Fats vs Oils

Butter is solid in the fridge; olive oil pours even when chilled. Lard is solid; sunflower oil is liquid. It’s tempting to think fats and oils are completely different substances, but chemically they’re the same kind of molecule. The difference comes down to the shape of their fatty acid chains and how well those chains pack together. This comparison explains what they share, how they differ, and why it matters in the kitchen and for health.

The short answer

In chemistry and nutrition:

• A fat is a triglyceride that is solid at room temperature (about 20 °C).
• An oil is a triglyceride that is liquid at room temperature.

Both are lipids, and both are made of the same building blocks (see lipids explained).

Side by side

Feature	Fats	Oils
State at 20 °C	Solid or semi-solid	Liquid
Main chemical type	Triglycerides	Triglycerides
Typical fatty acids	Mostly saturated (no C=C)	Mostly unsaturated (one or more C=C, usually cis)
Chain shape	Straight	Kinked at each cis double bond
Chain packing	Close, orderly	Loose, disordered
Intermolecular forces	Stronger London forces	Weaker London forces
Typical sources	Animals (butter, lard, suet); a few plants (coconut, palm kernel, cocoa butter)	Plants (olive, sunflower, rapeseed); fish
Stability to oxidation	More stable; slower to go rancid	Less stable (especially polyunsaturated); go rancid faster
Energy per gram	About 37 kJ	About 37 kJ

What they share: the triglyceride

Every fat and oil is mostly made of triglycerides. A triglyceride forms when one molecule of glycerol (propane-1,2,3-triol) reacts with three fatty acids, forming three ester bonds and releasing three molecules of water (a condensation reaction).

A fatty acid is a long hydrocarbon chain — typically 12 to 22 carbon atoms — with a carboxylic acid group (–COOH) at one end (see fatty acids). The three fatty acids in a triglyceride can be the same or different, and natural fats and oils are always mixtures of many different triglycerides.

Because the long hydrocarbon chains are non-polar, fats and oils are insoluble in water but dissolve in non-polar solvents. Both store the same energy per gram, about 37 kJ, because it depends on the number of C–H bonds, not on the physical state.

The key difference: saturated vs unsaturated chains

Saturated fatty acids: straight chains

In a saturated fatty acid, every carbon in the chain is joined to the next by a single bond, and carries as many hydrogen atoms as possible. Examples: palmitic acid (C16) and stearic acid (C18).

Single bonds allow free rotation, and in their most stable arrangement the chains are straight zig-zags. Straight chains can line up side by side, like pencils in a box. Close packing means lots of contact between chains and strong London (dispersion) forces. More energy is needed to separate them, so the melting point is higher — and the triglyceride is solid at room temperature.

Unsaturated fatty acids: kinked chains

In an unsaturated fatty acid, the chain contains one (monounsaturated) or more (polyunsaturated) C=C double bonds. In natural fats, these are almost always in the cis arrangement, which puts a bend of about 30° in the chain. Oleic acid (C18, one double bond) is the main fatty acid in olive oil; linoleic acid (C18, two double bonds) is abundant in sunflower oil.

Kinked chains can’t pack neatly. There are gaps between them, less surface contact, and weaker London forces. Less energy is needed to separate them, so the melting point is lower — the triglyceride is liquid at room temperature.

Data point: stearic acid (saturated, C18) melts at about 69 °C; oleic acid (one cis double bond, C18) melts at about 13–14 °C; linoleic acid (two double bonds) at about −5 °C. Same number of carbons, very different melting points — all down to shape.

Exceptions that prove the rule

• Coconut oil is solid in a cool room (it melts around 24 °C) despite being a plant fat, because it’s rich in saturated fatty acids — though they’re relatively short (mainly lauric acid, C12), which keeps its melting point lower than animal fats.
• Fish oils are liquid even in cold water because they’re rich in highly unsaturated omega-3 fatty acids. That keeps fish membranes flexible at low temperature.
• Chain length matters too: shorter chains have weaker London forces, so they lower the melting point.

Stability: why oils go rancid faster

The C–H bonds next to C=C double bonds are weaker, so they’re more easily attacked by oxygen and free radicals. Oils rich in polyunsaturated fatty acids go rancid faster through a radical chain reaction (lipid peroxidation), forming smelly aldehydes and ketones (see antioxidants). That’s why oils are sold in dark bottles and why manufacturers add antioxidants such as tocopherols. Saturated fats are more stable, which is one reason they were traditionally preferred for deep-frying and baking.

Fats and oils can also be broken down by hydrolysis of their ester bonds, releasing free fatty acids. This is the reaction behind the sour smell of old butter (butanoic acid) and, with sodium hydroxide, the making of soap (saponification).

Turning oils into fats: hydrogenation

Oils can be made solid by hydrogenation: reacting them with hydrogen gas over a nickel catalyst at around 150–200 °C (see nickel). Hydrogen adds across C=C double bonds, making the chains more saturated and straighter:

–CH=CH– + H₂ → –CH₂–CH₂–

This was used for over a century to make margarine and shortening. But partial hydrogenation has a side effect: some cis double bonds flip to the trans form. Trans fatty acids have a straighter shape, similar to saturated chains, and they raise LDL (“bad”) cholesterol and lower HDL cholesterol. Many countries have now restricted or banned partially hydrogenated oils, and manufacturers use other methods — such as full hydrogenation blended with oil, or interesterification (rearranging fatty acids between triglycerides) — to make spreads.

Health: the nuances

• Saturated fats tend to raise LDL cholesterol, which is linked to heart disease; health bodies generally recommend limiting them (see cholesterol chemistry).
• Unsaturated fats, especially when they replace saturated fats, are generally associated with better heart health.
• Omega-3 and omega-6 polyunsaturated fatty acids are essential: the body can’t make them, so they must come from food.
• Trans fats from partial hydrogenation are the most harmful.
• All fats and oils are energy-dense, at about 37 kJ per gram.

When to use which

Situation	Better choice	Why
Flaky pastry, biscuits	Solid fat (butter, lard)	Solid layers trap steam and create texture
Salad dressing	Oil	Liquid when cold, mixes into emulsions
High-heat frying	Stable oils (e.g. high-oleic oils) or refined oils with a high smoke point	Resist oxidation and smoking
Lower saturated fat intake	Oils such as olive or rapeseed	More unsaturated fatty acids

Key takeaways

• Fats and oils are both triglycerides: glycerol + three fatty acids joined by ester bonds.
• Fats (solid) are rich in saturated, straight chains that pack closely with stronger London forces.
• Oils (liquid) are rich in cis-unsaturated, kinked chains that pack loosely.
• Oils oxidise (go rancid) faster; hydrogenation makes them solid but partial hydrogenation creates harmful trans fats.

For a detailed look at the chains themselves, see saturated vs unsaturated fats.