Mineral Oil
Formula: CₙH₂ₙ₊₂ — Paraffin oil, liquid paraffin, white mineral oil
Appearance: Colorless, odorless oily liquid
Hazard: Very low hazard · Non-toxic
Properties
A mixture of saturated hydrocarbons (alkanes) with chain lengths typically C15–C40, derived from petroleum refining. Chemically inert under ordinary laboratory conditions — does not react with acids, bases, or most oxidizers. Completely immiscible with water (density ~0.85 g/mL, slightly less than water). Miscible with most organic solvents. Very low vapor pressure — essentially non-volatile at room temperature. Food-grade mineral oil is widely used as a laxative and in food processing. The “light mineral oil” sold for lab use is essentially the same product as pharmaceutical-grade liquid paraffin.
Historical Context
Mineral oil was a byproduct of the early petroleum industry. Before kerosene lamps dominated in the mid-19th century, illuminating oil came from whale blubber and vegetable oils — expensive and increasingly scarce as whale populations collapsed. The Pennsylvania oil rush (1859, Drake’s well) made petroleum-derived lamp oil cheap and abundant, and the subsequent fractional distillation of crude oil produced mineral oil as a less volatile fraction alongside kerosene.
Pharmaceutical liquid paraffin was promoted as a laxative in the early 20th century and became one of the first “internal lubricants” sold commercially. Its chemical inertness made it attractive as a base for cosmetics, ointments, and baby oil. The same inertness that makes mineral oil medicinally useful also makes it a useful chemistry lab reagent — it seals reactive metals (sodium, potassium) from air and moisture without reacting with them.
In physics demonstrations, mineral oil’s transparency and well-defined density make it a standard component of density column demonstrations — one of the clearest ways to visualize that liquids form distinct layers when immiscible and of different densities.
Experiments
Density Column: Build a layered column of liquids by carefully pouring, in order: honey, corn syrup, dish soap, water (colored), vegetable oil, mineral oil (these have progressively lower densities). Drop small objects of different densities — a grape, a cork, a coin, a cherry tomato — to see each one settle at its density layer. Mineral oil and vegetable oil form visually distinct layers and together demonstrate that even two similar-looking oils can separate.
Surface Chemistry: Place a thin layer of mineral oil on the surface of water in a wide dish. Add a drop of food coloring to the water below — it diffuses in the water but not into the oil layer. Then add a drop of dish soap, which disrupts the oil layer and the interface dramatically, spreading the dye. A vivid demonstration of how surfactants disrupt hydrophobic barriers.
Inert Seal: Use mineral oil to store small pieces of reactive metal (sodium, potassium, calcium) safely — the oil prevents air and moisture contact without reacting. This is standard lab practice and illustrates how chemical reactivity can be controlled by controlling access to reactants.
Experiments using this chemical:
- Density Columns — Layered immiscible liquids by density
Safety
Very low hazard — essentially non-toxic and chemically inert.
Mild laxative effect if ingested in quantity (this is its pharmaceutical use). Combustible — keep away from open flames in bulk, though flash point is high enough (~150 °C) that it is not a hazard at room temperature. Slippery on floors if spilled — wipe up promptly. Dispose of small quantities with general waste; larger volumes should be collected as waste oil.
Incompatible with: Strong oxidizers at elevated temperatures; chlorine gas (reaction of organic substrate at high temp/UV)