the fastest molecules leave a liquid they take their energy with them, and what remains is on average slower β cooler. Evaporation removes heat from whatever the liquid rests on. That is why a drop of a volatile oil on the back of the hand feels cool for a few seconds before the sensation fades.
Keep that separate from an effect people merge with it. Peppermint reads as cold long after evaporation has finished, because menthol interacts with the nerve endings in skin that report temperature β the sensation is produced at the receptor, not by a liquid leaving. Both happen on the same drop, and they are not the same event.
A pure substance is one kind of particle throughout. A mixture is two or more sitting together without having combined: each keeps its own properties, the proportions can be anything, and the parts can be separated again by physical means β which is the whole reason a still works.
Element
One kind of atom, not reducible further by chemical means. Carbon, hydrogen, oxygen β the next lesson.
Compound
Elements bonded in fixed proportions, with properties belonging to neither ingredient. Limonene is ten carbons and sixteen hydrogens in one arrangement; rearrange them and it smells of something else.
Mixture
Pure substances physically together in no fixed ratio, each still what it was. An essential oil is a mixture, and so is the sea. A suspension is the version whose parts stay separate enough to see β oil shaken into water.
So an essential oil is a mixture β dozens of pure substances, sometimes hundreds. Lemon at 60β75 per cent limonene means a quarter to two-fifths of the bottle is everything else, including compounds at a fraction of one per cent that carry much of the aroma. Every oilβs list sits in its constituent profile.
That makes the word on the label easy to misread. βPureβ does not mean one compound β that would be a chemical isolate. It means nothing added and nothing taken away. Adding is the familiar adulteration. Removing is subtler: an oil can be fractionated, a valuable constituent distilled off and sold separately, and what remains still smells broadly right but is no longer the mixture the plant made. How that gets verified is a lesson of its own.
It is also why an oil cannot be reduced to its main constituent. Lemon is not limonene. Two lemon oils with the same headline percentage can smell noticeably different, because the difference lives in the tail.
Fill a glass with water and add one drop of lemon oil. It does not sink, dissolve or disperse. It sits on the surface and spreads into a film thin enough to bend the light. Stir it and it breaks into beads; stop, and the beads find one another.
The property responsible is polarity. A water molecule is bent, and the oxygen at its centre pulls the shared electrons harder than the two hydrogens do, leaving a molecule with ends β negative at the oxygen, positive at the hydrogens. A glass of water is a crowd of small magnets gripping each other.
Most essential oil constituents have no ends at all. Limonene is carbon and hydrogen only, sharing electrons evenly enough that no part of it carries a meaningful charge β nothing for a water molecule to hold. And water molecules have something better to hold: each other. They close ranks and the oil is squeezed into the smallest space it can occupy. It is not repelled. It is excluded. Like dissolves like.
Diagram to come
Three panels reading left to right. One: a water molecule drawn bent, oxygen marked with a partial negative, each hydrogen with a partial positive, noted as having ends. Two: a limonene skeleton with no charge marks anywhere, noted as having nothing to grip. Three: the observed result β a cross-section of a glass with a lemon drop spread as a film across the surface and the water molecules beneath drawn linked to each other, turning their backs on it. The reader should come away understanding the oil is excluded rather than repelled.
That one rule turns most of the handling advice a healer is given into reasoning. A carrier oil is non-polar like the constituents it carries, so the two genuinely mix, and diluting spreads the drop and slows its evaporation. One to two drops in a carrier oil is the ordinary way to apply anything topically, and oils known to be hot on skin β oregano, thyme, cinnamon bark, cassia and clove, and peppermint on a child β need considerably heavier dilution. Dilution and skin sensitivity have a lesson to themselves.
One property of citrus belongs beside any topical use. Expressed citrus oils carry compounds that make skin more reactive to ultraviolet light. Avoid direct sunlight and UV for at least twelve hours on skin where you have applied lemon, lime, grapefruit, bergamot or wild orange.
Water does not dilute an essential oil
A drop of oil in a glass of water is undiluted oil floating on water β not weakened, only concentrated into a film at the surface. The same applies if an oil reaches the eye: water spreads it rather than lifting it, while a carrier oil or whole milk will take it up. Whether an oil is meant for internal use at all is settled by the label on your own bottle β labelling, and the product range itself, differ from market to market.
A bath is the same problem larger. Drops added straight to bathwater float, gather, and arrive on skin at full strength. Blend them into a dispersant first β whole milk, unscented bath gel, a spoonful of carrier oil.
The same property makes citrus useful around the house: sticky residue, adhesive and grease are non-polar, and limonene dissolves them where water cannot. It is also why the condensate leaving a still parts on its own β oil above, hydrosol below. The recipes that use this sit in their own course.
Density is mass divided by volume, and it is why oil sits on water: most essential oils are slightly less dense. A few sink β clove, cinnamon bark, wintergreen. Viscosity is resistance to flow: lemon and peppermint are thin, myrrh and vetiver thicker. Density decides what floats. Viscosity decides the size of a drop.
A drop is not a unit of measurement but an outcome, and its size shifts with the bottle. The working figure is twenty drops to the millilitre, which puts a 15 mL bottle at roughly three hundred.
Oil gathers at the orifice of the reducer, held by surface tension, and grows until its own weight overcomes the tension holding it up. The size is settled by whatever that balance happens to be: the width of the orifice, the viscosity of that oil, the temperature of the bottle, the angle you hold it at.
So a bottle brought in from a cold car hangs a long time and then releases one fat drop; warm hands correct it. Held vertically it tends to release a rush; held at forty-five degrees it gives one drop at a time. None of that changes the recipe β it changes how carefully you have to watch it.
It is still the unit every recipe is written in, and at twenty drops to the millilitre, one drop in 5 mL of carrier is about one per cent.
About one per cent
One drop per 5 mL of carrier, or six in a 30 mL bottle. A sensible starting point on adult skin, and a ceiling for the face.
About two to three per cent
Two to three drops per 5 mL. For a small area rather than the whole body, and for a short run.
Children, older skin, sensitive skin
Considerably lower β often a quarter to a half of an adult dilution, and always tested on a small patch first.
Hot oils
Oregano, thyme, cinnamon bark, cassia and clove, and peppermint on a child, need heavier dilution than any figure above.
Two extra drops on a six-drop recipe is a third more oil than asked for. If they were vetiver rather than lemon they carry more mass and stay far longer, so the result is not merely stronger β it is a different blend, weighted towards its base. How to use an oil is built on the counting habit.
Keep oils away from eyes and inner ears, and out of reach of children. If you are pregnant, nursing, under medical care or taking medication, speak to your healthcare practitioner before use. Nothing here is intended to diagnose, treat, cure or prevent any disease.
Everything here has been matter in bulk: crowds of particles, and what they do when warmed or mixed. What the particles are made of narrows fast. Nearly every compound in every bottle you own is carbon, hydrogen and oxygen, with traces of nitrogen and sulfur β carbon alone is 70 to 80 per cent of an essential oil.
Each rung rests on the one below: elements, atoms, the table that sorts them by behaviour, then bonding, which sets a moleculeβs shape. Then terpenes β the ten- and fifteen-carbon skeletons you met here as light and heavy β and the groups attached to them. The next lesson is elements, the shortest rung. Once you know which atoms are in play, the number of things a molecule can be drops sharply.
Lesson 1 of 18 Β· Science